Question

When is the ventilation-perfusion ratio (Va/Q) infinity?

• A. Partial pressure of O2 becomes zero
• B. No exchange of O2 and CO2
• C. Partial pressure of CO2 alone becomes zero
• D. Partial pressure of both CO2 and O2 remain normal

Hint:

Infinite Va/Q means ventilation without perfusion - alveolar dead space.

Answer 1

The Correct Option is B

Step 1: The ventilation-perfusion ratio is \[\frac{Va}{Q} = \frac{\text{alveolar ventilation}}{\text{alveolar blood flow}}\] where Va is the air reaching the alveolus and Q is the blood perfusing it. A normal ratio means matched ventilation and perfusion. Step 2: When Va/Q is infinity, the denominator Q (perfusion) is zero while ventilation continues. This is the situation of alveolar dead space - the alveolus is ventilated but receives no blood. Step 3: With no blood flowing past the alveolus, there is no gas exchange across the respiratory membrane. The alveolar gas simply equilibrates with humidified inspired air, giving alveolar PO2 of about 149 mmHg and PCO2 of about 0 mmHg. Step 4: Both at Va/Q = 0 (shunt) and Va/Q = infinity (dead space) there is no effective exchange of oxygen and carbon dioxide. So the defining feature here is no exchange of O2 and CO2, making option (b) correct.