A newborn with a severe cardiopulmonary problem should have the catheter inserted into either of the umbilical arteries. Usually, a long, flexible umbilical artery catheter (UAC) is placed into the patient and advanced into the aorta (see Figure 1-7). If indicated, this should be done as quickly as possible after birth, before arterial spasm prevents the catheter from being advanced. If the patient weighs more than 1250 g, a 6-French (6 Fr) catheter is used; a 3.5 Fr catheter is used if the neonate weighs less than 1250 g. Besides obtaining blood samples and monitoring the blood pressure, one can give the newborn glucose or a blood transfusion through the catheter. A neonatal patient also may have the umbilical vein catheterized.

b. Put the equipment together and make sure that it works properly

Arterial catheters come as single units in sterile packaging. Additional stopcocks, tubing, flush solution, and an automatic solution drip device are needed. These are discussed in Chapter 5 in some detail.

c. Troubleshoot any problems with the equipment

See the discussion in Chapter 5.

3. Get a blood sample from an arterial catheter (Code: IB9f and IIIE2b) [Difficulty: ELE: R, Ap; WRE: An]

The steps for obtaining an arterial blood sample include the following:

1. Tell a conscious adult patient that you are going to take a blood sample from the arterial catheter.

2. Put gloves on both hands.

3. Remove the dead-ender cap (or syringe) from the sample (side) port on the three-way stopcock between the catheter and the intravenous (IV) tubing.

4. Screw a sterile 5- to 10-mL syringe to the sample port for removing the IV solution from the catheter. (A smaller syringe should be used for a neonate.)

5. Turn the stopcock off to the IV tubing (Figure 3-1).

6. Pull a waste sample of IV solution and blood into the syringe. (The amount withdrawn and discarded depends on the dead space volume from the tip of the catheter to the side port. Studies indicate that between 2.5 and 6 times this volume should be removed. Typically, this is ≤5 mL in an adult and even less in a neonate.)

7. Turn the stopcock off to all ports by turning it halfway between any two ports.

8. Attach a preheparinized sterile syringe to the sample port, turn the stopcock open to the syringe, and withdraw about 2 to 3 mL of arterial blood to be analyzed.

9. Turn the stopcock off to all ports by turning it halfway between any two ports.

10. Remove the blood-sample syringe, and cap off the syringe to seal it.

11. If liquid heparin is used in the syringe, roll the syringe to mix it with the blood.

12. If it will be more than 30 minutes until analysis, place the blood sample into an ice-water bath.

13. Turn the stopcock toward the sample port so that the IV solution runs into the catheter.

14. Fast flush any blood in the catheter back into the patient.

15. Screw the dead-ender cap (or syringe) onto the sample port.

16. Remove gloves, and properly dispose of them and any other waste materials. Wash your hands.

Figure 3-1 A three-way stopcock for use in an arterial line system. A, The normal operating position of the stopcock that allows fluid to flow to the patient (and the blood pressure to be monitored if assembled for continuous blood pressure monitoring). B, The stopcock position that allows blood to be withdrawn from the patient through the sample port. The flush solution port is closed. C, The stopcock position for flush solution to go to the sample port to clear out any blood. When the stopcock is turned to a 45-degree angle between any two ports, all of the ports are closed.

(From Wilkins RL, Stoller JK, Kacmarek RM: Egan’s fundamentals of respiratory care, ed 9, St Louis, 2010, Mosby.)

4. Perform an arterial puncture to obtain a blood sample for analysis (Code: IB9f and IIIE2a) [Difficulty: ELE: R, Ap; WRE: An]

A number of possible variations on the technique exist. The following is a general but thorough listing of the steps and any important related information.

a. Check for a valid physician order.

b. Check the patient’s chart for pertinent information on supplemental oxygen being used, bleeding disorders such as hemophilia, and use of anticoagulant medications. It is important to check the patient’s clotting time because a hematoma will result in a patient with a slow clotting time if extra time is not spent holding the puncture site.

d. Introduce yourself and your department to the patient. Identify the patient. Explain what you are there to do. Gain the patient’s confidence so that he or she will offer full cooperation.

e. Select the puncture site. The following choices are listed in order from most to least favorable: radial, brachial, dorsalis pedis, and femoral. If the radial site is selected, try to puncture the left wrist if the patient is right-handed or vice versa.

f. If the radial or pedal sites are selected, the Allen’s test (also known as the modified Allen’s test) must be performed to ensure that adequate collateral flow is present in case the artery becomes clotted because of the procedure.

1. Radial artery site

See Figure 3-2 for the basic procedure. Circulation to the hand is stopped by pressing both the radial and the ulnar arteries closed. Releasing the pressure over the ulnar artery should result in the hand flushing within 5 to 15 seconds. This is a positive test result and proves that the ulnar artery has adequate circulation to the hand. If the hand does not flush within 15 seconds of the release of the ulnar artery, the circulation is inadequate, and the radial artery of that wrist must not be punctured. Another site must be evaluated for puncture.

Figure 3-2 The modified Allen’s test. A, The hand is clenched into a tight fist, and pressure is applied to the radial and ulnar arteries. B, The hand is opened (but not fully extended); the palm and fingers are blanched. C, Removal of the pressure on the ulnar artery should result in flushing of the entire hand.

(From Shapiro BA, Peruzzi WT, Kozlowski-Templin R: Clinical application of blood gases, ed 5, St Louis, 1994, Mosby.)

2. Dorsalis pedis artery site

Press down on the dorsalis pedis artery to occlude it. Press on the nail of the great toe so that it blanches. Release the pressure on the nail, and watch for a rapid return of color. This normal test finding confirms that a good blood flow exists through the posterior tibial and lateral plantar arteries. It is safe to draw a sample from the site. A slow return of blood flow indicates poor circulation; another site must be chosen.

g. Prepare the equipment and the puncture site by using sterile technique.

1. If necessary, draw up the heparin solution, flush the syringe with it, and discard the excess.

2. If a radial or brachial site is selected, the joint should be hyperextended with a folded towel to help stabilize it.

3. Clean the site by wiping the area with an alcohol or iodophor swab in a widening spiral motion that starts at the desired puncture site.

4. Put on gloves and goggles.

5. Some prefer to anesthetize the puncture site with a 0.8- to 1.0-mL injection of 0.5% to 2% lidocaine (Xylocaine) into the skin. Others think that this is unnecessary because the lidocaine injection itself will cause pain. If an anesthetic is used, wait about 2 minutes for it to take effect before puncturing the skin.

h. Draw the blood sample.

1. Hold the syringe like a pencil. The radial and dorsalis pedis arteries should be entered from a 45-degree angle; the brachial and femoral arteries should be entered from a 90-degree angle (Figure 3-3). Use the first two fingers of your free hand to palpate the pulse and hold the artery still. The bevel of the needle should be up as it enters the skin.

2. Tell the patient that he or she will “feel a stick.”

3. The needle should enter the skin quickly to minimize pain. Carefully advance the needle into the artery. A pulsatile flow is seen with each heartbeat. If unsuccessful, withdraw the needle to the skin, change the angle as needed, and reinsert it into the artery.

4. Withdraw about 1 to 2 mL of blood before removing the needle.

5. Press the sterile gauze onto the puncture site for 2 to 5 minutes. Check the site to ensure that clotting has occurred. Hold longer if necessary. An assistant may help with this.

6. While holding the site, seal the syringe or safely cover the needle.

7. If liquid heparin was added to the syringe, roll it to mix the heparin and blood.

8. If the blood sample will not be analyzed within 30 minutes, place the syringe into an ice-water mix. (Failure to cool the blood sample results in a decrease in the Pao₂ value, an increase in the Paco₂ value, and a decrease in the pH value.)

9. Label the syringe with the date, time, patient’s name, oxygen percentage, and temperature if abnormal. Some departments may also add the patient’s age and the position in which he or she was sitting or lying when the sample was drawn because of the effects they may have on oxygenation.

10. Have the sample promptly analyzed. The patient’s blood gas results should be accurate if the blood sample is analyzed within 10 to 15 minutes if not iced and within 60 minutes if iced.

11. Properly dispose of gloves and waste materials, and wash your hands.

12. Return in 20 minutes to recheck the puncture site for a hematoma and good distal circulation.

Figure 3-3 A, Radial arterial position in the lower arm and wrist. B, Bevel and needle positioning for radial arterial puncture and other arterial punctures, respectively.

(From Lane EE, Walker JF: Clinical arterial blood gas analysis, St Louis, 1987, Mosby.)

Exam Hint 3-2 (ELE, WRE)

Commonly tested areas involve the ABG technique and possible hazards to it. Patient hazards include blood vessel trauma, hematoma, arterial clot formation, arteriospasm, and infected puncture site. In addition, the therapist may receive an accidental needle puncture injury.

5. Perform arterialized capillary blood gas sampling (Code: IB9g and IIIE2c) [Difficulty: ELE: R, Ap; WRE: An]

Occasionally, a sample of arterialized capillary blood from a patient must be obtained for blood gas analysis. The usual clinical situation is a neonate who has a pulmonary problem that warrants evaluation. However, because of the neonate’s small arteries, a sample cannot be drawn. The steps and key points to keep in mind during the sampling procedure follow.

a. Select a highly vascularized and well-perfused site. Usually the heel is selected, but the great toe, earlobe, and finger also are acceptable.

b. Warm the heel (or other site) with a warm towel or heat lamp for about 5 to 10 minutes; 42° C is ideal. Warming vasodilates the vessels and “arterializes” the capillary blood supply.

c. After warming, unwrap the site, and wipe it with an antiseptic pad.

d. Use a pediatric lance to deeply puncture the outer edge of the heel (Figure 3-4). Blood should flow freely without squeezing the area. Wipe away the first blood drop with a sterile gauze pad. The blood will be “dearterialized” if it is squeezed out with venous blood, and the sample will be useless for blood gas analysis.

e. Insert a preheparinized capillary tube (0.075 to 1.0 mL) deeply into the drop of blood. The blood should easily flow through the tube and, ideally, a second sample tube is filled.

f. Seal both ends of the tubes.

g. Use a magnet to draw the metal filing (flea) back and forth in the tubes to mix the blood, if needed.

h. Unless the blood samples will be analyzed within 30 minutes, place the sealed tubes into an ice-water bath.

i. Send the samples to the laboratory for analysis along with the proper paperwork unless point-of-care analysis is performed.

j. Apply pressure to stop the bleeding. Complications include infection, bone spurs, and laceration of the posterior tibial artery.

Figure 3-4 Puncture sites on an infant’s heel for obtaining an arterialized capillary blood sample. Avoid the posterior tibial artery that runs through the center of the foot.

