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 (The amount withdrawn and discarded depends on the dead space volume from the tip of the catheter to the side port.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.