Which phenomenon describes perfusion without ventilation in atelectasis, leading to a right-to-left shunt?

• A. Diffusion impairment
• B. Perfusion without ventilation leading to a right-to-left shunt
• C. Hypercapnic respiratory failure
• D. Ventilation-perfusion mismatch

Answer: (B)

Intrapulmonary shunt is the idea being tested here. In atelectasis, portions of the lung collapse and stop receiving air, so ventilation to those regions is essentially zero, while blood flow through the surrounding capillaries continues. That means blood passes through the lung without coming into contact with air for oxygen to diffuse in and carbon dioxide to diffuse out. The result is blood that remains deoxygenated by the time it reaches the left heart—a right-to-left shunt.

This is a specific form of a ventilation–perfusion problem: V is zero in the affected units, so it behaves as a shunt. Diffusion impairment would involve a problem across the alveolar–capillary membrane, not the complete absence of ventilation. Hypercapnic respiratory failure arises from overall hypoventilation and CO2 retention, not the selective perfusion of nonventilated alveoli. Ventilation–perfusion mismatch is broader and includes scenarios where some ventilation and perfusion are mismatched, but the described phenomenon is the classic shunt where perfusion occurs without ventilation.