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clinical skills
assessment
respiratory conditions
patient safety
critical care

Happy hypoxia in COVID-19: pathophysiology and pulse oximetry accuracy

02 July 2021
Features
02 July 2021
Volume 13 · Issue 7

Abstract

Many patients with COVID-19 have presented to emergency departments with arterial hypoxaemia but without breathlessness; this is called ‘happy hypoxia’ or, more accurately, ‘silent hypoxaemia’. Hypoxaemia needs to be identified correctly in patients with COVID-19 as it is associated with in-hospital mortality. The aetiology of silent hypoxia is unclear, and the pathophysiological processes involved in the relationship between the response to hypoxaemia and the sensation of dyspnoea may explain its clinical presentation. Pulse oximetry is used routinely to measure oxygen saturation. However, recent literature has questioned its accuracy in patients with COVID-19. Inaccuracies in readings, which arise for several reasons, could in part explain silent hypoxaemia. Caution should be taken when interpreting pulse oximeter readings or patients could be given a higher inspired oxygen fraction than necessary. Silent hypoxaemia may also mask disease severity in patients with COVID-19.

The ongoing coronavirus disease 2019 (COVID-19) pandemic caused many challenges. Among these is the phenomenon of ‘happy hypoxia’ or, more precisely, silent hypoxaemia. Many patients have presented to emergency departments with low oxygen saturations (SpO2) measured with pulse oximetry yet had minimal signs of respiratory distress (Tobin et al, 2020). Although happy hypoxia has gained extensive media coverage, its aetiology remains unclear.

A pathophysiological explanation of silent hypoxaemia in COVID-19 is a dissociation between hypoxaemia and the sensation of breathlessness. The process could involve an idiosyncratic action of COVID-19 on chemoreceptors sensitive to oxygen (Tobin et al, 2020), intrapulmonary shunting, relatively preserved lung compliance, and dysfunctional hypoxaemic vasoconstriction (Dhont et al, 2020). Tobin et al (2020) presented three cases where patients with a reduced partial pressure of arterial oxygen (PaO2) ranging between 4.8kPa and 6kPa denied any having an difficulty in breathing.

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Happy hypoxia
Hypoxaemia
COVID-19
Pulse oximetry
Dyspnoea
Oxygen saturation
Emergency care