True asystole and electromechanical dissociation do not respond to electrical shock. Nevertheless there are anecdotal stories of physicians who, out of frustration, delivered defibrillation shocks to patients "in asystole" only to find that "defibrillation" resulted in the restoration of normal sinus rhythm. The explanation for this apparent paradox perhaps relates to our recent observation that ventricular fibrillation (VF) may masquerade as asystole if only one electrocardiographic (ECG) lead is monitored. We noted that in patients with atrial fibrillation, fibrillatory waves were often of very low amplitude or were not apparent in lead I, yet were obvious in lead VI. This suggests an anterior electrical vector in atrial fibrillation. To determine whether there was a dominant electrical vector in subjects in VF, 21 dogs were placed in VF and standard 12-lead ECGs were recorded at intervals. In two-thirds of the animals one lead showed no electrical activity (electrical asystole), while obvious VF was apparent in all the other leads. Thus, if only one lead is monitored, VF may masquerade as asystole. McDonald recently confirmed this phenomenon in man. Before one decides not to defibrillate a patient with an asystolic rhythm, one should quickly monitor another lead or change the monitoring electrode paddles 90 degrees to look at the electrical activity in a different plane.