ICU · respiratory

Hypoxaemia Mechanisms — Comprehensive (5 Causes, A-a Gradient, Shunt vs V/Q Mismatch)

Also known as Hypoxaemia · Mechanisms of hypoxaemia · Five causes of hypoxaemia · V/Q mismatch · Shunt · Venous admixture · Diffusion impairment · Hypoventilation · Type 1 respiratory failure · Type 2 respiratory failure · A-a gradient · Alveolar gas equation · Shunt equation · P/F ratio · Berlin ARDS definition · Oxygenation index

There are exactly FIVE mechanisms of hypoxaemia: (1) ventilation-perfusion (V/Q) MISMATCH — the commonest cause, which RESPONDS to oxygen; (2) SHUNT — blood bypasses ventilated alveoli, which does NOT respond to oxygen and needs PEEP; (3) DIFFUSION IMPAIRMENT — a thickened blood-gas barrier, which responds to oxygen and worsens with exercise; (4) HYPOVENTILATION — type 2 failure with a NORMAL A-a gradient; and (5) LOW INSPIRED PO2 — altitude. Two bedside discriminators separate them. The ALVEOLAR GAS EQUATION gives PAO2 = FiO2(Patm − PH2O) − PaCO2/RQ (~100 mmHg on room air), so the A-a GRADIENT (PAO2 − PaO2) is normal (<15 young, <25 elderly) in hypoventilation and low inspired PO2, and ELEVATED in V/Q mismatch, shunt, and diffusion impairment. The SHUNT EQUATION Qs/Qt = (CcO2 − CaO2)/(CcO2 − CvO2) is normally <5% and 10% is significant. The definitive V/Q-mismatch-vs-shunt test is 100% FiO2 for 15 minutes: if PaO2 rises above ~350 mmHg the problem is V/Q mismatch (correctable), if it stays below ~350 mmHg there is a significant SHUNT (needs PEEP, proning, ECMO). The P/F RATIO (PaO2/FiO2) is the Berlin ARDS yardstick: <300 mild, <200 moderate, <100 severe. Oxygenation indices rank severity: P/F ratio, A-a gradient, and the Oxygenation Index (OI = FiO2 × MAP × 100 / PaO2). The clinical approach is stepwise: (1) calculate the A-a gradient; (2) if elevated, get a chest X-ray and echocardiogram to localise the lung or heart problem; (3) treat the cause — oxygen for V/Q mismatch, PEEP/recruitment/proning for shunt, ventilation for hypoventilation.