Left ventricular systolic resistance in rats with hypertension and hypertrophy.

Traditional indexes of ventricular performance often fail to identify differences between the normal and hypertrophied ventricle. This may not be the case for load-independent mechanical properties, elastance, and resistance. Accordingly, we derived these properties of the intact left ventricle (LV) in 25-wk-old male spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto controls (WKY) using in-situ and isolated hearts. We found that 1) pump performance was similar in SHR and WKY, both at base line and after dextran; 2) the peak systolic elastance (Emax) was higher and theoretical maximum flow (Qmax, inverse of ventricular resistance) was lower in SHR; (3) slopes of peak isovolumetric pressure-volume and stress-strain relations were significantly higher in SHR; and 4) although end-diastolic pressure-volume relation for SHR was shifted to the right, there was no difference in end-diastolic stress-strain relations. Thus elastance in hypertrophied LV is augmented due to both an increase in muscle mass and the force-generating capacity of the myocardium. Furthermore, we propose that the decrease in Qmax seen in SHR reflects a change in certain velocity-dependent properties of the myocardium, whereas the preservation of pump performance is a result of the opposing effects of increased Emax and decreased Qmax. These observations underscore the importance of quantifying systolic resistance, together with elastance, for a better assessment of the LV as a mechanical pump.