Effects of altered preload on left ventricular systolic time intervals in acute myocardial infarction.

Acute effects of altered preload on systolic time intervals were examined in I7 patients; 7 subjects without acute myocardial infarction and normal haemodynamics (post aortic valve surgery-Class I), 6 patients with acute myocardial infarction and raised left ventricular filling pressure, and 4 patients with acute myocardial infarction and normal filling pressure. Alterations in preload were brought about by applications of venous cuffs on both thighs. Observations were also made on the systemic artery and pulmonary artery pressures, cardiac output, and heart rate before and after application of venous occlusive cuffs. In Class I postsurgical patients with normal haemodynamics, pre-ejection period (PEP) lengthened, left ventricular ejection time (LVET) shortened, and PEPILVET ratio was increased after application of thigh cuffs. While heart rate was held constant by atrial pacing, stroke index decreased, pulmonary artery systolic and diastolic pressures fell, but there was no significant change in systemic artery pressure. In patients having acute myocardial infarction with raised left ventricularfilling pressure, PEP was shortened, LVET remained unchanged, and PEPILVET ratio decreased after application of venous cuffs. No significant changes in heart rate, cardiac output, and systemic artery pressures were observed. However, both the pulmonary artery systolic and diastolic pressures fell. In patients having acute myocardial infarction with normal left ventricular filling pressure, there was no significant change in any of the aforementioned parameters. These differences in responses to applications of venous occlusive cuffs are explained on the basis of alterations in left ventricular performance. It is concluded that in patients with acute myocardial infarction, observations on the effects of altered preload on systolic time intervals may provide a more accurate index of left ventricular performance than measurements of systolic time intervals alone.