ABG INTERPRETATION

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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?

A. pH
B. PaCO2
C. PaO2
D. HCO3−

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

A. pH
B. PaCO2
C. PaO2
D. PvO2

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

pH 7.42
PaCO2 36 mm Hg
PaO2 122 mm Hg
HCO3− 26 mEq/L

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

A. 45 mm Hg
B. 77 mm Hg
C. 113 mm Hg
D. 235 mm Hg

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

A. Hypercapnia
B. Hypothermia
C. Alkalemia
D. HbCO

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

pH 7.51
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.)

image 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.

image 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
(1) FiO2
(2) VT
(3) Respiratory rate
(4) Mode of ventilation (e.g., CMV, SIMV)
(5) PEEP level
(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

A. PaO2
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)

image

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

image

image

image

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.

image 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)

image

image

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
image

FIGURE 10-1

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
a. Hypercapnia
b. Acidosis
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
a. Hypocapnia
b. Alkalosis
c. Hypothermia
d. Decreased levels of 2,3 DPG
e. HbCO
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.
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