Changes in the inotropic state of the left ventricle during isometric exercise.

Abstract

Left ventricular function during sustained isometric handgrip exercise was studied at the time of cardiac catheterization in 8 normal subjects and 26 patients with heart disease.Both groups showed increases in heart rate, aortic mean blood pressure, and cardiac minute output with no change in systemic vascular resistance.Left ventricular filling pressure and stroke work were measured before and during isometric exercise in I7 cases.In 7 normal subjects, stroke work increased (8i + 8 8 to I04 ± II g m, P< O OI) without a significant change in left ventricularfilling pressure (6-8 + o 8 to 7.5 ± I -2 mmHg, NS) suggesting a shift to a higher ven- tricularfunction curve.Patients with heart disease, however, showedgreat variability in their stroke work res- ponse to isometric exercise (5 with increase, 3 with no change, and 2 with decrease), despite a significant increase in left ventricular filling pressure (9X6 9 mmHg to I5 5±+i6 mmHg, P< o.oi).Myocardial mechanics during isometric exercise were studied in 6 normal subjects and 8 patients with heart disease.Left ventricular pressure-velocity curves were shifted upwards and to the right in all 6 normal subjects and in 7 of the 8 patients with heart disease.Vmax (developed pressure) increased in both normal subjects (74 ±6 to I09± 5 sec -1, + 47%, P< O OI) and to a lesser extent in the patients with heart disease (59 ± 8 to 82+I2 sec + 39%,3 P< o.oi) in response to isometric exercise.Similarly, maximum left ventricular dp/dt increased in both normals (i88i + I63 to 2408+ I29 mmHg/sec, P< o oi) and patients with heart disease (I540 ± I40 to 1976 24I mmHg/sec, P < o oi).These observations suggest that the normal physiological response to isometric exertion includes a major increase in left ventricular myocardial contractility.For the normal heart, this increase appears sufficient to account completely for the increased ventricular performance associated with handgrip exercise.For the diseased heart, increased ventricular performance induced by handgrip appears also to be mediated by increased contractility, but in addition by varying degrees of reliance upon the Frank-Starling mechanism, depending upon the adequacy of inotropic (ccntractility) reserve to meet the stress imposed by isometric exercise.In studies of the circulatory response to exercise, attention has been largely directed towards under- standing the effects of dynamic exercise (Dexter et al., I95I).Recently, however, interest has focused on the cardiovascular effects of sustained voluntary muscle contraction, commonly termed isometric