['Predictors of severity of cardiac arrest or efficacy of cardiopulmonary resuscitation are few. Respiratory end tidal CO2 (ETCO) is a marker of pulmonary blood flow and, possibly, cardiac arrest. The purpose of this study was to evaluate ETCO as a quantitative marker of cardiac arrest in a human model of ventricular fibrillation (VF).', 'Thirty-one cardiac arrest/VF episodes (mean BP < 40 mmHg) in 8 men and 3 women mean age = 42 +/- 24 years, mean left ventricular ejection fraction = 39%) undergoing defibrillator (ICD) implant for ventricular tachycardia or previous cardiac arrest were evaluated with continuous ETCO monitoring during defibrillation threshold testing. All patients but one were intubated.', 'Significant differences (P < 0.001) were noted between ETCO values prior (mean 37.2 +/- 6.8 mmHg) versus during VF (mean 27.1 +/- 5.9 mmHg), and during VF versus return of spontaneous circulation (mean 36.6 +/- 6.6 mmHg). ETCO decreased by 23% +/- 8% from pre-VF to during VF. It increased by 37% +/- 16% during VF to return of spontaneous circulation. These changes were significantly different (P < 0.001).'] Is end tidal CO2 a quantitative measure of cardiac arrest?