Association between pH and neurological outcome among cardiac arrest patients treated by extracorporeal CPR: a prospective observational cohort study in Japan

Background We aimed to identify the association of pH value in blood gas assessment with neurological outcome among out-of-hospital cardiac arrest (OHCA) patients treated by extracorporeal cardiopulmonary resuscitation (ECPR). Methods We retrospectively analysed the database of a multicentre prospective observational study to collect pre-hospital and in-hospital data among OHCA patients in Osaka prefecture, Japan (CRITICAL study), from 1 July 2012 to 31 December 2016. We included the adult OHCA patients treated by ECPR with blood gas assessment before starting ECPR. Patients with OHCA of external causes, such as trauma, were excluded. The patients were divided into three approximately equal groups based on the pH values. We conducted logistic regression analysis to identify the odds ratio (OR) and 95% confidence interval (CI) of the groups for the 1-month neurological favourable outcome adjusted by the potential confounders of sex, age, witnessed by bystander, CPR by bystander, pre-hospital initial cardiac rhythm, and cardiac rhythm on hospital arrival. Results Among the 9,822 patients in the database, 260 patients were finally included in the analysis. The three groups were Tertile 1: pH ≥7.030, Tertile 2: pH 6.875-7.029, and Tertile 3: pH<6.875. The adjusted OR of Tertiles 2 and 3 compared with Tertile 1 for 1-month favourable neurological outcome were 0.26 (95% CI: 0.10-0.63) and 0.24 (95% CI: 0.09-0.61), respectively. Conclusion This multi-institutional observational study showed that low pH value (<7.03) before the implementation of ECPR was associated with 1-month unfavourable neurological outcome among OHCA patients treated with ECPR. It may be helpful to consider the candidate for ECPR.