THE INFLUENCE OF PRELOAD ON POST-SYSTOLIC SHORTENING IN ISCHEMIC MYOCARDIUM

Post-systolic shortening is a wall motion abnormality defined as short ening of cardiac muscle after the end of ejection and usually regarded as a manifestation of ischaemia. This study was designed to determine whether changes in preload may alter the magnitude of ischaemia-induc ed post-systolic shortening. Eleven beagles were anaesthetized (haloth ane 0.8%) and instrumented for measurement of pressures, flows and dim ensions in the apical subendocardium supplied by the left anterior des cending coronary artery. Myocardial ischaemia was obtained by tighteni ng a micrometer-controlled snare around the left anterior descending c oronary artery. Post-systolic shortening, calculated as end-systolic l ength minus minimum length divided by end-systolic length, was measure d at different levels of preload. Increasing the preload from 4 to 17 mmHg caused a significant reduction in post-systolic shortening (8.9% vs. 12.9%, P<0.05, Student's paired t-test); post-systolic shortening was negatively correlated with coronary perfusion pressure (r=0.35, P< 0.01) and positively correlated with systolic bulging. This study demo nstrates that the amount of post-systolic shortening depends on the vo lume status, which therefore has to be taken into account in interpret ing regional wall motion abnormalities, such as those detected by echo cardiography.