(From Walsh B, Czervinske MP, Di Blasi RM: Perinatal and pediatric respiratory care, ed 3, St Louis, 2010, Mosby.)

MODULE C

Analyze blood gas values

1. Manipulate a point-of-care blood gas analyzer by order or protocol (Code: IIA10) [Difficulty: ELE: R, Ap; WRE: An]

a. Select the appropriate point-of-care blood gas analyzer

A point-of-care (POC) blood gas analyzer can be used as an alternative to a centrally located analyzer. This is especially helpful when a patient is being transported, or when the data are very quickly needed at the bedside. The blood gas values of Pao₂, Paco₂, and pH are typically obtained from either a POC analyzer or a standard blood gas analyzer. These standard units are acceptable for all patient care situations except when carbon monoxide poisoning is known or suspected. Neither the POC nor the standard blood gas analyzer is able to measure carboxyhemoglobin (COHb). A CO-oximeter is needed to measure the patient’s level of carboxyhemoglobin in carbon monoxide cases. Some POC analyzers also can measure levels of serum electrolytes and other commonly needed patient values such as blood glucose, hemoglobin, and hematocrit levels.

b. Put the equipment together and make sure that it works properly (CSE code: IIA10)

POC analyzers can be powered through a self-contained battery or by plugging the unit into a standard alternating current electrical outlet. The units have single-use disposable cartridges that include the electrodes and calibration reagents and will hold waste samples. The following discussion covers the basic principles of operation of the electrodes in POC and standard centrally located analyzers.

1. pH electrode

The modern pH electrode has existed since the mid-1950s and is usually referred to as the Sanz electrode after its principal inventor. The basic principle behind the pH analyzer is its ability to measure the voltage (potential for electrical flow) between two different solutions. This is based on the different hydrogen ion (H+) concentrations between the solutions that reflect their relative pH values. The reference electrode is immersed in a solution with a pH of 6.840 that fills a glass or plastic chamber. The blood sample, of unknown pH, is placed in a separate measuring chamber called a cuvette. These two chambers are separated by a special glass membrane that contains metals and sodium ions (Na+), thus making it pH sensitive. Both chambers are kept at a stable 37° C temperature. See Figure 3-5 for a graphic representation of the pH electrode. When blood or a quality control (QC) material is introduced into the cuvette, the potential exists for hydrogen ions to replace the sodium ions in the pH-sensitive glass if the two pHs are different. The replacement is proportional to the difference in the two pHs.

Figure 3-5 Key components of the pH electrode. A, A voltage develops across the pH-sensitive glass when a difference exists in the hydrogen ion concentration between the two solutions. B, Two separate half-cells are used for the measuring electrode and the reference electrode. C, The addition of a KCl contact bridge and voltmeter completes the electrical circuit and enables the pH of the patient’s blood sample to be measured.

(From Harrison BA, Shapiro C: Clinical application of blood gases, ed 4, St Louis, 1989, Mosby.)

2. Pco₂ electrode

The partial pressure of carbon dioxide (Pco₂) is measured in a modified pH electrode. This was first designed in the mid-1950s by Stow and further perfected by Severinghaus. Accordingly, these units are now referred to as Severinghaus electrodes, or sometimes as Stow-Severinghaus electrodes. In Figure 3-6, the electrode is depicted in cross section. It has a reference half-cell and a measuring half-cell that are enclosed within pH-sensitive glass and electrically connected by an electrolyte contact bridge. The blood sample is introduced into a cuvette heated to 37° C.

Figure 3-6 Schematic illustration of the modern Pco₂ electrode. (Note that the space between the elastic membrane and the nylon spacer is greatly enlarged for clarity.)

(From Harrison BA, Shapiro C: Clinical application of blood gases, ed 4, St Louis, 1989, Mosby.)

The principle of operation is based on the amount of CO₂ found in the blood sample that diffuses through the silicone elastic membrane. The CO₂ chemically combines with the bicarbonate solution to change the pH of the solution by the release of H+. This change in H+ concentration creates a voltage difference between the measuring and reference half-cells that is proportional to the amount of CO₂ found in the patient’s blood sample.

3. Po₂ electrode

This unit is completely different from the others mentioned. It was developed in the late 1950s by Clark and thus is usually called a Clark electrode. It is also sometimes known as a polarographic electrode because of the basis of its operation. Figure 3-7 is a drawing of key features of the unit. A phosphate-KCl buffer solution surrounds the silver anode. A thin membrane separates the blood-filled cuvette from direct contact with the electrode but allows oxygen molecules to diffuse slowly through to contact the platinum wire cathode. The whole unit is heated to 37° C. The term polarographic comes from the addition of about 0.7 volt to the cathode to make it slightly “polarized” or negative compared with the anode. This is needed to ensure that oxygen is rapidly chemically reduced (that it gains electrons) at the cathode. This creates an electrical current directly proportional to the number of reduced oxygen molecules.

Figure 3-7 Schematic illustration of the Clark electrode for measuring PO₂.

(From Harrison BA, Shapiro C: Clinical application of blood gases, ed 4, St Louis, 1989, Mosby.)

It must be understood that the partial pressure of oxygen (Po₂) being measured is derived from oxygen that is dissolved in the plasma. It does not come from the hemoglobin found in the erythrocytes (red blood cells). The reported value for the saturation of oxygen in the hemoglobin (Sao₂) is calculated by using a mathematical table. Under normal conditions, the calculated Sao₂ value is the same as or close to the true Sao₂ value. Carbon monoxide (CO) poisoning is the only commonly seen clinical situation during which a calculated saturation can be incorrectly high. If carbon monoxide poisoning is suspected or known, the patient’s blood sample should be analyzed on a CO-oximeter unit.

c. Troubleshoot any problems with the point-of-care blood gas analyzer

POC analyzers are designed to self-calibrate and display information on their operation. Replace a testing cartridge that is not properly functioning. Recharge a battery that is running low on reserve power.

d. Perform quality control procedures for a blood gas analyzer (Code: IIC1) [Difficulty: ELE: R, Ap; WRE: An]

1. Quality control

Quality control (QC) refers to creating a measurement and documentation system to confirm the accuracy (precision) and reliability of all blood gas measurements. Accuracy or precision means that the measured physiologic values truly reflect the actual physiologic values. Reliability means that a high degree of confidence exists so that the measured values represent the patient’s actual physiologic values. Both are critically important if the blood gas results are to be used to make correct clinical decisions.

2. Quality assurance

Quality assurance (QA) refers to the broader concern that the results of the blood gas measurement are not only accurate and reliable but also clinically useful. To help ensure this, the Clinical Laboratory Improvement Amendments of 1988 (CLIA 88) require that the department have written policies and procedures on items including record keeping, equipment maintenance, staff training, and the correction of errors.

3. Calibration

Calibration is the systematic standardization of the graduations of the blood gas analyzer against known values to ensure consistency. Proper calibration of the electrodes is essential to the accuracy of the blood gas values. Some general calibration steps are discussed later. The manufacturer’s guidelines must be followed for the specific steps in calibration.

4. Quality control materials

A variety of QC materials are available to calibrate the electrodes for Po₂, Pco₂, and pH measurements. Their uses vary. Each of the following materials has its advantages, disadvantages, and limitations. The manufacturer of a particular brand or model of blood gas analyzer may require that a specific type of material be used in its units.

Aqueous buffers are water based and are used to check pH and Pco₂ measurements; they cannot be used to check Po₂ measurements. Commercially prepared gases are used to check Po₂ and Pco₂ measurements; they cannot be used to check pH measurements. The following CO₂ mixes may be used: 0, 5%, 10%, and 12%. The following O₂ mixes may be used: 0, 12%, 20%, 20.95% from room air, 21%, and 100%.

Tonometered liquids are exposed in the laboratory to known oxygen and carbon dioxide gas mixes until the liquids are saturated and have the same partial pressures as the gases. Three types of tonometered liquids are used. First, using human or animal serum or whole blood is the most accurate method available and is used mainly to obtain Po₂ and Pco₂ levels. Although whole human blood cannot be used for pH measurements, a bovine blood product can be used to obtain all three values. Second, assayed liquids are non–water-based liquids that are pretonometered by the manufacturer and are available in sealed glass vials. They can be used to obtain Po₂, Pco₂, and pH measurements. These are very popular because of the advantages of speed and simplicity. Third, oxygenated fluorocarbon-based emulsions (perfluorinated compounds) can be used to obtain Po₂, Pco₂, and pH measurements. They are considered as accurate as whole blood without its associated risks.

5. Levey-Jennings charts

The Levey-Jennings charts (also known as Shewhart/Levey-Jennings or quality control [QC] charts) are used to record the results of each calibration procedure. They are similarly designed with time plotted on the horizontal scale and the analyte (Po₂, Pco₂, or pH) on the vertical scale. The vertical scale for each analyte has a central value for what is normally expected. On both sides of this normal value are standard deviation (SD) points, showing movement away from what is expected. When an analyte electrode is operating within the acceptable limits, it is said to be in control. In general, if an analyte is within 2 SDs of the normal value, it is considered to be in control.

An out-of-control situation exists whenever a single calibration value or a series of calibration values is outside established limits. A random error is an unpredictable aberration in precision that occurs when the QC material is sampled. A systematic error shows an accuracy problem and is much more serious. It must be investigated, corrected, and documented. Figure 3-8 shows an example of both random error and systematic error. Rules have been established for determining whether the error is random or systematic (Table 3-1).

Figure 3-8 Levey-Jennings quality control chart for Paco₂. The central horizontal line represents the mean value of 40 torr. The next lower and higher horizontal lines show 1 standard deviation (1 SD) value from the mean. The second most distant lower and higher horizontal lines are 2 SDs from the mean. The most distant horizontal lines are 3 SDs from the mean. The bottom number scale indicates 8-hour work shifts. Open circles, Calibration values that are within 2 SDs of the mean value. They are considered to be in control. Black circles, Calibration values that are out of control. A represents a random error and B a systematic error.

(From Shapiro BA, Peruzzi WT, Kozlowski-Templin R: Clinical application of blood gases, ed 5, St Louis, 1994, Mosby.)

TABLE 3-1 Westgard’s Rules for Determining When an Aalyzer Is Not Functioning Properly

Rule name	Levey-Jennings chart
RANDOM ERROR
1-2 SDs	The measurement is more than 2 SDs but not more than 3 SDs from the mean.
1-3 SDs	The measurement is more than 3 SDs from the mean.
R^*-4 SDs	Two consecutive measurements are 4 SDs or more apart.
SYSTEMATIC ERROR
2-2 SDs	Two consecutive measurements are either 2 SDs above or 2 SDs below the mean.
4-1 SDs	Four consecutive measurements are either 1 SD above or 1 SD below the mean.
7-trend	Seven consecutive measurements are on only one side of the mean; each measurement is progressively more out of control.
10-mean	Ten consecutive measurements are on only one side of the mean.

