ABG INTERPRETATION

Published on 12/06/2015 by admin

Filed under Pulmolory and Respiratory

Last modified 22/04/2025

Print this page

This article have been viewed 2220 times

CHAPTER 10 ABG INTERPRETATION

PRETEST QUESTIONS

Answer the pretest questions before studying the chapter. This will help you determine your strong and weak areas in the material covered.

1. Which of the following blood gas measurements determines how well a patient’s lungs are being ventilated?

2. Which of the following blood gas measurements determines the level of tissue oxygenation?

3. The information below has been obtained from a patient using an aerosol mask at 40% oxygen.

PaCO₂ 36 mm Hg

PaO₂ 122 mm Hg

HCO₃⁻ 26 mEq/L

What is this patient’s A − a gradient? PB = 747 mm Hg

4. Which of the following conditions shifts the HbO₂ dissociation curve to the right?

5. A patient with a 2 L/min nasal cannula has the following ABG results.

PaCO₂ 27 mm Hg

PaO₂ 62 mm Hg

HCO₃⁻ 23 mEq/L

These results indicate which of the following conditions?

A. Uncompensated respiratory acidosis

B. Chronic respiratory alkalosis

C. Compensated metabolic alkalosis

D. Acute respiratory alkalosis

6. The respiratory therapist has received an order to obtain ABG levels from a patient, but an Allen test indicates collateral circulation is not present in the right wrist. At this time, the therapist would

A. Obtain blood from the right radial artery.

B. Obtain blood from the right brachial artery.

C. Wait for the physician to evaluate collateral circulation.

D. Check collateral circulation in the left wrist.

See answers and rationales at the back of the text.

REVIEW

I. ABG ANALYSIS

CRT Exam Content Matrix: IB9f, IB10f, IIIE2a

RRT Exam Content Matrix: IB9f, IB10f, IIIE2a

A. Blood gas analysis monitors the following physiologic variables

1. Arterial oxygenation: PaO₂

2. Alveolar ventilation: PaCO₂

3. Acid–base status: pH

4. O₂ delivery to tissues: PvO₂

B. Arterial samples are used because the values reflect the patient’s total cardiopulmonary status.

C. Mixed venous blood, obtained from the pulmonary artery via a Swan-Ganz catheter, is used to determine O₂ delivery to the tissues (see Chapter 11 on ventilator management).

D. Blood gases are tested to determine whether to change current therapy or to maintain it.

E. Common sites from which to obtain arterial blood are the radial, femoral, or dorsalis pedis arteries.

1. The radial artery is the most common site because of the presence of good collateral circulation and easy access.

2. A modified Allen test is performed to determine collateral circulation.

a. Instruct the patient to close his or her hand tightly as you occlude both the radial and ulnar arteries.

b. Instruct the patient to open his or her hand as you release the pressure on the ulnar artery while watching for the hand to regain normal color.

c. The hand should “pink up” within 10 to 15 s. This is considered a positive Allen test, and the radial artery may be punctured to obtain arterial blood.

d. If color is not restored within 10 to 15 s, the test is negative, which means that collateral circulation is not present and blood must not be obtained from this radial artery. Perform an Allen test on the opposite hand to assess collateral circulation.

F. ABG Sampling (via radial artery puncture)

1. Explain the procedure to the patient.

2. Perform a modified Allen test.

3. Place a folded towel under the patient’s wrist to keep the wrist hyperextended.

4. Clean the puncture site with isopropyl alcohol (70%) or some other appropriate disinfectant.

5. The practitioner must wear gloves for this procedure.

6. A local anesthetic, such as lidocaine (Xylocaine), may be administered subcutaneously around the puncture site, especially in patients who have been punctured several times. Allow 3 to 5 min for the anesthetic to take effect.

7. Aspirate 0.5 mL of a 1 : 1000 solution of heparin into the syringe using a 22- or 23-gauge needle. Pull the plunger of the syringe back and forth so that the entire portion of the syringe is exposed to the heparin. Then push the plunger all the way in to expel the heparin, making sure there are no air bubbles in the syringe. (Most syringes are previously treated with heparin.)

The heparin lubricates the syringe, but it is primarily used to prevent the blood from clotting once in the syringe.

