Effects of preload alteration on the degree of ischemia and function of ischemic myocardium under constant mean aortic pressure, coronary perfusion pressure and heart rate in isolated perfused canine heart.

Abstract

We examined the effects of preload alteration on global and regional (i.e., non-ischemic and ischemic areas) function in the presence of regional myocardial ischemia and on the degree of ischemia using 18 isolated, metabolically supported canine left ventricles. For this purpose, cardiac output (CO), systolic segment length change (SL), myocardial CO2 tension (PmCO2) and ST level of epicardial ECG were measured at 3 levels of left ventricular end-diastolic pressure (LVEDP), i.e., approximately 7 (low LVEDP), 11 (middle LVEDP), and 16 mmHg (high LVEDP) without and with left circumflex artery (LCx) stenosis under a constant mean aortic pressure (90 mmHg), mean coronary perfusion pressure (90 mmHg) and heart rate. In the Pre-ischemic stage, CO and SL increased significantly when LVEDP was elevated in a stepwise fashion by changing the height of the reservoir connected to the left atrium. There were no significant changes in PmCO2 or ST level. On the other hand, with LCx stenosis, CO did not show a subsequent increase at higher LVEDPs (i.e., from 796 +/- 103 ml/min at middle LVEDP to 931 +/- 153 ml/min at high LVEDP). Furthermore, there was no significant SL response in the LCx area following alterations of LVEDP, although there was considerable lengthening of end-diastolic length. Both increased PmCO2 and ST level of the LCx area, following LCx stenosis, further increased significantly with elevation of LVEDP. These results suggest the possibility that considerable elevation of LVEDP worsens the degree of ischemia and does not significantly augment ischemic regional myocardial function or global function, while mild elevation of preload improves or tends to improve simultaneously regional ischemic and global functions without aggravating the ischemic injury significantly. Therefore, we conclude that the preload level is quite important in managing ischemia induced myocardial dysfunction.