SDs, standard deviations.

Modified from Lane EE, Walker JF: Clinical arterial blood gas analysis, St Louis, 1987, Mosby.

6. pH electrode

NOTE: These and the following guidelines are based on CLIA standards or widely reported industry standards. The various brands and models of blood gas analyzers may have different frequencies of calibration requirements, depending on their Food and Drug Administration (FDA)-approved manufacturers’ studies.

a. One-point calibration

1. Should be done before every sample is analyzed if one-point calibration is not automatically performed every 30 minutes

2. Performed with the near-normal QC material of 7.384 ± 0.005 pH used to set the balance potentiometer

3. Recommended every 30 minutes

4. Should be rechecked after a suspicious pH result; the blood sample should then be rerun

b. Two-point calibration

1. Should be done at least once every 8 hours when patient samples are being analyzed

3. Performed with the slope potentiometer set with a QC material of 6.840 ± 0.005 pH (this is the same pH as the reference electrode solution)

4. Performed with the balance potentiometer set with a QC material of 7.384 ± 0.005 pH

c. Three-point calibration

1. Should be done at least every 6 months on existing equipment

2. Should be done whenever a new electrode is put into use

3. Covers the physiologic range to confirm linearity: QC materials of 6.840 ± 0.005, 7.384 ± 0.005, and 7.874 ± 0.005 pH are used

7. Pco₂ electrode

a. One-point calibration

1. Should be done before every sample is analyzed if one-point calibration is not automatically performed every 30 minutes

2. Performed with 5% ± 0.03% CO₂ used to set the balance potentiometer

3. Recommended every 30 minutes

4. Should be rechecked after a suspicious Pco₂ result; the blood sample should then be rerun

The CO₂ can be directly in contact with the electrode, tonometered with an aqueous material or blood, or premixed in aqueous buffers, assayed liquids, or fluorocarbon-based emulsion.

b. Two-point calibration

1. Should be done at least once every 8 hours when patient samples are being analyzed

3. Performed with 10% ± 0.03% CO₂ to set the slope potentiometer

4. Performed with 5% ± 0.03% CO₂ to set the balance potentiometer

c. Three-point calibration.

1. Should be done at least every 6 months on existing equipment

2. Should be done whenever a new electrode is put into use

3. Covers the physiologic range to confirm linearity; three Pco₂ values between 0 and 80 mm Hg should be determined

4. Room air or a gas cylinder containing up to 0.03% CO₂ can be used to set the 0 point

Exam Hint 3-3 (WRE) (ELE, WRE)

Past examinations have required that the following equation be solved. Remember that more than one carbon dioxide percentage can be used. NOTE: The atmospheric pressure of a gas (e.g., oxygen, carbon dioxide) may be listed by the equivalent terms of torr or mm Hg. For example, a Pco₂ of 40 torr is the same as a Pco₂ of 40 mm Hg. However, it is important to note that on the NBRC exams, the term torr is used for gas pressure.

The predicted Pco₂ value at a given CO₂ percentage is calculated with this formula:

In which Pco₂ is the predicted Pco₂ in mm Hg or torr; P_B is the barometric pressure at the institution where the analysis is being performed; P_H₂O is the water vapor pressure based on the patient’s temperature, 47 torr at 37° C/98.6° F; and % CO₂ is the percentage of CO₂ (also listed as FCO₂).

Example for one-point (balance) potentiometer calibration at sea level in which Pco₂ is the predicted Pco₂ in torr, P_B is 760 torr, P_H₂O is 47 torr, and % CO₂ is 5%:

Therefore set the Pco₂ control at 36 torr.

8. Po₂ electrode

a. One-point calibration

1. Should be done before every sample is analyzed if one-point calibration is not automatically performed every 30 minutes

2. Performed with 12% ± 0.03% O₂ used to set the balance potentiometer; some analyzers are designed to use 20% ± 0.03% O₂ from a gas cylinder or draw room air (20.95% oxygen) into the unit

3. Recommended every 30 minutes

4. Should be rechecked after a suspicious Po₂ result; the blood sample should then be rerun

The O₂ can be directly in contact with the electrode, tonometered with an aqueous material or blood, or premixed in aqueous buffers, assayed liquids, or fluorocarbon-based emulsion as discussed.

b. Two-point calibration

1. Should be done at least once every 8 hours when patient samples are being analyzed

2. Should be done whenever readjustment of one-point calibration is greater than 3 torr

4. Performed with 0 ± 0.03% O₂ to set the slope potentiometer

5. Performed with 12% ± 0.03% O₂ to set the balance potentiometer; some analyzers are designed to use 20% ± 0.03% O₂ from a gas cylinder or draw room air (20.95% oxygen) into the unit

c. Three-point calibration

1. Should be done at least every 6 months on existing equipment

2. Should be done whenever a new electrode is put into use

3. Should be done to confirm linearity whenever the Po₂ value could be more than 150 torr, assuming that the balance point is set on room oxygen content; the third point should be set on 100% ± 0.03% O₂ from a gas cylinder

Exam Hint 3-4 (ELE, WRE)

Past examinations have required that the following equation be solved. Remember that more than one oxygen percentage can be used.

The predicted Po₂ value at a given O₂ percentage is calculated with this formula:

In which Po₂ is the predicted Po₂ in torr, P_B is the barometric pressure at the institution where the analysis is being performed, P_H₂O is the water-vapor pressure based on the patient’s temperature, 47 torr at 37° C/98.6° F; and % O₂ is the percentage of O₂ (also listed as FO₂).

Example for one-point (balance) potentiometer calibration at sea level in which Po₂ is the predicted Po₂ in torr, P_B is 760 torr, P_H₂O is 47 torr, and % O₂ is 12%:

Therefore set the Po₂ control at 86 torr.

Exam Hint 3-5 (WRE)

Often a question relates to QC. It could include reanalyzing a blood gas sample with suspicious results, rechecking out-of-control calibration results (greater than 2 SDs), or using two analyzers for cross referencing.

9. Miscellaneous topics

10. Calibration gas cylinders

For economic reasons, the low-percentage oxygen and carbon dioxide gases are placed together in one cylinder, and the high-percentage oxygen and carbon dioxide gases are placed together in a second cylinder. Box 3-1 summarizes the normal precision of the electrodes discussed and the gases used in their calibration. A cylinder containing 100% oxygen and no carbon dioxide can be used for three-point calibration.

11. Temperature correction

Temperature correction refers to mathematical adjustment of a patient’s Pao₂, Paco₂, and pH values if his or her temperature is not 37° C. Remember that blood gas analyzers are calibrated at 37° C because it is normal body temperature. If the patient has a fever, the oxygen and carbon dioxide partial pressures in the blood will be greater than those found during the blood gas analysis. Conversely, the hypothermic patient will have lower oxygen and carbon dioxide partial pressures in the blood than those found during the blood gas analysis. The pH value will shift in the opposite direction from the Pco₂ value. Usually this small shift in values is ignored. However, because some physicians may specify that their patient’s blood gases be temperature corrected, the patient’s temperature should be listed on the blood gas slip. It is a simple mathematical process to temperature correct the blood gas results. Most modern analyzers perform temperature correction automatically when programmed to do so. However, because the patient’s values are not usually affected in a clinically significant way, temperature correction is not currently recommended.

e. Perform quality control procedures for a point-of-care blood gas analyzer (Code: IIC3) [Difficulty: ELE: R, Ap; WRE: An]

The general principles of QC and QA discussed with standard blood gas analyzers apply to point-of-care (POC) analyzers as well. Remember to flush the electrode membrane after each use, if possible, to prevent protein buildup. If protein buildup occurs, the response time is longer than normal. Follow the manufacturer’s guidelines to change an electrode membrane as needed. Make sure that no air bubbles are under the membrane or within the tubing through which the blood travels. Rerun the calibration for any of the electrodes and reanalyze the sample if you are suspicious of the result. If the electrode does not calibrate close to the reference buffer solutions or gases, it should not be used.

In a random-error situation, the practitioner likely made a simple error when introducing the material or running the analyzer. Common problems include an air bubble injected into the unit or incomplete flushing of the previous sample. Usually flushing out any residual blood and then carefully injecting more of the current patient blood samples will correct the problem. Run the analyzer again to obtain new patient values or run the same patient sample through another analyzer and compare the two sets of results.

A systematic error usually indicates a problem with the analyzer, QC materials, or processes. Examples of systematic errors include misanalyzed CO₂ or O₂ standards for calibration; contaminated QC materials; or deteriorated oxygen, carbon dioxide, or pH electrode function. Each of these must be investigated until the problem is found and corrected. The unit cannot be used again until after it is proven to work properly and to give accurate results.

f. Perform blood gas analysis (Code: IB9j and IIIE3c) [Difficulty: ELE: R, Ap; WRE: An]

Modern blood gas analyzers are simple to operate. Follow the manufacturer’s guidelines for insertion of the blood sample. Perform the specified steps in the analysis. Print out the results. Many current units run a self-diagnosis if any problems appear. Correct any identified problems as discussed above.

1. CO-oximeter/hemoximeter

a. Perform quality control procedures on the CO-oximeter/hemoximeter (Code: IIC1) [Difficulty: ELE: R, Ap; WRE: An].

The unit is preassembled by the manufacturer. Practical experience with a unit is recommended to understand how to add a patient blood sample and perform calibration duties. (See Figure 3-9 for a schematic drawing of a CO-oximeter.) A thallium-neon hollow cathode lamp emits light in the infrared-visible range. A device called a monochromator contains four filters and rotates through the light beam. Each filter allows only one specific wavelength to pass through it. These four monochromatic wavelengths correspond to the three isosbestic points, discussed later (shown in Figure 3-10) and 626.6 nm. This last wavelength is poorly absorbed by all four hemoglobin moieties. It is used to find the maximal difference in absorption so that the relative amounts of the hemoglobin species can be determined.

Figure 3-9 CO-oximeter basic components. A, Thallium-neon hollow cathode light source. B, Lens and mirror. C, Monochromator with four specific wavelength filters. D, Light-beam splitter that diverts half of the light to the reference detector and half to the cuvette. E, Reference wavelength detector. F, Patient sample cuvette. G, Sample wavelength detector. H, Temperature-regulated block set at 37° C.

(From Shapiro BA, Peruzzi WT, Kozlowski-Templin R: Clinical application of blood gases, ed 5, St Louis, 1994, Mosby.)

