Chapter 17: Blood Gas Analyses and Their Interpretation

Chapter 17. Blood Gas Analyses and Their Interpretation

Arterial blood gases and serum chemistry data for a 71-year-old male patient show the following results: [Na⁺] = 139 mM; [K⁺] = 4.4 mM; [Cl^–] = 85 mM; pH_a = 7.37; P_aCO₂ = 79 mm Hg; and [HCO₃^–] = 43 mM. No other reliable history or physical exam information is available regarding this patient. Which of the following is the most likely explanation for these results?

a. Diabetic ketoacidosis

b. Excessive vomiting

c. Renal tubular acidosis

d. Emphysema

e. Septicemia

The most correct answer is d.

With pH_a = 7.37, this is a mild acidosis within the reference range. However, [HCO₃^–] and P_aCO₂ are clearly abnormal, suggesting a compensated acid-base abnormality; because P_aCO₂ >40 mm Hg, a respiratory acidosis is most likely. Among causes offered in the differential diagnosis, only emphysema would cause respiratory acidosis, probably of the type 2 variant associated with airway obstruction, reduced V̇_A, and increased physiological shunt (Chap. 8). Thus the increased [HCO₃^–] reflects its retention by the kidneys to offset the hypoventilation-induced elevation of P_aCO₂.

Which of the following situations would be most likely to cause a decrease in the P_aO₂ of a 5 mL arterial sample just drawn for blood gas analysis?

a. There is a 0.5 mL air bubble in the syringe.

b. The blood sits for 45 minutes at 27°C before delivery to the lab.

c. The blood sample is stored on ice for 30 minutes.

d. A needle is left on the syringe when delivered to the lab.

The most correct answer is b.

Of the four choices, only storage on ice for up to 30 minutes (answer c) is the appropriate way to hold and/or transfer such an ABG sample. However, leaving a needle on the syringe or introducing an air bubble (answers a and d) will raise the sample's apparent P_aO₂ and its pH_a, and lower its P_aCO₂ due to the higher ambient PO₂ in air with its low PCO₂ (Chap. 8). Storage of the sample at room temperature will not prevent ongoing aerobic metabolism by the sample's ...