Which of the following gas exchange abnormalities occur with worsening airflow obstruction? I. Asthma patients can hypoventilate, producing hypercapnic respiratory failure. II. Overdistended alveoli in certain lung regions together with capillary loss increase capillary shunting in emphysema patients. III. Hypoxemia-induced pulmonary vasoconstriction increases capillary shunting. IV. Hypoxemia

can develop from V/Q mismatching caused by maldistribution of ventilation.

A. I, II only
B. I, IV only
C. I, II, III only
D. II, III, IV only

ANS: B
A. Incorrect response: See explanation B.
B. Correct response: Several gas exchange abnormalities occur with worsening airflow obstruction. First, worsening respiratory failure in obstructive lung disease generally is characterized by hypoventilation, causing hypercapnic respiratory failure. Second, overdistended lung regions (auto-PEEP) coupled with underlying emphysematous changes in some patients result in capillary loss and increasing alveolar dead space. Third, hypoxemia can develop as a consequence of V/Q mismatching produced by maldistribution of ventilation. Fourth, hypoxemic-induced pulmonary vasoconstriction, along with chronic pulmonary vascular changes overloads the right ventricle, further decreasing blood flow to the lung and increasing alveolar dead space.
C. Incorrect response: See explanation B.
D. Incorrect response: See explanation B.?