Adaptations of the left ventricle to chronic pressure overload

Left ventricular (LV) function during the adaptation to chronic pressure overload produced by an ascending aortic constriction was analyzed in conscious dogs, instrumented with intraventricular micromanometers and pairs of ultrasonic crystals for measurement of LV wall thickness (WTh) and internal LV chamber diameter. During inflation of the cuff to produce LV pressures averaging 220 mg Hg, calculated peak wall stress (WSt) increased by 55% above control while percent shortening decreased by 24% and mean circumferential shortening velocity (VCF) decreased by 39% from control. By 9 days (mean) after aortic constriction, the cross-sectional area (CSA) of the LV wall increased by 10% and peak WSt fell to 37% above control. End-diastolic diameter (EDD) increased to 4% above control, while percent shortening and mean VCF remained reduced at -12% and -20% of control, respectively. During the phase of concentric hypertrophy (mean 2 1/2 weeks), CSA increased further to 15% above control and WSt fell to 22% above control, while EDD and percent shortening returned to control and mean VCF increased to -7% of control (not significant). At 24 hours after release of the cuff WSt, percent shortening, mean VCF, and peak velocity of LV pressure rise (peak dP/dt) were not significantly different from control. Rapid, partial regression of hypertrophy was observed in some dogs. Thus, the left ventricle responds to chronically elevated pressure by initial dilation with increased WSt followed by gradual wall thickening and consequent reduction of WSt to near normal. After successful adaptation to the pressure overload, hypertrophy per se did not produce intrinsic depression of the myocardial inotropic state.