Association Between Dyscapnia, Ventilatory Variables, and Mortality in Patients With Acute Respiratory Distress Syndrome-A Retrospective Cohort Study

Abstract

Background: This study aimed to investigate the associations between dyscapnia, ventilatory variables, and mortality. We hypothesized that the association between mechanical power or ventilatory ratio and survival is mediated by dyscapnia. Methods: Patients with moderate or severe acute respiratory distress syndrome (ARDS), who received mechanical ventilation within the first 48 h after admission to the intensive care unit for at least 48 h, were included in this retrospective single-center study. Values of arterial carbon dioxide (PaCO₂) were categorized into "hypercapnia" (PaCO₂ ≥ 50 mm Hg), "normocapnia" (PaCO₂ 36-49 mmHg), and "hypocapnia" (PaCO₂ ≤ 35 mm Hg). We used path analyses to assess the associations between ventilatory variables (mechanical power and ventilatory ratio) and mortality, where hypocapnia or hypercapnia were included as mediating variables. Results: Between December 2017 and April 2021, 435 patients were included. While there was a significant association between mechanical power and hypercapnia (B_EM = 0.24 [95% CI: 0.15; 0.34], P < .01), there was no significant association between mechanical power or hypercapnia and ICU mortality. The association between mechanical power and intensive care unit (ICU) mortality was fully mediated by hypocapnia (B_EM = -0.10 [95% CI: -0.19; 0.00], P = .05; B_MO = 0.38 [95% CI: 0.13; 0.63], P < .01). Ventilatory ratio was significantly associated with hypercapnia (B = 0.23 [95% CI: 0.14; 0.32], P < .01). There was no significant association between ventilatory ratio, hypercapnia, and mortality. There was a significant effect of ventilatory ratio on mortality, which was fully mediated by hypocapnia (B_EM = -0.14 [95% CI: -0.24; -0.05], P < .01; B_MO = 0.37 [95% CI: 0.12; 0.62], P < .01). Conclusion: In mechanically ventilated patients with moderate or severe ARDS, the association between mechanical power and mortality was fully mediated by hypocapnia. Likewise, there was a mediating effect of hypocapnia on the association between ventilatory ratio and ICU mortality. Our results indicate that the debate on dyscapnia and outcome after ARDS should consider the impact of ventilatory variables.

Keywords: hypercapnia; hypocapnia; mechanical power; mechanical ventilation; respiratory failure; ventilatory ratio.