ICU · Respiratory / gas exchange
Mechanisms of Hypoxaemia — V/Q Mismatch, Shunt, Diffusion, Hypoventilation
Also known as Hypoxaemia · Mechanisms of hypoxaemia · V/Q mismatch · Shunt · Diffusion impairment · Hypoventilation · Type 1 respiratory failure · Type 2 respiratory failure · A-a gradient · Alveolar gas equation · Oxygen cascade
There are five mechanisms of hypoxaemia: low inspired oxygen, hypoventilation (type 2 failure, with a NORMAL alveolar-arterial gradient), ventilation-perfusion mismatch (the commonest cause, correctable by 100 per cent oxygen), shunt (does NOT correct with 100 per cent oxygen), and diffusion impairment (worsens with exercise). The alveolar-arterial oxygen gradient distinguishes hypoventilation (normal gradient) from a lung problem (high gradient), and the response to 100 per cent oxygen distinguishes V/Q mismatch (corrects) from true shunt (does not).
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Meet the patient
It's 2am on the ward. A 64-year-old woman with community-acquired pneumonia is hypoxaemic on 4 L/min nasal cannula — SpO2 88%, ABG shows pH 7.38, PaCO2 36, PaO2 56 on FiO2 0.36. She looks mildly tachypnoeic, no chest pain, no CO2 retention.[1]
The first question the senior is going to ask at the bedside is the one that decides everything: is this a lung problem or a pump problem? Five mechanisms; one calculation; one test. Two minutes from now you will know whether to give oxygen, ventilate, or recruit. The whole topic fits into the next page.[1]
References15ShowHide
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- [13]Wiedemann HP, Wheeler AP, Bernard GR, et al. Comparison of two fluid-management strategies in acute lung injury N Engl J Med, 2006.PMID 16714767
- [14]Peek GJ, Mugford M, Tiruvoipati R, et al. Efficacy and economic assessment of conventional ventilatory support versus extracorporeal membrane oxygenation for severe adult respiratory failure (CESAR): a multicentre randomised controlled trial Lancet, 2009.PMID 19762075
- [15]Goligher EC, Tomlinson G, Hajage D, et al. Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome and Posterior Probability of Mortality Benefit in a Post Hoc Bayesian Analysis of a Randomized Clinical Trial JAMA, 2018.PMID 30347031