Arterial Blood Gas as a Predictor of Mortality in COVID Pneumonia Patients Initiated on Noninvasive Mechanical Ventilation: A Retrospective Analysis

Abstract

Background: The alveolar-arterial oxygen (A-a) gradient measures the difference between the oxygen concentration in alveoli and the arterial system, which has considerable clinical utility. Materials and methods: It was a retrospective, observational cohort study involving the analysis of patients diagnosed with acute COVID pneumonia and required noninvasive mechanical ventilation (NIV) over a period of 3 months. The primary objective was to investigate the utility of the A-a gradient (pre-NIV) as a predictor of 28-day mortality in COVID pneumonia. The secondary objective included the utility of other arterial blood gas (ABG) parameters (pre-NIV) as a predictor of 28-day mortality. The outcome was also compared between survivors and nonsurvivors. The outcome variables were analyzed by receiver-operating characteristic (ROC) curve, Youden index, and regression analysis. Results: The optimal criterion for A-a gradient to predict 28-day mortality was calculated as ≤430.43 at a Youden index of 0.5029, with the highest area under the curve (AUC) of 0.755 (p <0.0001). On regression analysis, the odds ratio for the A-a gradient was 0.99. A significant difference was observed in ABG predictors, including PaO₂, PaCO₂, A-a gradient, AO₂, and arterial-alveolar (a-A) (%) among nonsurvivors vs survivors (p-value <0.001). The vasopressor requirement, need for renal replacement therapy, total parenteral requirement, and blood transfusion were higher among nonsurvivors; however, a significant difference was achieved with the vasopressor need (p <0.001). Conclusion: This study demonstrated that the A-a gradient is a significant predictor of mortality in patients initiated on NIV for worsening respiratory distress in COVID pneumonia. All other ABG parameters also showed a significant AUC for predicting 28-day mortality, although with variable sensitivity and specificity. Key messages: COVID-19 pneumonia shows an initial presentation with type 1 respiratory failure with increased A-a gradient, while a subsequent impending type 2 respiratory failure requires invasive ventilation. A significant difference was observed in ABG predictors, including PaO₂, PaCO₂, A-a gradient, AO₂, and a-A (%) among nonsurvivors vs survivors. (p-value <0.001). The vasopressor requirement, need for renal replacement therapy, total parenteral requirement, and blood transfusion need were higher among nonsurvivors than survivors; however, a significant difference was achieved with the vasopressor need (p <0.001). How to cite this article: Gupta B, Jain G, Chandrakar S, Gupta N, Agarwal A. Arterial Blood Gas as a Predictor of Mortality in COVID Pneumonia Patients Initiated on Noninvasive Mechanical Ventilation: A Retrospective Analysis. Indian J Crit Care Med 2021;25(8):866-871.

Keywords: COVID-19 Acute Respiratory Distress Syndrome; Critically ill adults; Mortality predictors.

Figs 1A to H

Receiver-operating characteristic curve for (A) a gradient; (B) pO₂/FiO₂; (C) respiratory index; (D) a–A (%); (E) PaO₂; (F) PaCO₂; (G) pH; (H) HCO₃