Efficient gas exchange requires matching of ventilation (V) to perfusion (Q) in pulmonary units. The ideal V/Q ratio is approximately 1.0. Pathological states disrupt this relationship through three mechanisms: (1) V/Q mismatch — the most common cause of hypoxemia — occurs when ventilation and perfusion are mismatched in degree (pneumonia with partially ventilated but perfused alveoli, or PE with ventilated but underperfused regions); (2) True shunt (V/Q = 0) — blood passes through completely unventilated alveoli (atelectasis, ARDS, pneumonia with complete consolidation) or through anatomic shunts (intracardiac right-to-left shunts); shunt is characterized by hypoxemia refractory to supplemental oxygen; (3) Dead space (V/Q = infinity) — ventilated alveoli without perfusion (pulmonary embolism is the classic pathological cause; anatomic dead space is the conducting airway volume ~150 mL). The alveolar-arterial (A-a) gradient quantifies gas exchange efficiency: normal is <10 mmHg in young adults, increasing with age (expected = age/4 + 4). Elevated A-a gradient indicates parenchymal lung disease or shunt; normal A-a gradient with hypoxemia suggests hypoventilation (neuromuscular disease, opioid overdose). The NP uses ABG analysis, A-a gradient calculation, and response to...