8. With the needle/syringe in one hand, palpate the artery with the other hand. The needle should enter the skin at a 45 degree angle with the bevel pointed up. Advance the needle until blood pulsates into the syringe.

Exam Note

Sometimes the needle passes through the artery and only a small amount of blood enters the syringe. If this happens, the needle should be slowly withdrawn until it is in the artery. If the needle needs to be redirected, it should first be withdrawn to the subcutaneous tissue.

9. After obtaining 2 to 4 mL of blood, apply a sterile gauze pad with pressure over the puncture site for 3 to 5 min or until the bleeding has stopped.

10. Air bubbles affect blood gas levels and should be removed from the syringe. Air in the blood causes increased PaO₂ levels and decreased PaCO₂ levels.

11. Place a cap or rubber stopper over the needle, or remove the needle and place a cap over the end of the syringe. This prevents air from entering the syringe.

12. Then roll the syringe back and forth in your hands to ensure proper mixing of the blood and heparin to prevent blood clotting. Place the syringe in ice to slow metabolism and keep the ABG levels accurate.

13. Record the following information after the sample is drawn

a. Patent’s name and room number

b. The patient’s FiO₂ level

c. If the patient is using a ventilator, record

(3) Respiratory rate

(4) Mode of ventilation (e.g., CMV, SIMV)

(6) Mechanical dead space

d. Patient’s temperature: A fever shifts the HbO₂ curve to the right, indicating that Hb more readily releases O₂ to the tissues but does not pick up the O₂ as easily. This may affect the PaO₂ value but not usually to a significant degree.

II. ARTERIAL OXYGENATION

CRT Exam Content Matrix: IB9k, IB10k

RRT Exam Content Matrix: IB10i

1. The PaO₂ is the portion of O₂ that is dissolved in the plasma of the blood. It is what is left over after the Hb molecules have been saturated.

2. For every 1 mm Hg of PaO₂, there is 0.003 mL of dissolved O₂.

B. PAO₂ (alveolar PO₂)

1. Is calculated by the following formula (alveolar air equation)

(47 mm Hg is the level of water vapor pressure at body temperature)

2. This value is often compared with PaO₂ to determine the P(A − a)O₂ gradient, which refers to the difference between alveolar O₂ tension and arterial O₂ tension. The normal gradient on room air is 4 to 12 mm Hg.

Exam Note

Because the exam generally uses a barometric pressure of 747 mm Hg, the corrected PB is 700 mm Hg. When multiplying 700 times 0.21, the math can be made simpler by multiplying 7 times 21. Furthermore, instead of multiplying the PaCO₂ by 1.25, simply add 10 to the PaCO₂. This does not provide an exact answer, but the answer is close enough to get the question correct on the exam. To make the math simpler and faster on the exam, use the equation below.

EXAMPLE:

A patient using a 50% Venturi mask has the following ABG levels

pH	7.36
PaCO₂	45 mm Hg
PaO₂	94 mm Hg

What is this patient’s A − a gradient? (PB = 747 mm Hg)

C. The majority of O₂ carried in the blood is bound to Hb. (See Chapter 1 on oxygen and medical gas therapy.)

D. HbO₂ Dissociation Curve

1. This curve plots the relationship between PaO₂ and SaO₂ and the affinity that Hb has for O₂ at various saturation levels.

2. This S-shaped curve indicates that at PaO₂ levels of less than 60 mm Hg, small increases in PaO₂ result in fairly large increases in SaO₂.

EXAMPLE:

Fifty percent of the Hb molecules would be carrying O₂ at a PaO₂ of only 26 mm Hg. As the PaO₂ increases to 40 mm Hg, the SaO₂ increases substantially, to about 75%. As the PaO₂ continues to increase to 60 mm Hg, the SaO₂ increases to approximately 90%.

3. The flat portion of the curve indicates that at PaO₂ levels above 60 mm Hg, saturation rises slowly: a PaO₂ of 70 mm Hg yields an SaO₂ of 93%, and PaO₂ levels between 80 and 100 mm Hg result in SaO₂ levels of 95% to 100%.

