RESPONSE TO MYOCARDIAL-ISCHEMIA AS A REGULATED PROCESS

There is increasing information leading to the conclusion that the coronary and myocardial responses to a reduction in coronary flow are graded and regulated. Support for this concept can be found in three areas. First, there is vasodilator reserve during ischemia. This residual tone of coronary resistance vessels may produce the most appropriate transmural distribution of myocardial blood flow and allow a reserve for a more severe challenge. Second, the mechanical myocardial response to ischemia is graded to the degree of ischemia and is greater than obligatory, since enhanced contractile function can be stimulated. It appears that chronic downregulation of mechanical function exists in ischemic myocardium. Last, there is evidence of metabolic regulation that allows repletion of some metabolic deficits and clearance of accumulated cellular lactate with moderate ongoing ischemia. These responses can be seen as protective for the physiological integrity of the cell. A more complete understanding of these processes may ultimately explain clinical phenomena such as the hibernating myocardium.