Progressive changes in pulmonary gas exchange during invasive respiratory support for COVID-19 associated acute respiratory failure: a retrospective study of the association with 90-day mortality

Background: Ratio of arterial pressure of oxygen and fraction of inspired oxygen (P/F ratio) together with the fractional dead space (V_d/V_t) provides a global assessment of pulmonary gas exchange. The aim of this study was to assess the potential value of these variables to prognosticate 90-day survival in patients with COVID-19 associated ARDS admitted to the Intensive Care Unit (ICU) for invasive ventilatory support. Methods: In this single-center observational, retrospective study, P/F ratios and V_d/V_t were assessed up to 4 weeks after ICU-admission. Measurements from the first 2 weeks were used to evaluate the predictive value of P/F ratio and V_d/V_t for 90-day mortality and reported by the adjusted hazard ratio (HR) and 95% confidence intervals [95%CI] by Cox proportional hazard regression. Results: Almost 20,000 blood gases in 130 patients were analyzed. The overall 90-day mortality was 30% and using the data from the first ICU week, the HR was 0.85 [0.77–0.94] for every 10 mmHg increase in P/F ratio and 1.61 [1.20–2.16] for every 0.1 increase in V_d/V_t. In the second week, the HR for 90-day mortality was 0.82 [0.75–0.89] for every 10 mmHg increase in P/F ratio and 1.97 [1.42–2.73] for every 0.1 increase in V_d/V_t. Conclusion: The progressive changes in P/F ratio and V_d/V_t in the first 2 weeks of invasive ventilatory support for COVID-19 ARDS were significant predictors for 90-day mortality.