End-systolic pressure-diameter relationships during pulsus alternans in intact pig hearts

Pulsus alternans has previously been ascribed to variations in end-diastolic fiber length and/or an alternating contractile state. In six paced, whole blood-perfused, intact, working pig hearts, left ventricular internal diameters were measured with ultrasonic crystals, and left ventricular pressures were measured with a micromanometer-tipped catheter. Pulsus alternans was induced with propranolol (24-51 micrograms X kg-1 X min-1). End-systolic pressure-diameter relationships were determined during pulsus alternans from a series of consecutive, variably volume-loaded beats using a previously described technique. Beats analyzed prior to any change in loading conditions revealed a left ventricular pressure of 62.7 +/- 8.7 mmHg for the strong beats and 55.2 +/- 7.0 for the weak beats (P less than 0.050). The end-diastolic diameter prior to the stronger beats was larger than that prior to the weaker beats (24.80 +/- 2.73 vs. 23.82 +/- 2.82 mm, P less than 0.025), but there was no significant difference in the corresponding end-diastolic pressures. The maximum dP/dt of the strong beats was 1,275 +/- 249 mmHg/s compared with 1,038 +/- 222 for the weak beats (P less than 0.025). The slope of the end-systolic pressure-diameter relationship, which provides an index of contractility independent of end-diastolic diameter, averaged 42.3 +/- 6.4 mmHg/mm for the strong beats and 24.4 +/- 6.8 for the weak beats (P less than 0.025). These data support an alternating contractile state as an independent mechanism for pulsus alternans in the intact animal.