Partial pressure of arterial carbon dioxide and survival to hospital discharge among patients requiring acute mechanical ventilation: A cohort study

Abstract

Purpose: To describe the prevalence of hypocapnia and hypercapnia during the earliest period of mechanical ventilation, and determine the association between P_aCO₂ and mortality.

Materials and methods: A cohort study using an emergency department registry of mechanically ventilated patients. P_aCO₂ was categorized: hypocapnia (<35mmHg), normocapnia (35-45mmHg), and hypercapnia (>45mmHg). The primary outcome was survival to hospital discharge.

Results: A total of 1,491 patients were included. Hypocapnia occurred in 375 (25%) patients and hypercapnia in 569 (38%). Hypercapnia (85%) had higher survival rate compared to normocapnia (74%) and hypocapnia (66%), P<0.001. P_aCO₂ was an independent predictor of survival to hospital discharge [hypocapnia (aOR 0.65 (95% confidence interval [CI] 0.48-0.89), normocapnia (reference category), hypercapnia (aOR 1.83 (95% CI 1.32-2.54)]. Over ascending ranges of P_aCO₂, there was a linear trend of increasing survival up to a P_aCO₂ range of 66-75mmHg, which had the strongest survival association, aOR 3.18 (95% CI 1.35-7.50).

Conclusions: Hypocapnia and hypercapnia occurred frequently after initiation of mechanical ventilation. Higher P_aCO₂ levels were associated with increased survival. These data provide rationale for a trial examining the optimal P_aCO₂ in the critically ill.

Keywords: Clinical outcomes; Hypercapnia; Mechanical ventilation.