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The following is a summary of “Partial pressure of carbon dioxide/pH interaction and its association with mortality among patients mechanically ventilated in the emergency department,” published in the May 2024 issue of Emergency Medicine by McCormick, et al.

Conflicting data exist regarding the effects of hypercapnia on clinical outcomes among mechanically ventilated patients in the emergency department (ED). For a study, researchers sought to investigate the hypothesis that the association between hypercapnia and clinical outcomes is modified by the degree of acidosis. Specifically, they sought to determine if hypercapnia was associated with increased in-hospital mortality and decreased ventilator-free days at lower pH levels, but with decreased in-hospital mortality and increased ventilator-free days at higher pH levels, among patients in ED requiring mechanical ventilation.

A secondary analysis was conducted using patient-level data from previous clinical trials and cohort studies involving adult patients in ED requiring mechanical ventilation. The included subjects had documented blood gas measurements while on mechanical ventilation in the ED. The primary outcome was in-hospital mortality, with ventilator-free days as a secondary outcome. Mixed-effects logistic, linear, and survival-time regression models were employed to assess if pH modified the association between the partial pressure of carbon dioxide (pCO₂) and outcome measures.

Among the 2,348 included subjects, the median pCO₂ was 43 (IQR 35–54) and pH was 7.31 (IQR 7.22–7.39). Overall in-hospital mortality was 27%. The analysis revealed that pH modified the association between pCO₂ and outcomes. Higher pCO₂ was associated with an increased probability of in-hospital mortality when pH was below 7.00, while it was associated with decreased mortality when pH was above 7.10. This trend persisted across sensitivity and subgroup analyses. Similar associations were observed with ventilator-free days.

The study findings suggested that pH levels influence the relationship between pCO₂ and clinical outcomes. Higher pCO₂ was associated with reduced mortality and increased ventilator-free days when pH exceeds 7.10 but with increased mortality and fewer ventilator-free days when pH is below 7.00. Targeting pCO₂ based on pH levels in the ED could be a potential intervention strategy in future clinical trials aiming to improve patient outcomes.

Reference: sciencedirect.com/science/article/abs/pii/S0735675724000858