Abstract
Recent observational studies have found associations between poorer outcomes and treatment that included mechanical CPR devices, contradicting findings from randomized trials. Resuscitation time bias is a systematic error occurring in observational studies of interventions applied to pulseless patients later in resuscitation attempts. Previous observational studies lack data on duration of resuscitation, a factor strongly related to outcome. We retrospectively analyzed cardiac arrest data to learn how resuscitation time and device use relate to clinical outcomes, and determine whether resuscitation time bias was present. Methods and Results: We analyzed data from all 49 patients with ventricular fibrillation, out-of-hospital cardiac arrest treated by our emergency medical service in one year. We compared 19 patients who received only standard manual CPR (the sCPR group) to 30 patients who received manual followed by mechanical CPR (the mCPR group). Response to CPR differed between groups even before device application. All sCPR patients achieved return of spontaneous circulation (ROSC), and did so after a median (IQR) of 3.3 (2.2-5.1) minutes of manual CPR. Patients in the mCPR group failed to get ROSC through 6.9 (5.3-11.0) min of manual CPR; mCPR patients that did get ROSC did so after 11.2 (5.7-23.8) additional minutes of CPR, delivered by a mechanical device. mCPR patients also received significantly more defibrillations and ALS drugs. ROSC and survival to hospital discharge were higher in the sCPR than the mCPR group (100% vs. 70%, P = 0.008; 74% vs. 43%, P = 0.045). Conclusion: Only patients remaining pulseless after early resuscitation efforts received mechanical CPR. Consequently, mechanical CPR devices assisted by facilitating prolonged treatment of patients who already had lower chances of survival before device application. Resuscitation time bias was present, and must be considered when interpreting registry reports comparing sCPR and mCPR.
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