The influence of preload on post-systolic shortening in ischaemic myocardium

Post-systolic shortening is a wall motion abnormality defined as shortening 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-induced post-systolic shortening. Eleven beagles were anaesthetized (halothane 0.8%) and instrumented for measurement of pressures, flows and dimensions in the apical subendocardium supplied by the left anterior descending coronary artery. Myocardial ischaemia was obtained by tightening a micrometer-controlled snare around the left anterior descending coronary artery. Post-systolic shortening, calculated as end-systolic length minus minimum length divided by end-systolic length, was measured 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 demonstrates that the amount of post-systolic shortening depends on the volume status, which therefore has to be taken into account in interpreting regional wall motion abnormalities, such as those detected by echocardiography.