For a study, researchers sought to see if asymptomatic (“happy”) hypoxia was a distinct physiological feature of COVID-19 ARDS and if it was connected with the requirement for ICU admission. They conducted an observational cohort analysis of all adult patients admitted to a large acute hospital Trust with hypoxaemic respiratory failure. Patients with COVID-19 were compared to those who did not. A linear mixed-effects model was used to simulate the physiological response to hypoxia.

About 75% of the 1,586 patients tested positive for SARS-CoV-2. When corrected for age and ethnicity, the ROX index in the COVID-19 group was 2.08 min−1 lower (1.56–2.61, p<0.001), indicating an improved respiratory response to hypoxia compared to non-Covid-19 individuals. The respiratory response to hypoxemia showed significant inter-and intra-patient heterogeneity. About 33% of the infected cohort required ICU care, and 31% died within 60 days. ICU admission and death were both linked to an improved respiratory response at all levels of hypoxia.

Individual patients with COVID-19 demonstrated a more symptomatic phenotype in response to hypoxemia than those with other causes of hypoxaemic respiratory failure, but there is a wide range of responses. As a result, whereas silent hypoxemia can occur in any patient suffering from hypoxaemic respiratory failure, it is not more common in individuals infected with SARS-CoV-2.