4. Various factors affect the affinity that Hb has for O₂. These factors shift the HbO₂ dissociation curve to the right or the left.

5. If the curve is shifted to the right, it indicates that Hb’s affinity for O₂ has decreased, or Hb will release O₂ to the tissues more readily. Factors that shift the curve to the right include

c. Hyperthermia

d. Increased levels of 2,3-diphosphoglycerate (DPG)

6. If the curve is shifted to the left, it indicates that the affinity of Hb for O₂ has increased, or Hb will not release O₂ to the tissues as readily. Factors that shift the curve to the left include

d. Decreased levels of 2,3 DPG

7. As O₂ diffuses from the alveoli to the blood (caused by the pressure gradient), it enters an RBC, where it combines with Hb.

8. As O₂ combines with the Hb, the release of CO₂ is enhanced. This is called the Haldane effect.

9. As the RBC travels to the tissue, it releases the O₂ because elevated CO₂ levels, which are present around tissues, decrease the affinity of Hb for O₂. This is known as the Bohr effect.

10. Levels of hypoxemia

60 to 79 mm Hg	Mild hypoxemia
40 to 59 mm Hg	Moderate hypoxemia
<40 mm Hg	Severe hypoxemia

11. Normal PaO₂ levels: Subtract 1 mm Hg from 80 mm Hg for each year over age 60 to determine normal PaO₂ by age.

Age (yr)	PaO₂ (mm Hg)
<60	80–100
60	80
65	75
70	70
75	65
80	60

a. The O₂ tension at which 50% of the Hb is saturated when the blood is at 37° C has a PCO₂ level of 40 mm Hg and a pH level of 7.40.

b. Normal P-50 is 26.6 mm Hg.

c. Used to describe affinity of Hb for O₂.

(1) Increased P-50 indicates decreased affinity.

(2) Decreased P-50 indicates increased affinity.

a. Refers to the quantity of O₂ being carried by the Hb compared with the maximum that may be carried.

b. Normal SaO₂ level is 95% to 99%.

III. CO₂ TRANSPORT AND ALVEOLAR VENTILATION

A. CO₂ makes up approximately 0.03% of inspired air.

B. CO₂ is the by-product of cellular metabolism, and it is by this mechanism that it enters the blood.

C. After CO₂ enters the blood, it takes one of two routes.

1. Five percent of the CO₂ dissolves in the plasma.

2. The remaining 95% enters RBCs.

a. Approximately 65% of the CO₂ entering the RBC is quickly converted to hydrogen and HCO₃⁻ ions.

b. The remaining CO₂ entering the RBC combines with Hb.

c. Therefore, CO₂ is carried in the blood three ways

(1) Dissolved in the plasma

(2) Bound to Hb

(3) As HCO₃⁻

D. The adequacy of ventilation is determined by the PaCO₂ level.

1. Normal PaCO₂ range is 35 to 45 mm Hg.

2. PaCO₂ levels <35 mm Hg indicate hypocapnia, which is excess CO₂ elimination or hyperventilation.

3. PaCO₂ levels >45 mm Hg indicate hypercapnia, which is inadequate CO₂ elimination or hypoventilation.

E. PaCO₂ increases when the respiratory rate or VT (minute volume) decreases or dead space increases. PaCO₂ decreases when the respiratory rate or VT increases or dead space decreases.

IV. ACID–BASE BALANCE (pH)

CRT Exam Content Matrix: IA5, IB9j, IB10j, IC7, IIIE4a

RRT Exam Content Matrix: IA5, IB9j, IB10j, IC8

A. In simple terms, the pH level is determined by the amount of carbonic acid (H₂CO₃) in the blood in relation to the amount of bicarbonate base (HCO₃⁻) in the blood. Mathematically, pH is the negative log of the hydrogen ion concentration. Therefore, as hydrogen ions increase, pH decreases.

