Electromechanical dissociation (EMD) implies organized electrical depolarization of the heart without synchronous myocardial fiber shortening and, therefore, without cardiac output. Experimentally EMD can be produced by inducing ventricular fibrillation, not performing cardiopulmonary resuscitation, and applying a defibrillating shock after a few minutes. The result is frequently the restoration of electrical activity of the heart; however, because of diffuse myocardial ischemia, there is no mechanical contraction. The simultaneous electrocardiogram and arterial pressure tracings can reveal a variety of organized ECG rhythms (sinus rhythm, all degrees of heart block, idioventricular rhythm, etc), all without an arterial pressure. Attempts to correct these rhythms with pharmacologic therapy or with pacemakers are of no avail. Therapy must be directed toward the restoration of myocardial perfusion. In contrast, the patient with heart block or other severe bradydysrhythmias in which there is a palpable pulse generated with the QRS complex usually respond well to pharmacologic intervention or pacing. The prognosis of patients with EMD is poor, but rational therapy implies the application of techniques that increase coronary perfusion and thereby decrease the diffuse myocardial ischemia.