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

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 (< 35 mm Hg), normocapnia (35–45 mm Hg), and hypercapnia (> 45 mm Hg). 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 66–75 mm Hg, 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.