1. Henderson-Hasselbalch equation

pK = dissociation constant = 6.1

2. Clinically, we may state

The relationship between pH, HCO₃⁻, and CO₂ is vitally important in interpreting blood gas values effectively. HCO₃⁻ and CO₂ determine the pH level. Understanding the relationship between these three values makes ABG interpretation much easier. To simplify, use the following equation

and use simple numbers to see how both HCO₃⁻ and CO₂ affect pH. If HCO₃⁻ is 12 and CO₂ is 4, then pH is 3 (12/4). If HCO₃⁻ increases to 16 and CO₂ does not change, then pH increases to 4 (16/4). Conversely, if HCO₃⁻ decreases to 8 and CO₂ remains at 4, then pH decreases to 2 (8/4). When HCO₃⁻ increases, pH increases; when HCO₃⁻ decreases, pH decreases. Therefore, pH and HCO₃⁻ have a directly proportional relationship.

Using the same numbers, make changes to the CO₂. If HCO₃⁻ is 12 and CO₂ is 4, then pH is 3 (12/4). If CO₂ increases to 6, then pH decreases to 2 (12/6). Conversely, if CO₂ decreases to 2, then pH increases to 6 (12/2). When CO₂ increases, pH decreases; when CO₂ decreases, pH increases. Therefore, CO₂ and pH have an indirectly proportional relationship.

Whenever ABG results reveal a decreased pH level, either the HCO₃⁻ is decreased or the CO₂ is increased, or both.

If the ABG result indicates an increased pH level, then either the HCO₃⁻ is increased or the CO₂ is decreased, or both. Practice examples follow in this chapter.

Exam Note

For every 20-mm Hg increase in PaCO₂, the pH decreases by 0.10. For every 10-mm Hg decrease in PaCO₂, the pH increases by 0.10.

B. The normal plasma pH range is 7.35 to 7.45.

1. A pH <7.35 is termed acidemia and indicates a higher than normal hydrogen ion concentration. Acidemia occurs as a result of

a. Increased PCO₂ levels

b. Decreased HCO₃⁻ levels

2. A pH >7.45 is termed alkalemia and indicates a below-normal hydrogen ion concentration. Alkalemia occurs as a result of

a. Decreased PCO₂ levels

b. Increased HCO₃⁻ levels

C. Respiratory Versus Metabolic Components

1. When the initial pH change is the result of a PCO₂ change, a respiratory disturbance has occurred.

a. An increased PCO₂ level (>45 mm Hg) decreases the pH (<7.35). This is respiratory acidemia, and if the HCO₃⁻ level is still within normal limits, it is acute or uncompensated. An example of acute (uncompensated) respiratory acidemia is as follows: pH, 7.25; PaCO₂, 60 mm Hg; HCO₃⁻, 25 mEq/L (normal, 22 to 26 mEq/L).

b. A decreased PCO₂ level (<35 mm Hg) increases the pH (>7.45). This is respiratory alkalemia, and if the HCO₃⁻ level is still within normal limits, it is acute or uncompensated. An example of acute (uncompensated) respiratory alkalemia is as follows: pH, 7.53; PaCO₂, 29 mm Hg; HCO₃⁻, 23 mEq/L.

2. When the initial pH change is the result of a change in HCO₃⁻, a metabolic disturbance has occurred.

a. A decreased HCO₃⁻ level (<22 mEq/L) decreases the pH (<7.35). This is metabolic acidemia, and if the PaCO₂ level is within normal limits, it is acute or uncompensated. An example of acute (uncompensated) metabolic acidemia is as follows: pH, 7.24; PaCO₂, 38 mm Hg; HCO₃⁻, 13 mEq/L.

b. An increased HCO₃⁻ level (> 26 mEq/L) increases the pH (> 7.45). This is metabolic alkalemia, and if the PaCO₂ level is within normal limits, it is acute or uncompensated. An example of acute (uncompensated) metabolic alkalemia is as follows: pH, 7.54; PaCO₂, 41 mm Hg; HCO₃⁻, 33 mEq/L.

D. pH Compensation

1. The levels of HCO₃⁻ and CO₂ will change to keep the pH within the normal range. This is called compensation. In severe renal disease and end-stage COPD, compensation may be minimal.

2. If the PCO₂ initially changes the pH, then the HCO₃⁻ changes accordingly to return the pH to normal.

EXAMPLE:

The (A) example is a partially compensated respiratory acidemia. The elevated PaCO₂ level caused the initial drop in pH. The HCO₃⁻ level is increasing to elevate the pH back to normal. Because the pH is approaching normal but is still low, this makes it partially compensated.

