A simple method for Emax trend evaluation: in vitro and in vivo results

The aim of this study is the evaluation of end systolic ventricular elastance trend (as a measure of heart contractility) by hemodynamic variables available in intensive care units or during heart surgery: heart rate, cardiac output, left atrial, mean and diastolic arterial pressure. Its basic assumption is the description of ejection as the interaction between variable left ventricular and arterial compliances (reciprocal of the corresponding elastances) connected in parallel. As pressure is the same in each compliance at systole beginning and ending, ventricular elastance can be estimated by assuming that energy variation is the same on both compliances. The algorithm has been tested on a numerical simulator of the circulatory system and on six sheep at basal conditions and during drug infusion. Correlation function in numerical simulation, between true and computed ventricular elastance (range 0.45 divided by 5 mm Hg-cm(-3)), yields 0.985. In vivo comparison between computed ventricular elastance trend and ventricular dp/dt trend yields a correlation function ranging between 0.87 and 0.99. The result of the algorithm cannot be assumed to be Emax value. However, it can be considered a contractility index as it closely follows any change in dp/dt. It can be computed by simple calculations and needs no variables other than those usually measured in intensive care . It allows the extrapolation of useful information for evaluating the trend in heart contractility and for setting up a control strategy for mechanical or pharmacological assistance during heart recovery.