Figure 3-10 Spectral analysis of the hemoglobin moieties (species). O₂Hb is oxyhemoglobin, COHb is carboxyhemoglobin, RHb is reduced hemoglobin, and MetHb (METHb) is methemoglobin. Point A shows the triple isosbestic point at 548 nm for O₂Hb, COHb, and RHb. Point B shows the double isosbestic point at 568 nm for O₂Hb and RHb. Point C shows the double isosbestic point at 578 nm for RHb and COHb. A fourth wavelength at 626.6 nm is used for comparison purposes.

(From Shapiro BA, Peruzzi WT, Kozlowski-Templin R: Clinical application of blood gases, ed 5, St. Louis, 1994, Mosby.)

When a blood sample is placed into the cuvette, the same four wavelengths are passed through it. The amount of absorbance at each wavelength is measured and compared with the absorbance at each wavelength by a reference sample solution (see the following discussion). The computer integrates the data and calculates the total hemoglobin concentration and the amounts of the four hemoglobin moieties.

Total hemoglobin (THb) concentration should be calibrated when the unit is installed, at regular intervals suggested by the manufacturer, after the sample tubing is changed, after the cuvette is disassembled or changed, and whenever a suspicious reading is seen. Calibration is done by filling the cuvette with a special dye produced by the manufacturer and analyzing it by following the prescribed steps.

Routine calibration is done every 30 minutes. The unit obtains and stores absorbance readings at the four different wavelengths from a “blank” solution in the reference detector. When the same blank solution is added to the sample cuvette, the same four wavelengths are measured. Their absorption levels are normally identical. The same procedure is performed after every patient sample is analyzed.

The following are examples of common problems and their solutions:

• Incomplete hemolysis of the blood sample causes the light to scatter off cell fragments and lipids. Sickle cells (as in sickle cell anemia) are difficult to disrupt and may cause false oxyhemoglobin and carboxyhemoglobin readings if extra time is not taken for hemolysis. Follow manufacturer’s guidelines on the procedure for hemolyzing the red blood cells.

• More than 10% methemoglobin may cause errors in the measurement of all hemoglobin moieties. Sulfhemoglobin also causes false readings. The practitioner may need to gather additional information from the chart or laboratory regarding the patient’s levels of these abnormal hemoglobin moieties. CO-oximetry should probably not be performed on blood samples with abnormal levels of methemoglobin or sulfhemoglobin.

• Intravenous dyes such as methylene blue, Evans blue, and indocyanine green used in various cardiac studies can absorb the same wavelengths of light used to identify the various forms of hemoglobin. The presence of these dyes results in a lower measurement of oxyhemoglobin than is actually present. Check the patient’s chart for a record of dyes used. CO-oximetry should probably not be used for blood gas analysis on patients who have these dyes in their systems.

• Failure to reprogram the analyzer for fetal hemoglobin instead of adult hemoglobin may produce false results. Remember to check the chart for the patient’s age. Reprogram the analyzer for fetal hemoglobin for any sample taken from an infant who is only a few weeks old.

• The presence of lipid particles in the blood causes light scattering and results in a reading that is falsely high in total hemoglobin and percentage of methemoglobin and falsely low in percentage of oxyhemoglobin and percentage of carboxyhemoglobin. Follow the laboratory’s guidelines to determine when a patient’s blood lipid value is too high for accurate use of the CO-oximeter.

• Air bubbles or incomplete hemolysis of blood in the cuvette causes an absorbance error. Air bubbles must be flushed out and the blood sample inserted again and reanalyzed. Make sure that all blood samples are hemolyzed according to the manufacturer’s guidelines.

• Blood clots in the sample tubing prevent blood from flowing through to the cuvette. If a sample cannot be inserted into the unit, suspect and check for a blood clot. Remove any clotted tubing, replace it, and confirm that the CO-oximeter is working properly.

b. Perform CO-oximetry/hemoximetry (Code: IB9j and IIIE3c) [Difficulty: ELE: R, Ap; WRE: An].

Most hospitals have a CO-oximeter in addition to a blood gas analyzer. A CO-oximeter should be used to analyze a blood gas sample whenever carbon monoxide poisoning is known or suspected. In addition, a CO-oximeter gives a complete analysis of the relative amounts of the different types of patient hemoglobin.

CO-oximeters also are called spectrophotometric oximeters and are the most accurate method available to measure the four different hemoglobin moieties (species or variations in the hemoglobin molecule). These hemoglobin species include the following:

• Oxyhemoglobin (HbO₂ or O₂Hb), which carries oxygen to the tissues, and reduced hemoglobin (HbR or RHb), which has given up its oxygen and picked up carbon dioxide

• Carboxyhemoglobin (HbCO or COHb), which is nonfunctional because of the tightness with which carbon monoxide binds to the hemoglobin

• Methemoglobin (HbMet or MetHb), which is nonfunctional because the Hb molecule is unable to combine reversibly with oxygen

• Sulfhemoglobin (HbS or SHb), which is similar to the HbMet and is also nonfunctional

• In addition, a CO-oximeter can measure the fetal hemoglobin (HbF or FHb) found in a newborn infant instead of adult oxyhemoglobin

Each of these hemoglobin moieties has a unique spectroscopic “fingerprint” of absorbed light-wave frequencies. See Figure 3-10 for the spectral analysis of the various forms of hemoglobin.

Follow the manufacturer’s guidelines on rewarming the blood sample to body temperature, hemolyzing the sample, and inserting it into the measurement cuvette. Failure to do so may result in incorrect patient values.

The principle of operation of a CO-oximeter is the comparison of the relative absorbances of four wavelengths of light by oxyhemoglobin, reduced hemoglobin, and carboxyhemoglobin. This is done by comparing the absorptions at the three isosbestic points (at which the moieties being compared have equal absorption) and a wavelength point at which the greatest difference in absorption between the two moieties is found. By computer integration of the data, the relative proportions of oxyhemoglobin, reduced hemoglobin, and carboxyhemoglobin are determined. If the total is less than 100% of the hemoglobin present, the difference has to be methemoglobin (or, rarely, sulfhemoglobin). The unit then provides data on total hemoglobin; percentages for oxyhemoglobin, reduced hemoglobin, carboxyhemoglobin, and methemoglobin; and total amounts for them in grams per deciliter of blood. Some units also calculate O₂ content.

Exam Hint 3-6 (ELE, WRE)

If a patient is known to have or suspected of having carbon monoxide poisoning (e.g., removed from inside a burning building), a CO-oximeter should be used for blood gas analysis. A standard or POC blood gas analyzer or pulse oximeter should not be used. These instruments cannot detect or measure carboxyhemoglobin.

MODULE D

Interpret blood gas analysis results to determine how the patient is responding to respiratory care

1. Review the chart for any blood gas results (Code: IA5) [Difficulty: ELE: R; WRE: Ap]

Look in the chart for any blood gas results, including arterial, capillary, or mixed venous. Any patient may have had an arterial sample analyzed. A neonate also may have had an arterialized capillary sample analyzed. A patient with a pulmonary artery catheter may have had a mixed venous sample analyzed.

2. Interpret the results of arterial blood gas analysis (Code: IB10j, IB10f, and IIIE4a) [Difficulty: ELE: R, Ap; WRE: An]

If the blood sampling equipment preparation, the sampling procedure, and blood gas analyzer operation were all done properly, the respiratory therapist and physician can believe that the blood gas results are accurate and reliable. However, if there was a problem with any aspect of obtaining and analyzing the blood gas sample, that sample must be discarded and a new sample obtained and analyzed.

A number of authors have written extensively on how to interpret ABGs. The system proposed by Shapiro and associates (1994) has been found to be both practical and relatively easy to understand. Most of the following discussion and tables are based on this system. This does not mean that if one has learned another system, he or she is at any disadvantage for taking the National Board for Respiratory Care (NBRC) examination.

Exam Hint 3-7 (WRE)

The Written Registry Examination includes specific questions about ABG interpretation. The exam also incorporates blood gas results into other questions that relate to any respiratory care technique or procedure, such as oxygen therapy for a patient with chronic obstructive pulmonary disease (COPD) and adjustment of mechanical ventilation. In addition, often a question requires the interpretation of mixed venous blood gases or arterialized capillary blood gases. The examinee must be proficient in all aspects of blood gas interpretation to do well on the advanced NBRC examinations.

a. Assessment of oxygenation

Hypoxemia or hypoxia can rapidly become life threatening. This will often be first seen when the patient develops cardiac arrhythmias, unstable vital signs, and mental confusion or unconsciousness. Table 3-2 shows the normal Pao₂ values for the newborn, child to adult, and the elderly when room air (almost 21% oxygen) is inhaled at sea level. These values decrease progressively as altitude increases. However, under most clinical conditions, this is not a factor unless working in a high-altitude setting.

TABLE 3-2 Age-Based Acceptable Levels of Partial Pressure of Oxygen in Arterial Blood (Pao₂) When Breathing Room Air (21% Oxygen) at Sea Level

Age	Pao₂
NEWBORN
Acceptable range	40 to 70 torr
CHILD TO ADULT
Normal	97 torr
Acceptable range	>80 torr
Hypoxemia	<80 torr
OLDER ADULT
60-year-old	>80 torr
70-year-old	>70 torr
80-year-old	>60 torr
90-year-old	>50 torr

Modified from Shapiro BA, Peruzzi WT, Kozelowski-Templin R: Clinical application of blood gases, ed 5, Chicago, 1994, Mosby-Year Book.

A general rule is that any patient is seriously hypoxemic if the Pao₂ is less than 60 torr on room air. See Table 3-3 for guidelines on judging the seriousness of hypoxemia. Once hypoxemia is recognized, it must be corrected. The most obvious way to correct hypoxemia is to give supplemental oxygen. The clinician must realize that oxygen alone will not correct the hypoxemia if the patient is hypoventilating (increased Paco₂), has heart failure, or is unable to carry or make use of the oxygen. In general, try to keep the patient’s Pao₂ level between 60 and 100 torr.

TABLE 3-3 Evaluation of Hypoxemia

CONDITIONS: ROOM AIR IS INSPIRED; THE PATIENT IS YOUNGER THAN 60 YEARS^*
Hypoxemia	Pao₂	Sao₂
Mild	60-79 torr	90-94%
Moderate	40-59 torr	75-89%
Severe	<40 torr	<75%

CONDITIONS: SUPPLEMENTAL O₂ IS INSPIRED; THE PATIENT IS YOUNGER THAN 60 YEARS
Hypoxemia	Pao₂
Uncorrected	Less than room air acceptable limit
Corrected	Within room air acceptable limit (<100 torr)
Excessively corrected	>100 torr

* Subtract 1 torr of O₂ from limits of mild and moderate hypoxemia for each year older than 60. A Pao₂ value of <40 torr indicates severe hypoxemia in any patient at any age.

