Pathophysiology
Clinical meaning
The ventilation-perfusion (V/Q) ratio describes the relationship between alveolar ventilation (V, normally ~4 L/min) and pulmonary capillary perfusion (Q, normally ~5 L/min), yielding a normal whole-lung V/Q ratio of approximately 0.8. Gas exchange efficiency depends on the matching of ventilation to perfusion in individual lung units. V/Q mismatch is the most common cause of hypoxemia and manifests as two physiological extremes. Dead space ventilation (high V/Q, approaching infinity) occurs when alveoli are ventilated but not perfused -- gas enters the alveolus but no capillary blood flow is present to participate in gas exchange. The classic pathological cause is pulmonary embolism (PE), where thrombus obstructs pulmonary arterial flow to ventilated segments. Anatomic dead space (conducting airways) is normal; alveolar dead space is pathological. Shunt (low V/Q, approaching zero) occurs when pulmonary capillary blood flows past non-ventilated alveoli, so deoxygenated blood returns to the left heart without participating in gas exchange. Causes include ARDS (alveolar flooding/collapse), pneumonia (consolidation), and atelectasis. True (anatomic) shunt is distinguished from V/Q mismatch shunt by its failure to correct with 100% supplemental oxygen -- in true shunt,...