OPEN American journal of respiratory and critical care medicine | 17 Jun 2020
MJ Tobin, F Laghi and A Jubran
Patients with COVID-19 are described as exhibiting oxygen levels incompatible with life without dyspnea. The pairing-dubbed happy hypoxia, but more precisely termed silent hypoxemia-is especially bewildering to physicians and is considered as defying basic biology. This combination has attracted extensive coverage in media but has not been discussed in medical journals. It is possible that coronavirus has an idiosyncratic action on receptors involved in chemosensitivity to oxygen, but well-established pathophysiological mechanisms can account for most, if not all, cases of silent hypoxemia. These mechanisms include how dyspnea and the respiratory centers respond to low levels of oxygen, how prevailing carbon dioxide tensions (PaCO2) blunt the brain’s response to hypoxia, effects of disease and age on control of breathing, inaccuracy of pulse oximetry at low oxygen saturations, and temperature-induced shifts in the oxygen dissociation curve. Without knowledge of these mechanisms, physicians caring for hypoxemic patients free of dyspnea are operating in the dark-placing vulnerable COVID-19 patients at considerable risk. In conclusion, features about COVID-19 that physicians find baffling become less strange when viewed in the light of long-established principles of respiratory physiology; an understanding of these mechanisms will enhance patient care if the much-anticipated second wave emerges. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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