Restoring forces assessed with left atrial pressure clamps.

A negative pressure (P) in the fully relaxed left ventricle (LV) indicates the presence of restoring forces generated during contraction. To assess restoring forces in the intact LV under physiological filling conditions, a servomotor system was used in anesthetized open-chest dogs (n = 8) to produce nonfilling diastoles by left atrial pressure (LAP) clamping during systole such that LAP was less than left ventricular pressure (LVP) during the subsequent diastole. Steady-state LV end-diastolic pressure (EDP) was varied by volume infusion from 4.0 +/- 1.5 (+/-SD) to 12.8 +/- 2.1 mmHg. The corresponding fully relaxed LVPs increased from -2.1 +/- 1.9 to 1.1 +/- 3.2 mmHg, P < 0.001. LAP clamping increased the rate of LVP fall by 34 +/- 28% (P < 0.001) during 10 ms after the LVP dropped below the level of the LVP-LAP crossover of the preceding normal beat. During clamped beats, two-dimensional echo revealed substantial downward displacement of the mitral valve (MV) leaflets despite the reversed LA-LV gradient and absence of filling. Thus 1) restoring forces are present at low physiological EDP but absent at high physiological EDP; 2) filling retards the rate of fall of LVP; 3) even in the absence of filling, the process of LV relaxation facilitates MV opening.