Estimating mortality risk is essential for prognostic enrichment. How various indices specific to respiratory compromise contribute to prognostication in patients with acute respiratory distress syndrome (ARDS) is not well-characterized in general clinical populations. The primary objective of this study was to identify variables specific to respiratory failure that add prognostic value to indicators of systemic illness severity. We tested the added benefit of respiratory variables in a representative observational cohort of patients with ARDS.
50 patients with ARDS were enrolled in a single-center, prospective, observational cohort. We tested the contribution of respiratory variables (oxygenation index, ventilatory ratio [VR], and the radiographic assessment of lung edema score) to logistic regression models of 28-day mortality adjusted for indicators of systemic illness severity (the Acute Physiology and Chronic Health Evaluation [APACHE] III score or severity of shock as measured by the number of vasopressors required at baseline). We also compared a model utilizing APACHE III with one including baseline number of vasopressors using the areas under their receiver operating curves.
VR significantly improved model performance by likelihood ratio testing when added to APACHE III (p = 0.04) or vasopressor number at baseline (p = 0.01). Adjusted for APACHE III, each 0.5-unit change in VR was associated with an odds ratio for 28-day mortality of 1.78 (95% CI = 0.78-3.23). The number of vasopressors required at baseline had similar prognostic discrimination to the multi-component APACHE III. A model including the number of vasopressors and VR (area under the receiver operating curve [AUROC] 0.77, 95% CI 0.64-0.90) was comparable to a model including APACHE III and VR (AUROC 0.81 (95% CI 0.68 – 0.93), p value for comparison = 0.58.)
In this observational cohort of patients with ARDS, the ventilatory ratio significantly improved discrimination for mortality when combined with indicators of severe systemic illness. Additionally, the number of vasopressors required at baseline and APACHE III had similar discrimination for mortality when combined with VR. The ventilatory ratio is easily obtained at the bedside and offers promise for both clinical prognostication and enriching clinical trial populations.