Modified from Shapiro BA, Peruzzi WT, Kozelowski-Templin R: Clinical application of blood gases, ed 5, Chicago, 1994, Mosby-Year Book.

Shapiro and associates (1994) suggested the following formula, in which FiO₂ is the fraction of inspired oxygen, for determining whether the patient will be hypoxemic on room air: “If Pao₂ is less than FiO₂ ×5, the patient can be assumed to be hypoxemic on room air.”

Figure 3-11 shows a normal oxyhemoglobin dissociation curve. The saturation value is important to know because it shows how much hemoglobin is saturated with oxygen. Several important points of correlation exist between the Sao₂ and the Pao₂. (Calculated saturation values can be misleadingly high if the patient has inhaled carbon monoxide. In this situation, it is best to measure the saturation directly on a CO-oximeter–type blood gas analyzer.)

Figure 3-11 The oxygen (oxyhemoglobin) dissociation curve plots the relation between hemoglobin saturation (y-axis) and plasma Pao₂ level (x-axis). A, 75% saturation and a Pao₂ of 40 torr are normally seen in venous blood. B, 85% saturation and a Pao₂ of 50 torr are the minimal levels that should be allowed in a chronically hypoxemic patient. C, 90% saturation and a Pao₂ of 60 torr are the minimal levels that should be allowed in an acutely hypoxemic patient. D, Hemoglobin in the pulmonary capillaries adjacent to normal alveoli will become 100% saturated when the Pao₂ level reaches 150 torr.

(Modified from Lane EE, Walker JF: Clinical arterial blood gas analysis, St Louis, 1987, Mosby.)

Figure 3-12 shows a number of factors that can influence the oxyhemoglobin dissociation curve and how oxygen associates and dissociates from hemoglobin. In a patient with normal oxygenation, these factors are not clinically significant. However, when the Pao₂ level is less than 60 torr and the Sao₂ level is less than 90%, these factors can become an important consideration. As can be seen, a left-shifted oxyhemoglobin dissociation curve results in a lower Pao₂ value at any given saturation. This results in even less oxygen being delivered to the tissues.

Figure 3-12 Conditions associated with altered affinity of hemoglobin for O₂. P₅₀ is the Pao₂ value at which hemoglobin is 50% saturated, normally 26.6 torr. A lower than normal P₅₀ value represents increased affinity of hemoglobin for O₂; a high P₅₀ value is seen with decreased affinity. Note that variation from the normal is associated with decreased (low P₅₀ value) or increased (high P₅₀ value) availability of O₂ to tissues (dashed lines). Shaded area, The entire oxyhemoglobin dissociation curve under the same circumstances.

(From Lane EE, Walker JF: Clinical arterial blood gas analysis, St Louis, 1987, Mosby.)

Exam Hint 3-8 (WRE)

True shunt or venous admixture leading to ventilation/perfusion mismatching are the two most common clinical findings leading to hypoxemia.

b. Assessment of carbon dioxide and pH

The pH is the next important value to interpret because extreme acidemia/acidosis and alkalemia/alkalosis can be life threatening. The carbon dioxide value is important to interpret because it has a direct effect on the pH and indirectly affects the oxygen level. A high or low Paco₂ level, by itself, is not life threatening.

Table 3-4 shows normal values for pH and Paco₂ and the acceptable ranges around the mean or average. Table 3-5 shows the most widely acceptable therapeutic ranges for pH and Paco₂. Values that fall outside of these ranges present a progressively greater risk to the patient.

TABLE 3-4 Normal Laboratory Ranges for Partial Pressure of Carbon Dioxide in Arterial Blood (Paco₂) and pH

TABLE 3-5 Acceptable Clinical Ranges for Partial Pressure of Carbon Dioxide in Arterial Blood (Paco₂) and pH

Paco₂	30-50 torr^*
pH	7.30-7.50

* This is the range for patients with an acute change. It does not apply to patients with long-standing disease, such as chronic obstructive pulmonary disease. These patients may have Paco₂ values greater than 50 torr.

Modified from Shapiro BA, Peruzzi WT, Kozelowski-Templin R: Clinical application of blood gases, ed 5, Chicago, 1994, Mosby-Year Book.

Table 3-6 shows the definitions of Shapiro and associates for alkalemia and acidemia from a respiratory cause. An acute change in the patient’s ventilation causes the following when starting from a Paco₂ of 40 torr:

• If the Paco₂ level increases by 20 torr, the pH decreases by 0.10 unit.

• If the Paco₂ level decreases by 10 torr, the pH increases by 0.10 unit.

TABLE 3-6 Naming Unacceptable Values for Partial Pressure of CO₂ in Arterial Blood (Paco₂) and pH

Paco₂ >45 torr	Respiratory acidosis/alveolar hypoventilation/ventilatory failure
pH <7.35	Acidemia
Paco₂ <35 torr	Respiratory alkalosis/alveolar hyperventilation
pH >7.45	Alkalemia

Modified from Shapiro BA, Peruzzi WT, Kozelowski-Templin R: Clinical application of blood gases, ed 5, Chicago, 1994, Mosby-Year Book.

Thus the body can be seen as better able to compensate with metabolic buffers for a respiratory acidosis than a respiratory alkalosis.

Metabolic effects are evaluated by interpreting either the bicarbonate (HCO₃⁻) value or the base excess/base deficit (BE/BD) value. Both reveal whether any metabolic effect on the pH exists. Normal values are as follows:

• HCO₃⁻: 24 mEq/L

• BE/BD: 0 mEq/L; ± 1 mEq/L is often listed as the normal range

Values indicating metabolic alkalosis of a primary or secondary nature are as follows:

• Bicarbonate level greater than 24 mEq/L

• BE greater than 0 or greater than +1 mEq/L

Values indicating metabolic acidosis of a primary or secondary nature are as follows:

• Bicarbonate level less than 24 mEq/L

• BE less than 0 or greater than −1 mEq/L (i.e., −5 mEq/L); some laboratories report this as a BD or negative BE.

Exam Hint 3-9 (ELE, WRE)

The NBRC uses BE for base excess (regardless of whether it has a positive or negative value) and HCO₃⁻ for bicarbonate.

Exam Hint 3-10 (ELE)

Expect to see several arterial blood gas questions that require the interpretation of acid-base balance and oxygenation. For example, identify that a patient with COPD has an elevated carbon dioxide level and a pH in the low-normal range with a compensated respiratory acidosis. Also, expect to see ABG values included in questions about the adjustment of oxygen therapy and mechanical ventilation.

Tables 3-7 to 3-9 show definitions of terms and classifications of the various acid-base states. As stated earlier, there are other systems for interpreting blood gases. All are probably satisfactory for interpretation purposes and preparing for the NBRC examinations.

TABLE 3-7 Clinical Terminology for Arterial Blood Gas Measurements

Clinical Terminology	Clinical Findings
Respiratory acidosis/alveolar hypoventilation/ventilatory failure	Paco₂ >45 torr
Acute ventilatory failure	Paco₂ >45 torr; pH <7.35
Chronic ventilatory failure	Paco₂ >45 torr; pH 7.36-7.40
Respiratory alkalosis/alveolar hyperventilation	Paco₂ <35 torr
Acute alveolar hyperventilation	Paco₂ <35 torr; pH >7.45
Chronic alveolar hyperventilation	Paco₂ <35 torr; pH 7.41-7.45
Acidemia	pH <7.35
Acidosis	Pathophysiologic condition in which the patient has a significant base deficit (plasma bicarbonate level below normal)
Alkalemia	pH >7.45
Alkalosis	Pathophysiologic condition in which the patient has a significant base excess (plasma bicarbonate level above normal)

Modified from Shapiro BA, Peruzzi WT, Kozelowski-Templin R: Clinical application of blood gases, ed 5, Chicago, 1994, Mosby-Year Book.

TABLE 3-8 Evaluation of Ventilatory and Metabolic Effects on Acid-Base Status

EVALUATION OF Paco₂
Paco₂ >45 torr	Respiratory acidosis/alveolar hypoventilation/ventilatory failure
Paco₂ 35-45 torr	Acceptable alveolar ventilation
Paco₂ <35 torr	Respiratory alkalosis/alveolar hyperventilation
EVALUATION OF Paco₂ IN CONJUNCTION WITH pH^*
Acceptable Alveolar Ventilation (Paco₂ from 35 to 45 torr)
pH >7.50	Metabolic alkalosis
pH 7.30-7.50	Acceptable pH
pH <7.30	Metabolic acidosis
Alveolar Hypoventilation (Paco₂ >45 torr)
pH >7.50	Partially compensated metabolic alkalosis
pH 7.30-7.40	Chronic ventilatory failure
pH <7.30	Acute ventilatory failure
Alveolar Hyperventilation (Paco₂ <35 torr)
pH >7.50	Acute alveolar hyperventilation
pH 7.40-7.50	Chronic alveolar hyperventilation
pH 7.30-7.40	Compensated metabolic acidosis
pH <7.30	Partially compensated metabolic acidosis

Paco₂, partial pressure of CO₂ in arterial blood.

* Some authors use a narrower pH range for these classifications.

Modified from Shapiro BA, Peruzzi WT, Kozelowski-Templin R: Clinical application of blood gases, ed 5, Chicago, 1994, Mosby-Year Book.

TABLE 3-9 Primary Blood Gas Classification

3. Interpret the results of CO-oximetry/hemoximetry blood gas analysis (Code: IB10j and IIIE4b) [Difficulty: ELE: R, Ap; WRE: An]

The CO-oximeter type blood gas analyzer gives values for oxyhemoglobin (O₂Hb), reduced hemoglobin (RHb), carboxyhemoglobin (COHb), and methemoglobin (MetHb)/sulfhemoglobin (SHb). Each of these hemoglobin moieties can be displayed in terms of grams per deciliter, percentage of the whole, and added together for a total hemoglobin (THb) value. Table 3-10 shows normal adult hemoglobin values. The amounts of carboxyhemoglobin and methemoglobin should be subtracted from the total hemoglobin amount to find the amount of functional hemoglobin. Any increase in the COHb or MetHb levels above those listed is abnormal and results in even less normal hemoglobin to carry oxygen. The patient with carbon monoxide poisoning is at greatest risk. A COHb level of 30% saturation or greater can be fatal. By subtraction, the O₂Hb (Sao₂) level can be no greater than 70% with a resulting Pao₂ of less than 40 torr.

