TIME-DEPENDENT SHIFTS OF THE LEFT-VENTRICULAR DIASTOLIC FILLING RELATIONSHIP IN CONSCIOUS DOGS

The viscous properties, creep and stress relaxation, have been demonstrated in isolated cardiac muscle preparations but have not been shown to occur in intact, conscious animals. In the present study, time-dependent shifts of the left ventricular (LV) diastolic pressure-dimension and stress-strain relationships consistent with creep and stress relaxation were observed in eight conscious dogs previously instrumented with LV micromanometers and ultrasonic dimension gauges. These changes were seen during recovery from major elevations of systolic and diastolic pressure produced by infusion of saline and phenylephrine. Thus, during recovery, at matched end-diastolic pressures (EDP) between 17 and 21 mm Hg, end-diastolic segment length was increased by an average of 1.6% (P < 0.001) and minor axis chord length by 2.4% (P < 0.01). These increases in LV dimensions were accompanied by reductions in diastolic wall thickness. When increases in EDP were limited by caval occlusion, such shifts no longer were apparent. In addition, 'rapid' elevation of LV pressures was associated with steeper filling relationship than with 'slow' elevation of LV pressures. In beats matched for EDP, systolic pressure, and heart rate, shifts in diastolic LV dimensions during recovery were associated also with increases in stroke volume and percent shortening. In two additional dogs, the presence of an increased end-diastolic volume during recovery was validated by angiographic techniques. The authors conclude that time-dependent shifts of the LV diastolic filling relationship can be demonstrated after major increases in systolic and diastolic pressure and can significantly influence both the shape of the filling relationship and systolic performance.