Question

Mismatch of ventilation/perfusion ratio is seen in?

• A. Apex
• B. Base
• C. Both
• D. None

Hint:

V/Q is about 3.0 at the apex and 0.6 at the base, both off the ideal 0.8.

Answer 1

The Correct Option is C

Step 1: The overall ventilation-perfusion (V/Q) ratio of the lung is about 0.8, since alveolar ventilation is roughly 4.2 L/min and cardiac output is about 5.0 L/min. Ideally every alveolus would have a V/Q of 0.8, but this is not the case even in normal lungs.

Step 2: Because of gravity, the apex is both underventilated and underperfused, while the base is both overventilated and overperfused. Gravity affects perfusion more than ventilation, so the apex is relatively underperfused and the base relatively overperfused.

Step 3: As a result the V/Q ratio is high at the apex (about 3.0) and low at the base (about 0.6). Both ends therefore deviate from the ideal 0.8, so the mismatch is seen at both apex and base. Hence the answer is both.

Step 4: At the apex ventilation exceeds perfusion, giving high PaO2 and low PaCO2; at the base perfusion exceeds ventilation, giving low PaO2 and high PaCO2. Since neither region alone explains the mismatch, options apex only and base only are incorrect.