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To compare the initial end-tidal CO2 (PetCO2) during cardiopulmonary resuscitation in asphyxial versus ventricular fibrillatory cardiac arrest.A cohort study.University research laboratory.Forty domestic piglets.Asphyxial cardiac arrest was produced by clamping the endotracheal tube in 20 piglets and was continued for 10 minutes after loss of aortic pulsations occurred. Ventricular fibrillation (VF) was induced by applying 60 Hz of alternating current via a pacing wire to the myocardium of the other 20 piglets, and continued for a 15-minute downtime. Cardiopulmonary resuscitation (CPR) was then provided to each group for two minutes, followed by standard advanced cardiac life support protocols.All piglets were instrumented for continuous monitoring of PetCO2, electrocardiogram, central venous pressure, and aortic pressure, PetCO2 of the first breath of CPR was 91 ± 20 mmHg in the asphyxial group versus 34 ± 14 mmHg in the VF group (P < 0.001). The asphyxial group continued to exhibit significantly greater PetCO2 for the first five breaths of resuscitation, after which there were no differences. The coronary perfusion pressures during the first breaths of CPR did not differ between the two groups. High initial PetCO2 did not correlate with return of spontaneous circulation.End-tidal CO2 during the first five breaths of CPR is much higher after an asphyxial cardiac arrest than VF. In each case, the initial PetCO2 appears to reflect alveolar CO2 prior to CPR. After one minute of CPR, PetCO2 is useful in monitoring the effectiveness of CPR.