Anaes · Applied cardiovascular & respiratory physiology

Oxygen cascade & hypoxic pulmonary vasoconstriction

Also known as Oxygen cascade · Alveolar gas equation · Alveolar oxygen · Hypoxic pulmonary vasoconstriction · HPV · One-lung ventilation

Oxygen steps down in partial pressure at each stage from the dry inspired air to the mitochondrion — the oxygen cascade — and the lung matches blood flow to that oxygen by a mechanism unique to the pulmonary circulation: hypoxic pulmonary vasoconstriction. The framework rests on five exam-critical ideas: the oxygen cascade falls from about 159 mmHg in dry inspired air at sea level, through 149 in humidified tracheal air, about 100 in alveolar gas, about 95 in arterial blood, about 40 in capillaries, to about 1 to 3 mmHg in the mitochondria; the alveolar gas equation (PAO2 equals FiO2 times the pressure of Patm minus PH2O minus PaCO2 over the respiratory quotient) gives the alveolar oxygen that sets the top of the arterial step; the alveolar-to-arterial difference is normally small (under about 15 mmHg breathing air) and rises with shunt, mismatch and diffusion impairment; hypoxic pulmonary vasoconstriction constricts pulmonary arterioles in response to ALVEOLAR (not mixed venous) hypoxia, diverting perfusion to better-ventilated lung and so defending V/Q matching; and HPV is impaired by volatile anaesthetics, systemic vasodilators and a low systemic vascular resistance, which is the mechanism of hypoxaemia during one-lung ventilation. Built on the alveolar-gas-equation review (Heymer 2026), the lung-isolation study (Brenn 2024), the microRNA-hypoxic-pulmonary-hypertension review (Ma 2026), the high-altitude cardiovascular review (Chacon-Diaz 2026), the respiratory-rate-and-ventilatory-efficiency study (Jung 2026), and the airway-pressure-release-ventilation gas-exchange recommendations (Nieman 2026).