TABLE 3-10 Normal Hemoglobin Values for Adults

Total hemoglobin (THb)	Men: 13.5-18.0 g/dL
	Women: 12.0-16.0 g/dL
	15.0 g/dL is often listed as an average for both
Oxyhemoglobin	94-100% of THb (reported as Sao₂^*) of 94-100%
Carboxyhemoglobin	Nonsmokers: <1.5% (0.225 g/dL) of THb
Carboxyhemoglobin	Smokers: 1.5-10% of THb
Methemoglobin	0.5-3% (0.075-0.45 g/dL) of THb
Oxygen content (arterial sample)	15-23 g/dL

g/dL, grams per deciliter (sometimes listed as g/100 mL); Sao₂, O₂ saturation in arterial blood.

EXAMPLE 1

The following example shows how to calculate the amount of functional hemoglobin and saturation for a patient with normal COHb and MetHb levels:

EXAMPLE 2

The following example is for a patient with an elevated COHb level and a normal MetHb level:

Exam Hint 3-11 (ELE, WRE)

If a significant difference is found in hemoglobin saturation values between the pulse oximeter, standard or POC analyzer, and a CO-oximeter, suspect that the patient has an elevated CO level. Only the CO-oximeter value will be accurate if the patient has CO poisoning.

4. Mixed venous blood gases

a. Review the patient’s chart for mixed venous blood gas results (Code: IA5) [Difficulty: ELE: R; WRE: Ap]

A mixed venous blood sample can be taken from the pulmonary artery of any patient who has a pulmonary artery (Swan-Ganz) catheter. This is a true mixed venous sample and should not be confused with a blood sample taken from an arm vein or other venous site. The symbol P is the prefix for venous blood gas values of oxygen and so forth. In general, a typical patient has the following mixed venous blood gas values: SO₂, 75%; PO₂, 40 mm Hg; PCO₂, 46 mm Hg; and pH 7.35. (Table 3-11 provides details on normal venous blood gas values and their interpretation in patients with cardiovascular disease.)

TABLE 3-11 Normal and Abnormal Mixed Venous Blood Gas Values

b. Interpret the results of mixed venous blood gas analysis to evaluate the patient’s response to respiratory care (Code: IB10j and IIIE4a) [Difficulty: ELE: R, Ap; WRE: An]

In the critically ill patient, it is just as important to measure the mixed venous blood gases as it is to measure the ABGs. The venous blood gas values reveal what has happened as the arterial blood has passed through the body. Oxygen has been extracted, and carbon dioxide has been added to the blood. The difference between the arterial and venous oxygen levels reflects oxygen consumption by the body as well as cardiac output.

Because of this, the most critical venous blood gas values to measure are the SO₂ and PO₂. It is generally accepted that a PO₂ value of less than 30 torr or an SO₂ value of less than 56% indicates that the patient has tissue hypoxia. Both values can be obtained by analyzing a mixed venous blood sample taken through a pulmonary artery catheter. If the patient has a fiberoptic catheter, the SO₂ value can be monitored continuously. This is extremely helpful if the patient is unstable or having frequent changes in inspired oxygen or ventilator settings. (Pulmonary artery catheters are discussed in more detail in Chapter 5.)

5. Arterialized capillary blood gases

a. Review the patient’s chart for arterialized capillary blood gas results (Code: IA5) [Difficulty: ELE: R; WRE: Ap]

This method of obtaining a blood gas sample is limited to neonatal patients when an arterial sample cannot be obtained.

b. Interpret the results of capillary blood gas analysis to evaluate the patient’s response to respiratory care (Code: IB10g and IIIE4a) [Difficulty: ELE: R, Ap; WRE: An]

Because this is not a true ABG sample, the following limitations are placed on interpreting the results.

• The capillary pH (c pH) value has a good correlation with the arterial pH value.

• The capillary CO₂ (Pcco₂) value correlates with the Paco₂ value about 50% of the time.

• The capillary O₂ (Pco₂) value does not correlate well with the Pao₂ value.

Based on these limitations, an arterialized capillary blood pH value can be clinically useful. The capillary CO₂ value should be viewed with suspicion. It should not be the only parameter monitored to judge the infant’s ability to ventilate; however, the combination of a low pH and elevated CO₂ value indicates hypoventilation. Evaluate the infant’s vital signs, breathing efforts, chest radiograph, and so on to determine respiratory status.

Unfortunately, the capillary O₂ value is practically useless for judging the infant’s oxygenation. Some infants may have a fairly close correlation with the Pao₂ level, whereas others do not. No way is known to predetermine those that will match and those that will not. Many practitioners view a low capillary oxygen level as a sign of clinical hypoxemia. The American Association for Respiratory Care (AARC) Clinical Practice Guideline Oxygen Therapy in the Acute Care Hospital notes that a Pco₂ level of less than 40 torr documents neonatal hypoxemia. An ABG sample remains the best way to determine the patient’s respiratory status.

MODULE E

Pulse oximetry

1. Check the patient’s chart for previous pulse oximetry results (Code: IA7e) [Difficulty: ELE: R; WRE: Ap]

Pulse oximetry (Spo₂) spectroscopically analyzes arterial blood to noninvasively determine the percentage of hemoglobin that is saturated with oxygen. It generally is indicated whenever a patient’s oxygenation must be monitored. An Spo₂ value of 92% or greater indicates that the patient is adequately oxygenated. Past values should be compared with current readings to monitor the patient’s progress or response to treatment.

2. Make a recommendation to perform pulse oximetry (Code: IC8) [Difficulty: ELE: R, Ap; WRE: An]

Pulse oximetry is indicated in the following situations: during anesthesia and intraoperative monitoring of oxygenation, postoperatively when the patient is still sedated, when the patient is receiving sedatives or analgesics that can blunt the airway protective reflexes, during bronchoscopy, during a sleep study, and for evaluating the effectiveness of oxygen therapy. An exception to the use of first-generation pulse oximetry is when there is known or suspected carbon monoxide poisoning.

3. Manipulate a pulse oximeter by order or protocol (ELE code: IIA22) [Difficulty: ELE: R, Ap, An]

a. Get an appropriate pulse oximeter and related equipment

Each pulse oximeter reports a percentage of hemoglobin saturation with oxygen (Spo₂). This information and the patient’s heart rate are displayed on a light-emitting diode (LED). More expensive units store the information in memory and/or print out a copy of the Spo₂ percentage and pulse rate to be placed into the patient’s chart if it is required. A variety of sensors fit the feet or hands of infants, an adult’s fingers, the bridge of the nose, the forehead, and the earlobe. Choose a sensor designed to fit the site that is selected.

In most patient care situations a first-generation pulse oximeter can be used. They are readily available and inexpensive to operate. A second-generation pulse oximeter will be needed if a patient is known to have carbon monoxide poisoning and continuous monitoring is required (for example, a Masimo Rad-57 [Irvine, CA] or Rainbow SET). Second-generation pulse oximeters can identify all types of normal and abnormal hemoglobin. However, because of their relative newness and expense, they may not be widely available.

b. Put the equipment together and make sure that it works properly

Follow the manufacturer’s suggestions for setup. The newer pulse oximetry systems visually display the strength of the pulse so that the best place for the probe can be found (Figure 3-13). Keep bright light away from the patient site and transducer. Figure 3-14 shows how to apply the finger probe properly.

Figure 3-13 Pulse oximetry signal strength. The strong surge of blood through the artery with each heartbeat results in variable absorption of the light emitted by the pulse oximeter. Venous and tissue absorption of light is stable when the heart is at rest. This absorption difference is used to find the patient’s artery and measure the heart rate. (AC, Variable absorption; DC, stable absorption.).

(From Ruppel GL: Manual of pulmonary function testing, ed 9, St Louis, 2009, Mosby.)

Figure 3-14 Pulse oximetry transducer properly placed on a finger. Fold the OXISENSOR over the end of the digit. Align the other end of the OXISENSOR so that the two alignment marks are directly opposite each other. Press the OXISENSOR onto the skin. Wrap the adhesive flaps around the digit.

(Reprinted with permission of Nellcor Puritan Bennett Inc., Pleasanton, Calif.)

c. Troubleshoot any problems with the equipment

The pulse signal will not be strong if the patient has poor circulation at the site of the oximeter probe. This can occur if the patient is hypothermic, hypotensive, or receiving a vasoconstricting medication. If the pulse signal is weak, the pulse oximetry values may not be accurate. See Table 3-12 for common sources of error and their solutions.

TABLE 3-12 Sources of Error in Pulse Oximetry

Sources of Error	Remedy
Light interference: xenon lamp, fluorescent light, infrared (bilirubin) light; probe fell off of patient	Cover the probe with an opaque wrap; put the probe back in place on the patient
Low perfusion: low blood pressure, hypothermia, vasoconstricting drugs	Use earlobe, bridge of nose, or forehead instead of finger or toe; discontinue use if still unreliable
Motion artifact	Secure the probe site; ensure that the Spo₂^* reading is synchronized with the heart rate
Darkly pigmented patient	Use lightly pigmented site such as tip of finger or toe; Spo₂ value may overestimate Pao₂; discontinue use if still unreliable
Artificial or painted fingernails	Remove acrylic nails; remove black, blue, green, metallic, or frosted nail polish; use a different site
Venous pulsation being read as an arterial pulsation	Loosen a tight sensor; change the finger sensor site every 2-4 hr; loosen the cause of a tourniquet-like effect
The following vascular dyes will cause low Spo₂ readings: methylene blue, indigo carmine, indocyanine green	Do not use Spo₂

Pao₂, partial pressure of O₂ in arterial blood; Spo₂, pulse oximeter.

4. Perform pulse oximetry on your patient (Code: IB9c and IIIE3b) [Difficulty: ELE: R; WRE: Ap, An]

First-generation pulse oximetry has gained wide acceptance because it offers a way to monitor continuously and noninvasively a patient’s oxygenation by following the percentage of hemoglobin saturated with oxygen. The reported Spo₂% is the percentage of oxyhemoglobin.

Pulse oximetry makes practical use of two physical principles. The first is spectrophotometry, which is used to analyze the transmission of wavelengths of light through the blood and body tissues. First-generation pulse oximeters analyze two wavelengths of light. One wavelength is 660 nm, and the other is between 920 and 940 nm, depending on the manufacturer. The 660-nm wavelength can be seen as red and is preferentially absorbed by oxyhemoglobin (O₂Hb). The 920- to 940-nm wavelength is not visible, because it is in the infrared range. It is preferentially absorbed by reduced hemoglobin (RHb). Second-generation pulse oximeters analyze several more wavelengths of light and can evaluate different forms of hemoglobin.

