Developed pressure data may provide misinformation when used alone to evaluate systolic function in isovolumetric left ventricle preparations

Abstract

We report data showing that developed pressure (DP max ) may lead to opposite conclusion with respect to maximal developed circumferential wall stress (s max ) when used to assess contractile function in left ventricle isovolumic preparations. Isovolumetric left ventricle preparations of rats with cardiac hypertrophy (H; N = 10) induced by isoproterenol administration showed higher DP max (174 ± 14 mmHg) than control (C; N = 8) animals (155 ± 12 mmHg) or rats with regression (R; N = 8) of hypertrophy (144 ± 11 mmHg). In contrast, the estimated s max for C (145 ± 26 kdynes/cm 2 ) and R (133 ± 17 kdynes/ cm 2 ) was higher than for H (110 ± 13 kdynes/cm 2 ). According to Laplaces law, the opposite results of DP max and s max may depend on the increased mass/volume left ventricle ratio of the hypertrophied hearts, which favored pressure generation. These results clearly show that DP max should be used with caution to analyze systolic function. Muscle contractile capacity can be measured by determining maximal force developed during contraction. Maximal intraventricular developed pressure (DP max ) is frequently utilized in isovolumetric left ventricle preparations as an index of myocardial contraction function, assuming that it correctly reflects maximal developed wall force (maximal developed wall stress -s max ) during contraction (1-3). This assumption is based on the fact that ventricular internal radius (R i ) and wall thickness (h) -the variables involved in the ventricular pressure and wall stress interact according to Laplaces law -do not vary during isovolumetric contraction. This seems to be an acceptable postulate when the changes in ventricular function are evaluated in the same heart and R i and h do not vary. However, comparison of DP max of different hearts with different R i and h values is frequently utilized to evaluate myocardial contractile function (1-3), an oversimplification that can distort the interpretation of the results.We have analyzed DP max and s max in isovolumetric left ventricle preparations from control rats (C; N = 8), rats submitted to cardiac hypertrophy (H; N = 10) by the ad-

Myocardial contractility of the isovolumetrically beating isolated rat heart

“…However, in the hypertrophied heart, the contractile state of the LV chamber may differ from that of the sarcomeres because of the change in ventricular geometry, allowing the LV to maintain a normal chamber performance even though the sarcomeres may have a depressed contractile state. On the other hand, the developed stress index, which indicates the myocardial capacity of force generation normalized by wall surface area, would be preferred (15). However, one would argue that even developed stress has inherent drawbacks.…”

“…In contrast, Brower et al (2,3) used peak isovolumetric pressure and the pressure-volume relation in the isolated hearts to measure myocardial contractility in volume overload-induced left ventricular hypertrophy in rats. In addition, maximal rate of pressure rise and stress-strain relationship have also been used by several investigators to assess myocardial contractility in the hypertrophied rat heart (6,7,(15)(16)(17). Both pressure-volume and stress-strain relationships provide a load-insensitive measure of contractile state.…”