‘Idiosyncratic Action’
The authors argued that, while “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.” Those mechanisms include:
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How dyspnea and the respiratory centers respond to low levels of oxygen,
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How prevailing carbon dioxide tensions (PaCO2) blunt the brain’s response to hypoxia,
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The effects of disease and age on control of breathing,
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Inaccuracy of pulse oximetry at low oxygen saturations, and
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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,” researchers conclude, adding that “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.”
One focus was the initial assessment of a patient’s oxygen level with a pulse oximeter.
“While a pulse oximeter is remarkably accurate when oxygen readings are high, it markedly exaggerates the severity of low levels of oxygen when readings are low,” Tobin points out. “Another factor is how the brain responds to low levels of oxygen. As oxygen levels drop in patients with COVID-19, the brain does not respond until oxygen falls to very low levels–at which point a patient typically becomes short of breath.”
Another factor was that low levels of carbon dioxide, which was seen in more than half of the patients, can minimize the effects of extremely low oxygen levels.
“It is also possible that the coronavirus is exerting a peculiar action on how the body senses low levels of oxygen,” Tobin suggested, nothing the high numbers of COVID-19 with taste and smell affected.
“This new information may help to avoid unnecessary endotracheal intubation and mechanical ventilation, which presents risks, when the ongoing and much anticipated second wave of COVID-19 emerges,” Tobin said.
The authors also looked at the bigger picture of how puzzling COVID-19 has been to clinicians.
“Given that hypoxemia is at the very heart of the most severe cases of COVID-19, one wonders if the lack of a widely accepted definition of hypoxemia contributes to some of the confusion and counterclaims associated with the disease,” they wrote, adding, “In conclusion, COVID-19 has engendered many surprises, but features that baffle physicians are less strange when contemplated through the lens of long-established principles of respiratory physiology.”
- Tobin MJ, Laghi F, Jubran A. Why COVID-19 Silent Hypoxemia Is Baffling to Physicians. Am J Respir Crit Care Med. 2020;202(3):356-360. doi:10.1164/rccm.202006-2157CP