Effect of abrupt mitral regurgitation after balloon valvuloplasty on myocardial load and performance. The concept that mitral regurgitation masks myocardial dysfunction by reducing afterload and augmenting ejection performance has not been well established in humans. The effect of abruptly produced mitral regurgitation on left ventricular loading and performance was therefore evaluated in five patients who developed this complication after an otherwise successful percutaneous balloon mitral valvuloplasty. Mitral valve area by Gorlin formula calculated with forward flow increased from 0.92 +/- 0.14 to 2.75 +/- 0.82 cm2. Mean left atrial pressure did not decrease (19 +/- 4 to 19 +/- 6 mm Hg). The size of the left atrial V wave relative to mean left atrial pressure (peak V - mean left atrial pressure) increased from 7 +/- 4 to 19 +/- 6 mm Hg. Angiographic mitral regurgitation increased from 0+ or 1+ to greater than 3+ in each patient and regurgitant fraction increased from 0.23 +/- 0.11 to 0.55 +/- 0.09 (p less than 0.01). End-diastolic volume increased modestly from 148 +/- 15 to 159 +/- 15 ml (p = NS). Heart rate increased from 54 +/- 5 to 71 +/- 8 beats/min (p less than 0.05), which may have prevented further increases in preload by shortening the filling period. End-systolic stress decreased by 32% from 277 +/- 34 to 188 +/- 52 kdyn/cm2 (p less than 0.01) as a result of a 25% decrease in end-systolic pressure from 121 +/- 8 to 91 +/- 7 mm Hg and a 16% decrease in end-systolic volume from 67 +/- 13 to 56 +/- 8 ml (p = NS). 