Hypothermia is associated with a low ETCO₂ and low pH-stat PaCO₂ in refractory cardiac arrest

The end-tidal carbon dioxide (ETCO₂) is frequently measured in cardiac arrest (CA) patients, for management and for predicting survival. Our goal was to study the PaCO₂ and ETCO₂ in hypothermic cardiac arrest patients.

We included patients with refractory CA assessed for extracorporeal cardiopulmonary resuscitation. Hypothermic patients were identified from previously prospectively collected data from Poland, France and Switzerland. The non-hypothermic CA patients were identified from two French cohort studies. The primary parameters of interest were ETCO₂ and PaCO₂ at hospital admission. We analysed the data according to both alpha-stat and pH-stat strategies.

We included 131 CA patients (39 hypothermic and 92 non-hypothermic). Both ETCO₂ (p < 0.001) and pH-stat PaCO₂ (p < 0.001) were significantly lower in hypothermic compared to non-hypothermic patients, which was not the case for alpha-stat PaCO₂ (p = 0.15). The median PaCO₂-ETCO₂ gradient was greater for hypothermic compared to non-hypothermic patients when using the alpha-stat method (46 mmHg vs 30 mmHg, p = 0.007), but not when using the pH-stat method (p = 0.10). Temperature was positively correlated with ETCO₂ (p < 0.01) and pH-stat PaCO₂ (p < 0.01) but not with alpha-stat PaCO₂ (p = 0.5). The ETCO₂ decreased by 0.5 mmHg and the pH-stat PaCO₂ by 1.1 mmHg for every decrease of 1° C of the temperature. The proportion of survivors with an ETCO₂ ≤ 10 mmHg at hospital admission was 45% (9/25) for hypothermic and 12% (2/17) for non-hypothermic CA patients.

Hypothermic CA is associated with a decrease of the ETCO₂ and pH-stat PaCO₂ compared with non-hypothermic CA. ETCO₂ should not be used in hypothermic CA for predicting outcome.