Estimation of maximum left ventricular inotropic response from changes in isovolumic indices of contractility in the dog

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Abstract
The effects of catecholamine infusion on three isovolumic indices of left ventricular (LV) contractility were studied under “steady-state” conditions in open-chest dogs with left atrial pressure held constant. The indices studied were (dP/dt)max; (dP/dt)DP40 where DP40 = developed LV pressure (LVP) of 5.3 kPa (40 mmHg) and (dP/dt/TP)max, where TP = total LVP above atmospheric. There was a curvilinear relationship between dose of noradrenaline and the rise of each index, but the magnitude of the rise differed considerably. When heart rate was allowed to rise (dP/dt)max rose to about 800% of the basal value, whilst the other two indices both increased to about 300% of basal. The reason for the difference appeared to be related to the earlier timing of (dP/dt)DP40 and (dP/dt/TP)max. When heart rate was controlled (dP/dt)max increased to about 500% of basal, similar to findings reported by others in the maximum rate of isometric force development in isolated myocardium. During noradrenaline infusion (dP/dt)max always occurred before aortic valve opening and provided a satisfactory measure of inotropic change whilst the other indices apparently occurred too early in systole to reflect full ventricular “activation”. However, during isoprenaline infusion blood pressure fell and estimates of inotropic reserve using (dP/dt)max were 40 to 100% in error due to early aortic valve opening until blood pressure was raised using methoxamine.