Oxygen and Gas Exchange

Also known as Oxygenation · Hypoxaemia · Hypoxia · Oxyhaemoglobin dissociation curve · High-flow nasal cannula · V/Q mismatch · Shunt · Dead space

Oxygen and gas exchange for the intensivist — hypoxaemia versus hypoxia, the alveolar gas equation and the A-a oxygen difference, the five mechanisms of hypoxaemia (V/Q mismatch, shunt, diffusion limitation, hypoventilation, low inspired oxygen) and how oxygen responsiveness separates them, the oxygen content equation and oxygen delivery (the critical DO2), venous saturations, oxygen-induced hypercapnia, oxygen devices and high-flow nasal oxygen (FLORALI), and the oxygen target (BTS 94–98 per cent, 88–92 per cent if at risk of hypercapnia; the IOTA meta-analysis — liberal oxygen increased mortality).

high18 referencesUpdated 2 Oct 202618 min readSource-verified · Oct 2026

Checked against its sources on 2 Oct 2026

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8 MCQs

Meet the patient

A 64-year-old man is brought to the ED at 2 am with central crushing chest pain. The monitor shows SpO2 88 per cent on 15 L/min, respiratory rate 30, heart rate 118, blood pressure 102/60. The pulse oximeter alarms but no one has yet sent an arterial blood gas. You ask for one before deciding anything.^[15]

Two questions decide him. Which mechanism is lowering the arterial oxygen — and is the tissue getting the oxygen it needs? Hypoxaemia and hypoxia do not always coexist, so the saturation alone answers neither question. Hold those two questions and everything in this chapter slots underneath.^[14]

References18Show

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