ICU · respiratory

Oxygen Therapy and Oxygen Delivery — Comprehensive ICU Physiology

Also known as Oxygen therapy · Oxygen delivery · DO2 · VO2 · Oxygen cascade · Alveolar gas equation · Oxygen content · High-flow nasal cannula · Oxygen extraction ratio

Oxygen therapy and delivery — the physiological principles governing oxygen movement from the atmosphere to the cell, and the ICU interventions to optimise it. The oxygen cascade: atmospheric PO2 (159 mmHg at FiO2 21%) → tracheal PO2 (149, after humidification) → alveolar PO2 (PAO2 ~100, after CO2 mixing — calculated by the ALVEOLAR GAS EQUATION: PAO2 = FiO2(Patm - PH2O) - PaCO2/RQ) → arterial PO2 (PaO2 ~95, after shunt/VQ mismatch) → capillary PO2 (40, after tissue extraction) → mitochondrial PO2 (1-3, for oxidative phosphorylation). Oxygen content: CaO2 = (1.34 × Hb × SaO2) + (0.003 × PaO2) — the HAEMOGLOBIN component dominates (99% of total O2 content — dissolved O2 is negligible at normal pressures). Oxygen delivery: DO2 = CO × CaO2 × 10 — normal ~1000 mL/min. Oxygen consumption: VO2 = CO × (CaO2 - CvO2) × 10 — normal ~250 mL/min. O2 extraction ratio (ER) = VO2/DO2 — normal ~25% (the body extracts 25% of delivered O2 at rest). ICU management of hypoxaemia: increase FiO2 → increase Hb (transfuse) → increase CO (inotrope/fluid) → reduce shunt (PEEP, proning, recruit alveoli). Hyperoxia is HARMFUL (ROS, absorption atelectasis, coronary vasoconstriction) — titrate FiO2 to lowest setting maintaining SpO2 92-96%.