The second principle is plethysmography. It is used to find and then evaluate the amplitude of the arterial pulse waveform. See Figure 3-13 for the plethysmographic arterial waveform. When the pulse oximeter sensor is placed on a patient site, the fingertip for example, the two wavelengths of light shine through the blood, tissues, and bone within the finger. It is important that the sending LED and receiving (photodiode) sensors be opposite each other (see Figure 3-14). Most units have a signal strength display that indicates when the photodiode is receiving a strong signal, and the patient’s pulse has been detected. The microprocessor is designed to detect a baseline level of light absorption by the tissues and venous blood, containing more RHb, as well as the light absorption of arterial blood, containing more O₂Hb. It can then compare the absorptions of the two wavelengths to determine the level of saturated oxyhemoglobin. This is displayed as saturation by pulse oximetry, or Spo₂.

It must be realized that because first-generation pulse oximetry samples only two wavelengths of light, the technology is unable to recognize the presence or quantity of the nonfunctional hemoglobin species of COHb and MetHb. Instead, an ABG sample should be drawn and sent to the laboratory to be passed through a CO-oximeter for a complete fractional hemoglobin analysis. Even healthy persons have small amounts of COHb and MetHB. For this and other technical reasons, manufacturers report the following Spo₂ values for general accuracy at 1 standard deviation (SD) for a general population: ± 2% from 100% to 70% saturation, and ± 3% from 70% to 50% saturation. Because of these limitations, the following clinical guidelines have been created by a number of authors.

• Do not use pulse oximetry on patients with significant levels of COHb or MetHB.

• If in doubt about abnormal types of hemoglobin, analyze an ABG through a CO-oximeter and compare the true Sao₂ with the Spo₂ from pulse oximetry. Pulse oximetry can be used if the correlation of values is within 4%.

• Question the Spo₂ value when the displayed heart rate is different from the actual heart rate.

• Do not use pulse oximetry when the Spo₂ reading is less than 70%.

• Pulse oximetry can be used on a term neonate or a neonate who weighs 1500 g or more. Smaller neonates should have oxygenation measured by a transcutaneous oxygen monitor. This is because it is too easy to hyperoxygenate a small neonate when small changes in saturation can result in wide swings in Po₂ level. The risk of retinopathy of prematurity (formerly called retrolental fibroplasia) from hyperoxia is too great in the small neonate.

The second-generation pulse oximeters (Masimo Rad-57 or Rainbow SET) analyze 7 to 12 wavelengths of light. Because of this, they can identify and measure the various normal and abnormal types of hemoglobin, such as carboxyhemoglobin (SpCO) from carbon monoxide poisoning and methemoglobin (SpMet) from nitric oxide therapy. These units provide the opportunity to continuously monitor patients with elevated levels of carboxyhemoglobin and methemoglobin as they are being treated. It may not be necessary to draw as many arterial blood gas samples for analysis through a CO-oximeter. If necessary, review the discussion on CO-oximeters and see Figure 3-10 since the principles apply to second-generation pulse oximeters.

Table 3-13 lists common clinical ranges for Spo₂ values. For the aforementioned reasons, with first-generation pulse oximeters the minimum safe values are 2% higher than the corresponding Sao₂ value by CO-oximetry. It is important that the patient have good pulsatile blood flow to the measurement site to obtain an accurate reading.

TABLE 3-13 Recommended Clinical Ranges for True Values in Saturation of O₂ in the Hemoglobin (Sao₂), Values in Pulse Oximetry (Spo₂),^* and Their Correlation with Values in Partial Pressure of O₂ in Arterial Blood (Pao₂)

5. Interpret a patient’s pulse oximetry value (Code: IB10c and IIIE4d) [Difficulty: ELE: R, Ap; WRE: An]

The previously healthy patient who has cardiopulmonary failure should have the Spo₂ value kept at 92% or greater to ensure adequate oxygenation. The patient with chronic obstructive pulmonary disease (COPD) can probably tolerate an Spo₂ value as low as 87%. In general, a neonate should have the Spo₂ level kept between 92% and 96%. See Table 3-13 for more specific guidelines. Saturations below these values indicate hypoxemia in most patients.

Note the site at which the saturation was measured. This is especially important in neonates who may have congenital heart defects. A higher saturation in the right fingers or right earlobe compared with the rest of the body is seen in a neonate who has patent ductus arteriosus (PDA). A higher saturation in the fingers and earlobes as compared with the toes is seen in a neonate who has coarctation of the aorta.

Exam Hint 3-12 (ELE, WRE)

The patient with carbon monoxide poisoning should not be evaluated with a first-generation pulse oximeter because the units are unable to distinguish functional oxyhemoglobin from nonfunctional carboxyhemoglobin. These pulse oximeters show the level of only functional O₂Hb. Use only a CO-oximeter blood gas analyzer on a patient with known or suspected CO poisoning. Clinical examples include a patient removed from an enclosed house fire or a patient removed from a car with the engine running.

Second-generation pulse oximeters can measure and display both functional hemoglobin (Spo₂) and nonfunctional hemoglobin (SpCO and SpMet). They can be used to monitor patients with carbon monoxide poisoning.

Exam Hint 3-13 (WRE)

Usually a question is related to an inaccurately low pulse oximeter reading caused by poor local perfusion. See Table 3-12 for this and other examples of causes of inaccurate readings.

MODULE F

Transcutaneous oxygen monitoring

NOTE: Transcutaneous monitoring (TCM) involves the continuous monitoring of oxygen, carbon dioxide, or both as they diffuse through the skin. Each gas has its own individual electrode, and a combined electrode exists for both gases.

1. Review the patient’s chart for a transcutaneous oxygen (Ptco₂) value (Code: IA7e) [Difficulty: ELE: R; WRE: Ap]

Transcutaneous oxygen monitoring (Ptco₂, tcPo₂, or TcO₂) enables any patient’s oxygenation status to be followed on a continuous basis. In practice, neonates are monitored much more often than adults. Check the chart for a record of the patient’s transcutaneous oxygen values. It is particularly important to compare the Pao₂ value with the Ptco₂ value. In addition, note what the Ptco₂ values are when the inspired oxygen is changed, the patient’s airway is suctioned, or changes are made in continuous positive airway pressure (CPAP) or mechanical ventilation. Avoid clinical situations that have previously resulted in hypoxemia.

2. Recommend transcutaneous oxygen monitoring for additional data (Code: IC8) [Difficulty: ELE: R, Ap; WRE: An]

Ptco₂ monitoring has been used for the following purposes:

• To monitor oxygenation during transportation of an unstable neonate within the hospital or between two hospitals

• To monitor intraoperative and postoperative oxygenation

• To monitor oxygenation during changes in the inspired oxygen and during changes in mechanical ventilation, and to detect hypoxemia during an equipment failure

• To help detect a right-to-left shunt. When a neonate has a patent ductus arteriosus (PDA), the Ptco₂ level is higher in the right upper chest than in the left upper chest, abdomen, or thighs.

• To help detect a coarctation of the aorta. When this defect is present, the Ptco₂ value is higher in the right and left upper chest than in the abdomen or thighs.

• To determine the response to an oxygen challenge test by a patient with congenital heart disease

3. Manipulate a transcutaneous oxygen monitor by order or protocol (ELE code: IIA22) [Difficulty: ELE: R, Ap, An]

a. Get the necessary equipment to perform transcutaneous oxygen monitoring

The electrode used for monitoring the patient’s transcutaneous oxygen level is a miniaturized and modified Clark-type polarographic electrode similar to that used in the blood gas analyzer (Figure 3-15). Some authors describe it as a Huch or Hellige electrode, named after two researchers who modified the original Clark electrode for their work with pediatric patients.

Figure 3-15 Schematic drawing of the modified Clark electrode used to monitor transcutaneous oxygen tension. 1, O-ring to hold the membrane to the electrode; 2, polypropylene membrane permeable to oxygen; 3, silver anode that surrounds the platinum cathode; 4, electrolyte chamber with solution held in place by the polypropylene membrane; 5, heating element; 6, platinum cathode; 7, electrolyte solution of sodium bicarbonate and sodium chloride held between the membrane and electrode; and 8, negative temperature coefficient (NTC) resistor that serves to regulate the temperature of the sensor.

(From Shapiro BA, Peruzzi WT, Kozlowski-Templin R: Clinical application of blood gases, ed 5, St Louis, 1994, Mosby.)

b. Put the transcutaneous oxygen monitor together and make sure that it works properly

Always follow the manufacturer’s recommendations for the assembly and care of the equipment. Select the proper electrode for the monitor based on the physician’s order for evaluating transcutaneous oxygen, carbon dioxide, or both together.

c. Troubleshoot any problems with the equipment

Problems usually associated with too much electrode drift include electrode or membrane surfaces contaminated by debris such as blood or sweat, an improperly applied membrane, a worn-out membrane, an air bubble beneath the membrane, improper gas exposed to the membrane during the calibration, exhausted electrolyte solution in the electrode, or inaccurate calibration values. After correcting a problem, the calibration procedure should be repeated.

4. Perform quality control procedures on a transcutaneous oxygen monitor (Code: IIC7) [Difficulty: ELE: R, Ap; WRE: An]

a. Calibration

Two-point calibration must be performed with oxygen percentages that will cause Po₂ values beyond the clinical range that can be expected. Usually the first calibration point is a “zero” point because the electrode is exposed to no oxygen in a nitrogen-filled chamber. This point is usually quite stable. The second calibration point is found when the electrode is exposed to room air (20.95% or 0.2095 oxygen). Always follow the manufacturer’s written procedures during the calibration process. Generally, when environmental conditions include a fairly stable room temperature of 25° C and 50% relative humidity, the following equation can be used to predict the room air calibration point:

in which P_B is the local barometric pressure and 0.2095 is the oxygen fraction found in room air. Expose the electrode to room air to determine whether it matches the calculated calibration value. Adjust the instrument to match the calibration Ptco₂ if necessary. It is recommended that the room air calibration point be rechecked every 24 hours when in continuous use, after changing the membrane, or after changing the electrolyte solution. A variation of up to ± 5 torr is acceptable and can be corrected by adjusting the reading on the instrument. If the variation is greater than ± 5 torr, the zero point and room air calibration procedures should be repeated.

5. Perform transcutaneous oxygen (Ptco₂) monitoring (Code: IB9b) [Difficulty: ELE: R, Ap; WRE: An]

a. Site selection

Care must be taken to select the best site for the electrode. A bad site will give incorrect information that can lead to mistakes in patient care. See Box 3-2 for a listing of site selection guidelines.

