American Board of Internal Medicine (ABIM) Certification Practice Exam

What is a characteristic feature of dead space physiology?

1. Inadequate oxygen delivery

2. Increased pulmonary arterial pressure

3. High V/Q ratio or (V/Q = infinity)

4. Decreased blood flow to the lungs

The correct answer is: High V/Q ratio or (V/Q = infinity)

A characteristic feature of dead space physiology is represented by a high ventilation-perfusion (V/Q) ratio, which can approach infinity. This condition occurs when there is ventilation without corresponding perfusion, meaning that air reaches the alveoli but does not undergo gas exchange with the blood due to a lack of adequate blood flow. In situations of dead space, the alveoli are ventilated but not perfused, leading to a significant amount of oxygen being present without being delivered to the tissues, resulting in a wasted effort in terms of ventilation. The ideal scenario for gas exchange happens with a balanced V/Q ratio, where ventilation is matched with perfusion. However, in dead space physiology, this balance is disrupted. Other options presented might involve factors related to pulmonary function or systemic circulation but do not specifically highlight the characteristic of dead space. For instance, inadequate oxygen delivery may occur as a consequence of dead space physiology but does not define it. Similarly, increased pulmonary arterial pressure and decreased blood flow to the lungs can be associated with other pulmonary conditions, rather than directly illustrating dead space mechanics. The hallmark of dead space is distinctly characterized by the high V/Q ratio or an infinite ratio, signifying that there is ample airflow relative to the limited blood flow, thereby