As the pH returns to normal (B), resulting from the continued increase in the HCO₃⁻ level, this is called chronic or compensated respiratory acidemia.

The (A) example is a partially compensated respiratory alkalemia. The decreased PaCO₂ level caused the initial increase in pH. The HCO₃⁻ level is decreasing to drop the pH back to normal. Because the pH is approaching normal but is still high, this makes it partially compensated.

As the pH returns to normal (B), resulting from the continued decrease in the HCO₃⁻ level, this is a chronic or compensated respiratory alkalemia.

3. If the HCO₃⁻ level initially changes the pH, then the PaCO₂ level changes accordingly to return the pH to normal.

EXAMPLE:

The (A) example is a partially compensated metabolic acidemia. The decreased HCO₃⁻ level caused the initial drop in pH. The patient is hyperventilating (decreasing PaCO₂ levels) to elevate the pH back to normal. Because the pH is approaching normal but is still low, this is partially compensated. As the pH returns to normal (B), resulting from the continuing drop in PaCO₂, this is a compensated metabolic acidemia.

The (A) example is a partially compensated metabolic alkalemia. The increased HCO₃⁻ caused the initial increase in pH. The patient is hypoventilating (increasing PaCO₂ levels) to drop the pH back to normal. Because the pH is approaching normal but is still high, this is partially compensated. As the pH returns to normal (B), resulting from the continuing PaCO₂ retention, this is a compensated metabolic alkalemia.

Exam Note

When a compensated blood gas measurement is interpreted, if the compensated pH is 7.35 to 7.40, the pH must be assumed to have been acidotic initially. Decide whether the PaCO₂ or HCO₃⁻ caused the initial acidemia. Similarly, if the compensated pH is 7.40 to 7.45, the pH must be assumed to have been alkalotic initially. Decide whether the PaCO₂ or HCO₃⁻ caused the initial alkalemia. Metabolic compensation takes several hours to occur, whereas respiratory compensation may occur in minutes.

E. Mixed Respiratory and Metabolic Component

1. When both the PaCO₂ and HCO₃⁻ cause the pH to move in the same direction, this is called a mixed or combined component.

2. An example of a mixed component is

pH	7.21
PaCO₂	55 mm Hg
HCO₃⁻	18 mEq/L

This is an example of mixed respiratory and metabolic acidemia. An elevated PaCO₂ level and decreased HCO₃⁻ level both contribute to acidemia.

V. ARTERIAL BLOOD GAS INTERPRETATION

CRT Exam Content Matrix: IA5, IB9j, IB10j, IC7, IIIE4a

RRT Exam Content Matrix: IA5, IB9j, IB10j, IC8

A. ABG Normal Value Chart Summary

pH	7.35–7.45
PaCO₂	35–45 mm Hg
PaO₂	80–100 mm Hg
HCO₃⁻	22–26 mEq/L
Base excess (BE)	−2 to +2

Total base deficit (BD): −2; total BE: +2

B. Basic Steps to ABG Interpretation

1. Determine the acid-base status by observing the pH.

a. Is the pH acidotic (<7.35)?

b. Is the pH alkalotic (>7.45)?

2. Determine whether the pH change is the result of a change in PaCO₂ or in HCO₃⁻.

3. When this is determined, observe for signs of compensation. If the PaCO₂ caused the initial pH change, is the HCO₃⁻ changing to return the pH to normal?

4. Determine oxygenation status by observing PaO₂.

C. ABG Example Problems

1. ABG levels are

pH	7.23
PaCO₂	57 mm Hg
PaO₂	81 mm Hg
HCO₃⁻	24 mEq/L
BE	−1

a. Acid-base status: acidemia

b. Ventilatory status: elevated PaCO₂; hypoventilation resulting in decreased pH

c. Metabolic status: normal HCO₃⁻; no compensation occurring at this time

d. Oxygenation status: normal PaO₂

e. Interpretation: uncompensated (acute) respiratory acidemia

f. To correct: Institute mechanical ventilation to increase the patient’s minute volume, or increase the ventilator rate or VT if the patient is already receiving mechanical ventilation.

