Fluctuations in body weight as occur with aging make body weight an unreliable reference for normalizing heart weight. We compared heart weight normalized by tibial length, which remains constant after maturity, with that normalized by body weight in 5- to 28-mo-old male Wistar rats. When normalized by tibial length or body weight, relative to the 5-mo heart, the senescent left ventricle undergoes 17 vs. 38% hypertrophy, respectively, and the right ventricle undergoes 0 vs. 28% hypertrophy, respectively. Histological measurements in the 25- compared with the 5-mo-old left ventricles reveal 6% larger myocyte diameters and 12% larger cellular cross-sectional areas, indicating about 15% hypertrophy; this value agrees more closely with the estimates based on tibial length than with those based on body weight. To allow prediction of left ventricular weight in a living rat, a regression equation using body weight, age, and tibial length was derived. This enabled us to perform a longitudinal aging study that verified that the above results were not biased by selective survival. Thus, in conditions in which body weight changes, cardiac hypertrophy can be more accurately quantified by relating heart weight to tibial length than to body weight. This approach may have applicability for assessing relative sizes of other organs as well.

IT IS WELL KNOWN that there are major anatomical and physiological differences between the right and left ventricles. Compared with the left ventricle, the right ventricle has a greater regional variation in wall thickness and a more complex geometric shape. The developed pressure in systole is much smaller, and intraventricular pressure falls considerably while ejection proceeds.Much recent research has been focussed on left ventricular function, yet there is little quantitative information concerning the right ventricle, particularly with respect to the question of how to characterize contractile state and pumping ability. Those indices of contractile state used in characterizing left ventricular function have not been quantified in and shown to be valid for the right ventricle.Since there are major differences between left and right ventricles, we investigated whether right ventricular contractile state could be described in a similar fashion as left ventricular contractile state. In this study we determined the time-varying ratio of instantaneous pressure to volume, which has been shown to be sensitive to changes in left ventricular contractile state and nearly independent of

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A B S T R A C T Isometric performance at 29'C was measured in left ventricular trabeculae carneae from young adult (6-mo) and aged (25-mo) rats (n = 18 in each group). Active tension and maximal rate of tension development did not differ with age, but contraction duration was 255±6 ms in the young adult and 283±6 ms in the aged group (P < 0.001). Although catecholamine content per gram heart weight was less in the aged myocardium, additional experiments showed that neither 1 X 10' M propranolol nor pretreatment with 6-hydroxydopamine eliminated the age difference in contraction duration. To determine if this age difference resulted from a prolonged active state, electromechanical dissociation and the overshoot of contraction duration during recovery from hypoxia were measured. During paired stimulation greater mechanical refractoriness was found in aged muscles (P < 0.01), but intracellular action potential recordings showed no age difference in the electrical refractory period. On recovery from hypoxia, contraction duration overshoot was 117±4% of control in the young and 138±4% of control in the aged muscles (P < 0.01). The greater electromechanical dissociation and greater overshoot in contraction duration following hypoxia in aged myocardium suggests that prolonged contraction duration in aged myocardium results from a prolonged active state rather than changes in passive properties or myocardial catecholamine content.

The end-systolic pressure-volume relationship (ESPVR) as derived from left ventricular pressure-volume loops has gained increasing acceptance as an index of ventricular contractile function. In animal experiments the ESPVR has been defined as a line connecting the upper left corners of several differently loaded pressure-volume (P-V) loops with a slope parameter Ees and a volume axis intercept parameter V.. In the clinical setting, several variants of the ESPVR have been determined with use of peak left ventricular pressure, end-ejection pressure, and end-ejection volume. The maximum P-V ratio has also frequently been measured. We attempted to determine which of these alternatives resulted in good approximations of the reference ESPVR in eight isolated canine ventricles that ejected into a simulated arterial impedance system with resistance, compliance, and characteristic impedance. We determined various versions of the ESPVR from the same set of beats quickly obtained with little change in inotropic background. To vary ventricular pressure wave forms, each of the arterial impedance parameters was independently controlled at 50%, 100%, and 200% of normal. Against each of the nine combinations of the impedance parameters four P-V loops were obtained under four preloads and from each of the sets offour P-V loops, the reference ESPVR, linear regression of the peak pressure on end-ejection volume (ESPVRPP-EEV), and linear regression of end-ejection pressure on endejection volume (ESPVREEPV) were determined. In addition, the maximum P-V ratio (MPVR) was calculated for each P-V loop. At all combinations of afterload impedance parameters ESPVRPP-EEV was shifted to the left (slope 5.4 vs 5.2 mm Hg/ml, intercept 6.6 vs 7.4 ml) and ESPVREEPV was shifted rightward (slope 5.0 mm Hg/ml, intercept 7.7 ml) from ESPVRREF. These differences, however, were quantitatively very small. MPVR was much smaller than the slope of ESPVRREF (4.0 vs 5.2 mm Hg/ml) and was load dependent. We conclude that as long as the P-V measurements are made under a fixed afterload system and different preloads, ESPVRpp EEV and ESPVREEPV, but not MPVR, can be used to approximate ESPVRREF.