Extracorporeal membrane oxygenation to support cardiopulmonary resuscitation: Useful, but for whom?

Extracorporeal membrane oxygenation has been successfully used to support patients with cardiac arrest failing to respond to conventional cardiopulmonary resuscitation (CPR) (1, 2). Extracorporeal membrane oxygenation is generally deployed in these patients when all conventional therapies have been exhausted and death without other methods of cardiopulmonary support is imminent. Survival for patients using extracorporeal membrane oxygenation to support failed conventional CPR (external CPR [ECPR]) is variable and is reported to be approximately 37% in children and 27% in adults. The American Heart Association in their 2005 CPR guidelines recommended consideration of ECPR at institutions capable of rapidly deploying ECPR for patients with inhospital cardiac arrest receiving good-quality CPR and thought to have a reversible reason for their cardiac arrest (3). The use of ECPR is increasing both in children and adults, although survival rates have not improved despite increasing experience (1, 2). Given the unpredictable nature of the need for ECPR, institutions providing ECPR services maintain extracorporeal membrane oxygenation teams and equipment to capable of rapid ECPR deployment at all times and is thus expensive. Before ECPR becomes more widely adopted, it is critically important to evaluate the efficacy of ECPR in promoting survival and good long-term outcomes compared with conventional CPR therapies. It is equally important to know who may benefit from such therapy and thus it is also crucial to refine patient selection for ECPR so that ECPR is used effectively and for the right patients. The emergent nature at which ECPR is deployed makes conducting a randomized trial of ECPR vs. conventional CPR challenging and very difficult. In this issue of Critical Care Medicine, Shin et al (4) add to the existing knowledge that ECPR may indeed help in some patients failing to respond to conventional CPR therapies. They compare outcomes of patients with cardiac arrest requiring CPR for 10 mins managed with conventional CPR and with ECPR in their institution during January 2003 to June 2009 using a novel case–control study design. Cases (ECPR patients) and control subjects (conventional CPR patients) for this study were selected after matching based on a propensity score predicting the probability to being a candidate for ECPR. In doing so, they attempt to balance important covariates between cases and control subjects to minimize bias resulting from confounding variables much similar to a randomization. Unlike randomization, propensity score matching can only balance measured confounders so residual confounding from unmeasured covariates could still be present (5). However, when randomization is not possible like in the case of ECPR, using propensity score-based matching may be an excellent alternative to minimize bias resulting from confounding. In this article, a cohort of patients receiving CPR for 10 mins for cardiac arrest treated with or without ECPR was used to select a propensity score-based matched cohort of cases (ECPR cases) and control subjects (conventional CPR patients) for analysis (4). Using this cohort of matched patients, the authors show that the odds of mortality and having more than minimal neurologic disability was lower (odds ratio, 0.17; 95% confidence interval, 0.04–0.71) for patients supported with ECPR compared with those managed with conventional CPR. These improved outcomes for ECPR patients persisted even at 6 months after cardiac arrest. Improved survival was also seen in a subgroup of patients with cardiac disease, the largest diagnostic group in their cohort. The authors conclude that ECPR improved survival in patients with refractory inhospital cardiac arrest. These results shown here by Shin et al are similar to those published recently by Chen et al in Lancet in 2009 (6). So is there sufficient evidence to promote the use of ECPR? Not yet. There are several issues to consider when evaluating results from this article. First, in this cohort of patients, 70% of patients had primary cardiac disease and thus generalization to patients with other diagnoses is difficult. Second, the decision to deploy ECPR in 85 (21%) of the 406 patients during the study period was at the discretion of the primary physician conducting CPR. Thus, the majority of patients in this cohort were deemed not suitable to be ECPR candidates by the primary physician managing the event, yet they form the chosen comparison group. Third, the median predicted probability of being an ECPR candidate in the cohort was low (0.32 0.18 for both cohorts) indicating that many patients both in the ECPR and control groups had a low probability of being an ECPR candidate, or in other words, many were not good candidates for ECPR. Thus, the authors are comparing outcomes in a cohort of patients, many of whom may not have been good candidates for ECPR. A stratified analysis showing differences in survival outcomes between the control and ECPR groups based on high or low probability of being an ECPR candidate would have been useful to better understand efficacy of ECPR in promoting survival. Finally, although survival is improved for ECPR patients compared with those who were not, survival to hospital discharge in the ECPR cohort was only 34% raising the question of who should be a candidate for ECPR. *See also p. 1.