An evaluation of derived aortic flow parameters as indices of myocardial contractility in rats

Potential indices of myocardial contractility derived from aortic blood flow, measured by means of electromagnetic flowmetry, were evaluated in rats under different ventricular loading conditions and were compared with the familiar contractility index dP/dt max. Aortic flow acceleration (dF/dt max) appeared to increase in a direct relationship to dP/dt max following inotropic stimulation by either dobutamine or isoprenaline. Since the former drug hardly influenced cardiac frequency, the increase in both variables could be attributed to a positive inotropic action. Also, afterload reduction by both drugs as a possible factor for increase in dF/dt max might be excluded since reduction in afterload by prazosin did not influence this variable, although dP/dt max was slightly reduced. However, dF/dt max was significantly decreased when afterload was markedly increased by methoxamine, whereas dP/dt max was slightly augmented. Therefore, this variable fails to satisfy the criterium of insensitivity to a certain level of aortic pressure rise. It is concluded that dF/dt max can serve as contractility index in rats, provided the influence of afterload conditions on this variable are taken into account.