AbstractTo assess the effect of rapid preload reduction on left ventricular performance in nonischemic cardiomyopathy, 11 patients were studied during inferior vena caval (IVC) balloon occlusion. Five developed sustained pulsus alternans.During pulsus alternans, the strong beats demonstrated systolic performance characteristics similar to baseline values, despite a drop in both left ventricular (LV) enddiastolic diameter (66 ± 13 to 61 ± 13 mm; p<0.05) and LV end‐diastolic pressure (21 ± 8 to 9 ± 6 mmHg; p<0.05). In contrast, the weak beats demonstrated a reduction in peak systolic pressure (130 ± 36 to 109 ± 33 mmHg; p<0.02), fractional shortening (20% ± 4% to 17% ± 9%; p<0.05) and peak positive dP/dt (1,006 ± 224 to 921 ± 287 mmHg; p<0.05).Measures of diastolic performance (peak negative dP/dt, the time constant of LV relaxation, the length of diastasis, and LV end‐diastolic stress) were not different between baseline beats and the strong beats; and only LV end‐diastolic stress differed when baseline beats were compared to the weak beats.When the strong beats were compared to the weak beats during induced pulsus alternans, significant differences were observed in peak systolic pressure, peak positive dP/dt, and fractional shortening, but no differences in any measured diastolic parameter was observed. A slight difference was noted in the left ventricular end‐diastolic diameters, with the weak beat consistently beginning at a slightly smaller diameter (61 ± 13 mm vs 59 ± 13; p<0.05).In summary, these data are consistent with an augmentation and deletion of intrinsic contractile forces in association with an alternation in preload on a beat‐to‐beat basis as best describing left ventricular performance