Nonischemic End-Systolic Performance

Nonischemic end-systolic performance decreases during ischemia. These changes in performance are likely to be dependent on the size and site of the ischemic zone, as well as the prevailing loading conditions. This study was designed to examine the effect of regional and generalized changes in inotropy on nonischemic end-systolic performance, independent of the ischemic zone size. Twenty dogs were instrumented with sonomicrometers and micromanometer pressure gauges. End-systolic pressure-thickness relationship data were obained during vena-caval balloon inflation. Measurements were obtained before and 90 s after left circumflex (LC) artery occlusion. Then, simultaneous with the occlusion of the LC artery, isoproterenol (0.04 μg/ml) was infused into the left anterior descending artery. After recovery, the same protocol was repeated before and after propranolol (0.5 mg/kg). In a separate set of animals, the same measurements were made following 2.5 and 5 μg/kg/min dobutamine. The effect of ischemia on the nonischemic end-systolic pressure-thickness relationship was expressed as the extent to which the relationship is shifted to the left. Infusion of intracoronary isoproterenol into the perfusion bed of the nonischemic zone produced a significant increase in the slope of the end-systolic pressure-thickness relationship. During ischemia, however, the extent of leftward shift of this relationship was less than that following β-blockade. Intravenous dobutamine resulted in a dose-dependent increase in the slope of the nonischemic end-systolic pressure thickness relationship, but the extent of leftward displacement of the relationship in response to regional ischemia was less than that following the control occlusion. The nonischemic segment is coupled with the nonfunctioning ischemic zone in such a way that it is required of the nonischemic segment to operate at decreased end-systolic thickness for any end-systolic pressure, the extent of which is to be determined, in part, by the size of the ischemic zone and the contractile state of the nonischemic myocardium. The lower the contractile state prior to coronary occlusion the greater extent of leftward shift of the pressure-thickness relationship.