Prevalence and outcome of silent hypoxemia in COVID-19

BACKGROUND: In the early stages of COVID-19 pneumonia, hypoxemia has been described in absence of dyspnea ("silent" or "happy" hypoxemia). Our aim was to report its prevalence and outcome in a series of hypoxemic patients upon Emergency Department admission. METHODS: In this retrospective observational cohort study we enrolled a study population consisting of 213 COVID-19 patients with PaO2/FiO(2) ratio <300 mmHg at hospital admission. Two groups (silent and dyspneic hypoxemia) were defined. Symptoms, blood gas analysis, chest X-ray (CXR) severity, need for intensive care and outcome were recorded. RESULTS : Silent hypoxemic patients (68-31.9%) compared to the dyspneic hypoxemic patients (145-68.1%) showed greater frequency of extra respiratory symptoms (myalgia, diarrhea and nausea) and lower plasmatic LDH. PaO2/FiO(2) ratio was 225 +/- 68 mmHg and 192 +/- 78 mmHg in silent and dyspneic hypoxemia respectively (P=0.002). Eighteen percent of the patients with PaO2/FiO(2) from 50 to 150 mmHg presented silent hypoxemia. Silent and dyspneic hypoxemic patients had similar PaCO2 (34.2 +/- 6.8 mmHg vs. 33.5 +/- 5.7 mmHg, P=0.47) but different respiratory rates (24.6 +/- 5.9 bpm vs. 28.6 +/- 11.3 bpm respectively, P=0.002). Even when CXR was severely abnormal, 25% of the population was silent hypoxemic. Twenty-six point five percent and 38.6% of silent and dyspneic patients were admitted to the ICU respectively (P=0.082). Mortality rate was 17.6% and 29.7% (log-rank P=0.083) in silent and dyspneic patients. CONCLUSIONS: Silent hypoxemia is remarkably present in COVID-19. The presence of dyspnea is associated with a more severe clinical condition.