Written by Clay Smith
The severe, yet clinically silent hypoxemia of COVID-19 seen in some patients may be explained by a few basic respiratory pathophysiological principles.
Why does this matter?
Dubbed “happy hypoxia,” the silent hypoxemia of COVID-19 has caused consternation among seasoned physicians. Rich Levitan remarked in a NYT op-ed that patients had profound hypoxia, “seemingly incompatible with life—but they were using their cellphones…” The authors offer some possible explanations for this phenomenon.
There’s nothing happy about hypoxia
The authors present three cases, all with COVID-19, minimal respiratory distress, yet profound hypoxemia. How can this be? The authors pose seven possible explanations.
Hypoxia alone does not trigger a large increase in minute ventilation until the PaO2 has dropped below 60; below that, it increases exponentially. See Figure 1.
Ventilatory drive is profoundly influenced by PCO2. “Severe hypoxia elicits an effective increase in ventilation only when background PaCO2 exceeds 39 mmHg.”
Increasing age and diabetes may decrease the responsiveness of the respiratory drive in response to hypoxia. Also, individuals vary widely in their sensitivity to the O2 and CO2 triggers of respiratory drive.
Pulse oximeters are notoriously inaccurate below 80%. SpO2 is also less accurate in critically ill patients and Black patients. See Figure 2.
Of course, profoundly low oxygen levels are deleterious for even a brief period of time, but humans can tolerate fairly low PaO2 for extended periods of time and may not feel bad. The authors remind us that tourists driving on Mt. Evans in Colorado routinely have a drop in SpO2 to 65%, and many feel no sense of dyspnea.
Fever shifts the oxyhemoglobin dissociation curve right, meaning a lower SaO2 for the same PaO2. The carotid body only senses PaO2.
They also remind us that hypoxemia must be defined in relation to the FiO2. Is a patient on 100% FiO2 with SpO2 92% hypoxic? Most physicians surveyed for this commentary would say yes. The authors comment that it may be difficult to judge severity based on number of liters of oxygen, given that it can be difficult to know the true FiO2 in non-intubated cases.
Why COVID-19 Silent Hypoxemia is Baffling to Physicians. Am J Respir Crit Care Med. 2020 Jun 15. doi: 10.1164/rccm.202006-2157CP. [Epub ahead of print]
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