MECHANICAL CHARACTERISTICS OF TACHYCARDIA-INDUCED LEFT-VENTRICULAR FAILURE AS EVALUATED IN ISOLATED DOG HEARTS

Left ventricles of control dog hearts and dog hearts failing due to ch ronic tachycardia were examined in vivo by echocardiography for systol ic function and size, then subsequently studied with an isolated-heart system (artificial perfusion, artificial loading). During 3 weeks of tachycardia (250 bt/min), area ejection fraction fell by 58%, while en d-diastolic transverse area increased by 56% (measurements at 120 bt/m in). Judging from post-perfusion left-ventricular weights, the dilatio n occurred with no hypertrophy, raising the question whether the failu re model may be associated with anabolic dysfunction. End-diastolic pr essure-volume (P-V) relations occurred at higher volumes in failing ch ambers than in controls, and this was marked by increases in two indic es of chamber size (candidate reference volumes): the volume resulting in a diastolic stress of 16 g/cm2, and the volume at which the nearly straight, low-stiffness segment of the end-diastolic P-V relation mee ts the upward bending, high-stiffness segment. Developed P-V relations of failing chambers were shifted to higher volumes and to lower press ures, the lower pressures being due more to reduced stress-developing ability (contractility) than to reduced wall/cavity ratio (pressure/st ress ratio). On average, shortening ability (normalized difference bet ween reference volume and extrapolated volume-axis intercept, i.e., ap parent ejection fraction from reference volume in absence of afterload ) was not different from that of controls. Isovolumic pressure waves o f the failing and dilated chambers were of almost normal duration and shape, extending further the range of conditions where isovolumic pres sure can be predicted by fitting a model isovolumic wave function to t he isovolumic phases of ejecting beats.