2. ABG levels are

pH	7.57
PaCO₂	25 mm Hg
PaO₂	98 mm Hg
HCO₃⁻	25 mEq/L
BE	0

a. Acid-base status: alkalemia

b. Ventilatory status: decreased PaCO₂; hyperventilation resulting in an increased pH

c. Metabolic status: normal HCO₃⁻; no compensation occurring at this time

d. Oxygenation: normal PaO₂

e. Interpretation: uncompensated (acute) respiratory alkalemia

f. To correct: Decrease the ventilator rate or VT or add mechanical dead space.

3. ABG levels are

pH	7.45
PaCO₂	35 mm Hg
PaO₂	53 mm Hg
HCO₃⁻	26 mEq/L
BE	+2

a. Acid-base status: normal pH

b. Ventilation status: normal PaCO₂

c. Metabolic status: normal HCO₃⁻

d. Oxygenation status: moderate hypoxemia

e. Interpretation: normal acid-base status with moderate hypoxemia

f. To correct: If the patient is using 60% or more O₂ mask, initiate CPAP, or add PEEP if ventilator patient is using 60% or more O₂.

4. ABG levels are

pH	7.38
PaCO₂	61 mm Hg
PaO₂	54 mm Hg
HCO₃⁻	33 mEq/L
BE	+9

a. Acid-base status: normal pH

b. Ventilatory status: increased PaCO₂; hypoventilation resulting in decreased pH

c. Metabolic status: elevated HCO₃⁻; compensation for initial acidemia

d. Oxygenation status: moderate hypoxemia

e. Interpretation: compensated (chronic) respiratory acidemia. Because the compensated pH is between 7.35 and 7.40, assume this was initially an acidemia caused by an elevated PaCO₂ level.

f. To correct: This is a classic example of “normal” ABG levels in a patient with chronic lung disease; therefore, no change in present therapy is needed.

5. ABG levels are

pH	7.29
PaCO₂	43 mm Hg
PaO₂	87 mm Hg
HCO₃⁻	16 mEq/L
BE	−7

a. Acid-base status: acidemia

b. Ventilatory status: normal PaCO₂

c. Metabolic status: decreased HCO₃⁻ resulting in decreased pH

d. Oxygenation status: normal PaO₂

e. Interpretation: uncompensated metabolic acidemia. No compensation is occurring because the PaCO₂ is normal.

f. To correct: Give HCO₃⁻. No ventilator variable changes or oxygenation modifications are necessary at this time.

6. ABG levels are

pH	7.20
PaCO₂	22 mm Hg
PaO₂	83 mm Hg
HCO₃⁻	15 mEq/L
BE	−10

a. Acid-base status: acidemia

b. Ventilatory status: decreased PaCO₂; hyperventilation compensating for initial acidemia

c. Metabolic status: decreased HCO₃⁻ resulting in decreased pH

d. Oxygenation status: normal PaO₂

e. Interpretation: partially compensated metabolic acidemia. This was an initial metabolic acidemia followed by hyperventilation. Because more CO₂ is being removed, the pH is returning to normal. It is not fully compensated, in that the pH is not within normal limits at this time. This is an example of a patient with diabetic acidosis (ketoacidosis).

f. To correct: May administer NaHCO₃⁻.

VI. ARTERIAL BLOOD GAS INTERPRETATION CHART

CRT Exam Content Matrix: IA5, IB9j, IB10j, IC7, IIIE4a

RRT Exam Content Matrix: IA5, IB9j, IB10j, IC8

Key for table: N, normal; I, increased; D, decreased.

VII. BLOOD GAS ANALYZERS

CRT Exam Content Matrix: IIA10, IIC1, IIC3

RRT Exam Content Matrix: IIA5, IIC1, IIC3

A. Currently, blood gas analyzers have the following capabilities

1. Accurate measurement of pH, PCO₂, and PO₂

2. Self-calibration

3. Accurate measurement of base excess or deficit

4. Accurate measurement of plasma bicarbonate (HCO₃⁻)

5. Correction for temperature

6. Self-troubleshooting abilities

7. Automated blood gas interpretation

B. Blood Gas Electrodes

1. Sanz electrode: measures pH by quantifying the acidity and alkalinity of a solution of blood. This is accomplished by the measurement of the potential difference across a pH-sensitive glass membrane.

