The CPR committee of the Dutch Heart Association currently recommends for the management of sudden cardiac arrest, the initiation of external cardiac compressions before airway control and ventilation, which may be termed the “CAB” sequence. This differs from the ABC sequence, recommended by the American Heart Association, in which the patient is ventilated first. Supporters of the CAB sequence reason that, since most sudden cardiac deaths are due to ventricular fibrillation, the blood in the arterial system should be well oxygenated at the onset of such an episode. We conducted these studies to characterize the decline in arterial blood gases during 5 min of cardiac arrest, followed by external cardiac compressions alone, vs followed by external cardiac compressions with ventilation. In addition, we studied the decline in arterial blood gases and ventricular fibrillation without therapy.