RESPIRATORY PHYSIOLOGY
Practice Questions: Answers -- Alveolar-Arterial Equilibration

ALVEOLAR-ARTERIAL EQUILIBRATION ANSWERS

1. A. A "right-to-left" shunt permits blood to flow from the right heart (or pulmonary artery or systemic veins) directly into the left heart (or systemic artery or pulmonary vein) without exchanging oxygen and carbon dioxide with pulmonary alveoli. Since an atelectatic region of the lungs consists of collapsed alveoli which are not ventilated, blood flowing through this region represents a "right-to-left" shunt.

2. A. Assuming the patient is breathing air, the low Pa-O2 must be due to ether hypoventilation (choice "A") or a lack of alveolar-arterial equilibration (due to choices "B", "C", and/or "D"). Since the AaDO2 is 4 mmHg, which is within the normal range of 0-5 mmHg, "B", "C", and "D" are ruled out, so the correct answer must be "A".  The low PA-O2 also indicates hypoventilation.

3. C. Chronic bronchitis could lead to hypoventilation but not to a high AaDO2.

4. B. DCO is a good index of DO2.

5. E. High oxygen extraction (the difference between Ca-O2 and Cv-O2) is characteristic of inadequate blood flow.

6. B. Since pulmonary venous blood has a normal oxygen content, equilibration between alveolar gas and pulmonary capillary blood must be normal, ruling out choices "A" and "C". The low systemic arterial oxygen means that systemic venous blood must have mixed with pulmonary venous blood to form systemic arterial blood ("venous admixture"), which is a type of right-to-left shunt.