Question

When the value of the ventilation-perfusion ratio (V/Q) is infinity, it means?

• A. No O2 goes from alveoli to blood and no CO2 goes from blood to alveoli
• B. Dead space
• C. The PO2 of alveolar air is 159 mmHg and PCO2 is 40 mmHg
• D. Partial pressures of O2 and CO2 are equal

Hint:

If Q (perfusion) drops to zero, what is left is air that never meets blood.

Answer 1

The Correct Option is B

Step 1: Read the ratio. V/Q is ventilation divided by perfusion. If V/Q goes to infinity, the denominator (perfusion, Q) is approaching zero while ventilation continues.
Step 2: Ventilation with no perfusion. An alveolar unit that is ventilated but receives no blood flow does no gas exchange. This is the definition of dead space, so option (b) is correct.
Step 3: Anchor the other values. V/Q = 0.8 is normal matching; V/Q > 0.8 means relatively more ventilation than perfusion; V/Q < 0.8 means a shunt-like mismatch (perfusion exceeding ventilation). At the extreme of perfusion = 0, you get dead space.
Ref: Arvind Arora Review Book of Physiology (p. 240-241).