The new resuscitation guidelines permit compressions before delayed, defibrillation, a change that has generally been welcomed. The benefits are generally assumed to relate to the immediate provision of limited coronary perfusion with protection or replenishment of myocardial metabolic reserves. In this paper we argue that the concept is inadequate to explain many experimental and clinical observations made during resuscitation attempts. We argue that changes in the size and shape of the ventricles are the most important reason for the narrow window of opportunity for defibrillation alone and for the value of compressions in extending this period. We also draw attention to the implication for clinical resuscitation and to one aspect of the current guidelines of the European Resuscitation Council that we believe to be inconsistent with the evidence that we review.