Left ventricular performance in rats with chronic cardiac overload due to arterio-venous shunt

The effects of experimental arterio-venous shunt (AV shunt) on left ventricular dynamics and myocardial mechanics were investigated in female Wistar rats, weighing approx. 240 g. The main goal of the study was to characterize ventricular and myocardial working capacity and to estimate the significance of the various processes involved in cardiac adaptation to the altered loading conditions. Twelve weeks after operation, systemic blood pressure was slightly elevated in AV shunt rats as compared with age-matched controls, although total peripheral resistance (as related to a common aortic pressure) was significantly reduced. Heart rate and cardiac output were considerably increased. Despite marked left ventricular dilatation with increased end-diastolic and systolic wall stress, no signs of resting insufficiency were present. Left ventricular weight, end-diastolic volume and pressure, stroke volume, and ejection fraction were increased. The area between the curve of isovolumic maxima and the end-diastolic P-V curve (up to a defined end-diastolic pressure) and the normalized area between the isovolumic and end-diastolic stress-length curves as well as the maximum rate of stress development were augmented, even in the specimen with the highest degree of ventricular dilatation--despite a shift of the myosin isoenzyme pattern towards VM3. The study reveals that besides the change in ventricular mass and configuration, augmented preload is involved in cardiac adaptation to the altered hemodynamic state. Furthermore, there are indications of an intensified sympathetic stimulation of the heart. Transformation of the myocardium towards a slower muscle seems to be of minor functional significance under the conditions of the present experiments.