Asymptomatic Hypoxemia as a Characteristic Symptom of Coronavirus Disease: A Narrative Review of Its Pathophysiology

Coronavirus disease (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has become a pandemic and caused a huge burden to healthcare systems worldwide. One of the characteristic symptoms of COVID-19 is asymptomatic hypoxemia, also called happy hypoxia, silent hypoxemia, or asymptomatic hypoxemia. Patients with asymptomatic hypoxemia often have no subjective symptoms, such as dyspnea, even though hypoxemia is judged by objective tests, such as blood gas analysis and pulse oximetry. Asymptomatic hypoxemia can lead to acute respiratory distress syndrome, and the delay in making a diagnosis and providing initial treatment can have fatal outcomes, especially during the COVID-19 pandemic. Thus far, not many studies have covered asymptomatic hypoxemia. We present a review on the human response to hypoxemia, focusing on the respiratory response to hypoxemia rather than the pathophysiology of lung injury arising from SARS-CoV-2 infection. We have also discussed whether asymptomatic hypoxemia is specific to SARS-CoV-2 infection or a common phenomenon in lung-targeted viral infections.