Postoperative ejection phase parameters and patient survival rates for mitral valve replacement surgery are considerably lower than for similar aortic valve surgery. While chordal transection probably is the major contributor to the lowered values, there is also evidence for decreased preoperative myocardial contractile reserve in mitral regurgitation patients. This study characterizes abnormalities in the force-frequency relation that may underlie impaired function of myocardium isolated from mitral regurgitation patients with New York Heart Association class II-III heart failure.Methods and Results.
Left ventricular epicardial myocardium was obtained by surgical biopsy during mitral valve replacement surgery in patients with mitral regurgitation heart failure (left ventricular ejection fraction, 0.64±0.05) and during coronary artery bypass surgery in patients with normal ventricular function. The steady-state twitch tension versus frequency relation was measured in myocardial strip preparations (37°C, 12 to 228 min−1) in the absence and presence of forskolin. Relative to normal function, peak isometric twitch tension in mitral regurgitation is depressed by 50% (P<.02) and 74% (P<.003) at contraction frequencies of 60 min−1 and 168 min−1, respectively. The slope of the tension-frequency curve is blunted and its peak is shifted to a lower frequency (mitral regurgitation: 134 min−1; normal function: 173 min−1;P<.02). The myosin heavy chain concentration did not differ between mitral regurgitation and normal function strips (53±4 versus 54±4 nmol/g blotted wt). Forskolin (0.5 μmol/L) completely reversed the tension depression, blunting, and the lowered peak frequency in the mitral regurgitation preparations.Conclusions.
Preoperatively, myocardial tension generation in mitral regurgitation patients is severely depressed, and the force-frequency curve is blunted and has a negative slope in the exercise range of heart rates. The reversal of these defects by forskolin suggests that abnormal excitation-contraction coupling may underlie the decreased contractile reserve in mitral regurgitation patients. (Circulation.1993;88:2700–2704.)