2. Severinghaus electrode: measures PCO₂ by causing the CO₂ gas to produce hydrogen ions by means of a chemical reaction.

3. Clark electrode: measures PO₂ as a result of a chemical reaction in which electron flow is measured.

C. Point of Care (POC) Analyzers

1. POC analyzers are portable blood gas analyzers that allow blood gas testing to be done at or near the patient bedside.

2. POC testing is accurate and results in a quicker response time for the analysis than when the blood is drawn from the patient and then transported to the laboratory.

3. POC analyzers are often handheld devices and usually require only a few drops of blood for analysis. The blood is placed in a disposable cartridge. Separate cartridges are used depending on the test to be done. Blood gas levels as well as hemoglobin, hematocrit, electrolytes, glucose, blood urea nitrogen, and creatinine may be analyzed.

4. The sensor on the POC analyzer is automatically calibrated. The POC analyzer calculates, displays, and stores the results. The results can also be placed into the hospital information system or central laboratory for storage and reporting.

D. Quality Control Procedures

1. Blood gas analyzers should undergo one- and two-point calibrations on a routine basis. The process should include high and low values for pH, PCO₂, and PO₂ electrodes.

2. A one-point calibration should be performed before a blood gas sample is run unless the analyzer automatically performs the calibration at programmed intervals.

3. A two-point calibration is usually performed every 8 h.

4. Two different buffers are used to perform a two-point calibration on the pH (one buffer for a one-point). The PCO₂ electrode is calibrated with the use of two gas concentrations (5% and 10%). The PO₂ electrode uses a gas mixture of 0% O₂ and another with either 12% or 20% O₂ for the two-point calibration.

5. The criteria for acceptable results are below

a. pH must be within ±0.04 of the target value.

b. PCO₂ must be within ±3 mm Hg of the target value.

c. PO₂ must be within ±3 mm Hg of standard deviations.

POSTCHAPTER STUDY QUESTIONS

1. Interpret the following arterial blood gas values

A. pH 7.21, PaCO₂ 43 mm Hg, PaO₂ 81 mm Hg, HCO₃⁻ 14 mEq/L

B. pH 7.36, PaCO₂ 62 mm Hg, PaO₂ 58 mm Hg, HCO₃⁻ 36 mEq/L

C. pH 7.57, PaCO₂ 27 mm Hg, PaO₂ 89 mm Hg, HCO₃⁻ 24 mEq/L

D. pH 7.22, PaCO₂ 51 mm Hg, PaO₂ 71 mm Hg, HCO₃⁻ 17 mEq/L

E. pH 7.44, PaCO₂ 28 mm Hg, PaO₂ 80 mm Hg, HCO₃⁻ 18 mEq/L

F. pH 7.32, PaCO₂ 52 mm Hg, PaO₂ 84 mm Hg, HCO₃⁻ 31 mEq/L

2. What do the results of an Allen test mean?

3. What conditions shift the HbO₂ dissociation curve to the right?

4. When the HbO₂ curve is shifted to the right, how is the affinity of Hb for O₂ affected?

5. Calculate the P(A − a)O₂, given the following data

ABG levels in a patient using 40% O₂

PaCO₂ 45 mm Hg

PaO₂ 80 mm Hg

HCO₃⁻ 25 mEq/L

6. Which ABG value best reflects the patient’s ability to ventilate?

7. List a set of ABG levels that are typical of a patient with diabetic ketoacidosis.

See answers at the back of the text.

Cairo JM, Pilbeam S. Mosby’s respiratory care equipment, ed 8. St Louis: Mosby; 2009.

Hess D, Respiratory care principles and practice, ed 1, Philadelphia, Saunders, 2002.

Wilkins RL, Stoller JK, Kacmarek R. Egan’s fundamentals of respiratory care, ed 9. St Louis: Mosby; 2009.

Wilkins R, Krider S, Sheldon R. Clinical assessment in respiratory care, ed 5. St Louis: Mosby; 2005.

Related

Respiratory Care Exam Review Review for the Entry Level and Adva