Mechanisms of forward flow during external chest compression

There are more than 300 000 victims of cardiac arrest each year, and attempts to resuscitate them are usually unsuccessful. Laboratory and clinical studies have shown that restoration of cardiac function after cardiac arrest is related to the level of coronary perfusion generated during resuscitation. It has also been shown that adequate cerebral perfusion is necessary for preservation of brain function. There is therefore a critical need for determining methods to augment blood flow generated during resuscitation. A key factor in enhancing flow is understanding the mechanisms of blood flow operative during chest compression, since it may be possible to optimize flow by exploiting those mechanisms. Fluid movement in any hydraulic system results from the interaction of a pump, or driving force for fluid movement, and a load. In the intact cardiovascular system, the pump is obviously the heart, and the load is the vascular resistance and compliance. During resuscitation, however, the pump and load are less well defined. Continuing controversy exists over the nature of the pump operative during chest compression. Pumps that have been proposed include direct cardiac compression, intrathoracic pressure rises, and a combination of both. It is important to determine the nature of the pump, since the method of chest compression for optimizing flow may be different depending on which pump is operative. For optimizing flow, it may also be important to understand the effects of a number of techniques that have been investigated which may modify the pump.