Alterations in Preload and Myocardial Mechanics in the Dog and in Man

Based on a myocardial mechanics approach to the analysis of left ventricular contraction, several indexes have been advanced as specific measures of myocardial contractility. Although two of these indexes, Vmax and (d P /d t )/ P max, have been shown to be appropriately responsive to interventions known to alter the inotropic state, controversy has arisen as to whether they are unaffected by alterations in preload or initial cardiac muscle fiber length. This study was undertaken to determine which, if any, of the indexes obtained from myocardial mechanics analysis of left ventricular systole is independent of preload in the intact heart. Increases in left ventricular enddiastolic pressure were produced in seven dogs by infusion of dextran or whole blood and in eight patients by sudden elevation of the legs during cardiac catheterization. In dogs, elevation of left ventricular end-diastolic pressure was accompanied by a progressive decline in both Vmax and (d P /d t )/ P max measured using total pressure ( P < 0.01). In contrast, Vmax measured using developed pressure was unaffected over a wide range of left ventricular end-diastolic pressures. Similarly, in the eight patients, increases in left ventricular end-diastolic pressure induced by sudden elevation of the legs were consistently associated with reductions in (d P /d t )/ P max measured using total pressure ( P < 0.01), but these changes in pressure had no effect on developed pressure Vmax. In the intact heart, measurements of myocardial mechanics using developed pressure appear to be independent of preload, but similar measurements using total pressure are inversely dependent on preload. This finding raises doubt as to the validity of total pressure measurements as indexes of myocardial contractility.