BOX 3-2 Optimal Sites and Sites to Avoid with Transcutaneous Gas Monitoring

OPTIMAL NEONATAL SITES

Upper part of the chest

Right upper chest if a preductal Ptco₂ value is desired

Abdomen

Inner aspect of either thigh

OPTIMAL ADULT SITES

Upper part of the chest

Inner aspect of the upper arm

SITES TO AVOID

CONDITIONS IN WHICH TRANSCUTANEOUS MONITORING SHOULD NOT BE USED

Locally cold skin or general, deep hypothermia

Locally decreased peripheral perfusion or general hypotension

Patient receiving vasoconstricting drugs such as tolazoline or dopamine

Cardiac index less than 1.9 L/min/m² of body surface area

Halothane anesthesia will give erroneously high transcutaneous O₂ values unless a Teflon membrane is used.

Ptco₂, Pressure of transcutaneous oxygen.

b. Skin and electrode preparation and application of the skin electrode

An airtight seal between the skin and electrode is necessary for accurate readings. An air leak (21% oxygen) will falsely increase or decrease the Ptco₂ reading from the patient’ s actual value. A room air leak will always cause a decrease in the PtcCO₂ reading. The following steps should be taken to ensure that the skin site and electrode are prepared and an airtight seal is ensured:

1. Clean the skin. Usually, cleaning with an alcohol swab is enough to remove perspiration. Oily skin should be cleaned with soap and water.

2. Adults may need to have hair shaved from the site.

3. Adults may have dead skin cells removed by placing sticky adhesive tape against the site and pulling it off.

4. Prepare the electrode according to the manufacturer’s guidelines. This usually includes placing a drop of the electrolyte solution on the electrode surface and placing a gas-permeable membrane with a double adhesive ring over the electrode.

5. The other side of the adhesive ring is pressed against the monitoring site to form an airtight seal.

6. As the electrode warms the skin, the patient values will fluctuate. Stabilization usually requires several minutes, after which the patient values should be clinically useful.

c. Limitations and patient precautions

Because the electrode is heated, it must be rotated to a different skin site on a routine basis. The general manufacturer’s guidelines for site rotation are every 4 to 6 hours for neonates weighing between 1000 and 2500 g; every 3 to 5 hours for neonates weighing between 2500 and 3500 g; and 2 to 4 hours for neonates weighing more than 3500 g, pediatric patients, and adult patients. As a safety precaution, change the electrode on all patients at least every 4 hours, or at least every 3 hours if the patient is hypothermic. The manufacturer’s range in times is based on the relative thickness of the patient’s skin and the different electrode temperatures. It is important to adjust the site-rotation times on an individual patient basis. Some may tolerate longer times, and others will need more frequent rotations.

Care must be taken when removing the adhesive ring and electrode. It is possible to tear the thin, delicate skin of a premature neonate. Loosen the adhesive by running the edge of an alcohol wipe along the side that is being gently pulled up. After the electrode is removed, it is important to examine the skin; it is normal to see a red circle. This warmed, vasodilated area will stay red for some time and will gradually fade away, with no scarring or permanent injury. Rarely, the skin will have overheated and a blister is seen; this is a second-degree burn. Obviously, future site rotations must be made more frequently. Do not use this site again. Treat it as a burn, and avoid any further injury that might break the skin and lead to an infection.

6. Interpret transcutaneous oxygen monitoring values to evaluate the patient’s response to respiratory care (Code: IB10b) [Difficulty: ELE: R, Ap; WRE: An]

It has been found that heating the electrode speeds the diffusion of oxygen through the skin. Heating also provides a closer correlation with the patient’s Pao₂ value. However, it must be remembered that the PtCO₂ value is not the same as the Pao₂ value. Current recommendations are that any unit should give PtCO₂ values that are within ± 15% of the Pao₂ values over the operating range of the instrument. The values should then correlate within ± 15% as the patient’s condition changes. This should always be confirmed.

An arterial blood gas sample should be drawn for Pao₂ every time transcutaneous oxygen monitoring is started. The Pao₂-PtCO₂ gradient can then be calculated as the difference between the two. For example, if the patient’s Pao₂ value is 100 torr, the PtCO₂ value should be no less than 85 torr. If the PtCO₂ level decreases to 70 torr, the Pao₂ level should have decreased to no lower than 85 torr. Because of this close correlation, the patient’s trend can be monitored with some assurance of accuracy. In addition, ABG samples do not need to be drawn as frequently for the Pao₂ measurement. This trending relation holds true for changes in the patient’s pulmonary condition. It does not, however, hold true when the patient has cardiovascular problems such as hypotension, hypothermia, peripheral vascular disease, or cardiogenic shock with decreased tissue perfusion.

Always follow the manufacturer’s recommendations for the proper electrode temperature. In general, the temperature ranges from 42.5° C for a 1000-g infant to 44° C for a 3500-g infant. A pediatric patient can tolerate a temperature of 44° C, whereas an adult can have an electrode temperature of 45° C. The higher temperatures are needed in older patients because their skin is thicker. Studies have shown that when the neonate’s Pao₂ level is greater than 100 torr, the PtCO₂ value underestimates it. This can lead to dangerous hyperoxemia. For this reason, it is recommended that the Ptco₂ level be kept at less than 90 torr. In the adult, less correlation is found between the Pao₂ and Ptco₂ values because of the thicker skin found in adults. The Ptco₂ value can still be used in the adult to follow trends in oxygenation, but the practitioner must realize that a decrease in the Ptco₂ value can be a result of hypoxemia, a decreased cardiac output, or cutaneous vasoconstriction. An arterial blood gas sample must be drawn to further evaluate the patient’s status.

a. Correlation of Ptco₂ and local power consumption

As mentioned previously, the electrode is heated to above body temperature. The amount of electrical current needed to keep the electrode at a constant temperature depends on the temperature of the blood and the speed at which the blood passes beneath the electrode. This is known as local power consumption or simply local power (LP). A change in the LP value and Ptco₂ value can be used to indicate what is happening to the patient. For example:

• A normal LP seen with a decreased Ptco₂ value indicates that the patient’s pulmonary problem has worsened.

• A normal LP seen with an increased Ptco₂ value indicates that the patient’s pulmonary problem has improved.

• A decreased LP seen with a decreased Ptco₂ value indicates that the patient’s cardiac output has worsened.

• An increased LP seen with an increased Ptco₂ value indicates that the patient’s cardiac output has improved.

Exam Hint 3-14 (WRE)

If a sick neonate has normal cardiovascular status, the Pao₂ and Ptco₂ values should correlate with each other (but not be identical) as the patient’s pulmonary condition changes. For example, the neonate has infant respiratory distress syndrome. The Pao₂ and Ptco₂ values probably will not correlate when the patient has cardiovascular problems. For example, the neonate is hypotensive with low cardiac output.

MODULE G

Transcutaneous carbon dioxide monitoring

1. Review the patient’s chart for a transcutaneous carbon dioxide value (Code: IA7e) [Difficulty: ELE: R; WRE: Ap]

Transcutaneous carbon dioxide monitoring (PtcCO₂, tcPco₂, or TcCO₂) enables any patient’s ventilation status to be monitored on a continuous basis. In practice, neonates are monitored much more often than adults. Check the chart for a record of the patient’s transcutaneous carbon dioxide values. It is particularly important to correlate these values with any ABG values, which allows comparison between the Paco₂ and the PtcCO₂ measurements. In addition, note the PtcCO₂ values when changes are made in CPAP or mechanical ventilation. Avoid clinical situations that have previously resulted in hypoventilation.

2. Recommend transcutaneous carbon dioxide monitoring for additional data (Code: IC8) [Difficulty: ELE: R, Ap; WRE: An]

PtcCO₂ monitoring has been used for the following purposes:

• To monitor ventilation during transportation of an unstable infant within the hospital or between two hospitals

• To monitor intraoperative and postoperative ventilation

• To monitor ventilation during changes in mechanical ventilation such as tidal volume, rate, minute ventilation, mechanical dead space, or a combination of these

• To detect hypoventilation during an accidental disconnection from the ventilator

3. Manipulate a transcutaneous carbon dioxide monitor by order or protocol (ELE code: IIA22) [Difficulty: ELE: R, Ap, An]

a. Get the necessary equipment to perform transcutaneous carbon dioxide monitoring

The electrode used for monitoring the patient’s transcutaneous carbon dioxide level is a miniaturized and modified Stow-Severinghaus–type electrode similar to the ABG electrode (Figure 3-16).

Figure 3-16 Schematic drawing of the modified Stow-Severinghaus electrode used to monitor transcutaneous carbon dioxide tension. 1, Epoxy resin; 2, glass electrode with a chlorinated silver wire, a buffer solution (the inner liquid), and a pH-sensitive glass membrane; 3, negative temperature coefficient (NTC) resistor that serves to regulate the temperature of the sensor; 4, O-ring to hold the membrane to the electrode; 5, electrolyte chamber with solution held in place by the polypropylene membrane; 6, electrolyte solution of sodium bicarbonate and sodium chloride held between the membrane and electrode; 7, polypropylene membrane permeable to carbon dioxide; 8, heating element; and 9, silver/silver chloride reference electrode.

(From Shapiro BA, Peruzzi WT, Kozlowski-Templin R: Clinical application of blood gases, ed 5, St Louis, 1994, Mosby.)

b. Put the transcutaneous carbon dioxide monitor together and make sure that it works properly

c. Troubleshoot any problems with the equipment

Problems usually associated with too much electrode drift include electrode or membrane surfaces contaminated by debris such as blood or sweat, an improperly applied membrane, a worn-out membrane, an air bubble beneath the membrane, improper gas sample exposed to the membrane during the calibration, exhausted electrolyte solution in the electrode, or inaccurate calibration values. After correcting a problem, the calibration procedure should be repeated.

4. Perform quality control procedures on a transcutaneous carbon dioxide monitor (Code: IIC7) [Difficulty: ELE: R, Ap; WRE: An]

Two-point calibration must be performed with carbon dioxide percentages that cause Pco₂ values beyond the expected clinical range. Usually the electrode is exposed to 5% and 10% carbon dioxide from prepared cylinders. Always follow the manufacturer’s written procedures during the calibration process. Generally, when environmental conditions include a fairly stable room temperature of 25°C and 50% relative humidity, the following equation can be used to predict the two calibration points:

Calibration PtcCO₂ = (P_B × CO₂% used) − XCO₂ × electrode temperature factor)

in which P_B is the local barometric pressure; CO₂% is the carbon dioxide fraction exposed to the electrode, either 10% (0.10) or 5% (0.05); and XCO₂ is the correction factor used to equilibrate the PtcCO₂ to Paco₂. (This is discussed later.)