Assessment of dσ*/dtmax, a Load Independent Index of Contractility, in the Canine

The search for a load-independent index of myocardial contractility has been a focus for nearly 100 years. Nearly all of the parameters developed have yielded insight into cardiac function but their clinical utility has been limited. A new index, dσ*/dt _max, has been proposed to be useful in the clinic. This parameter is expressed as the maximum time rate of change of the pressure normalized circumferential wall stress (σ* = σ _θ /P, where σ _θ is circumferential wall stress and P is pressure) for a thick walled sphere model of the left ventricle (LV). This definition for a contractility index renders dσ*/dt _max dependent only on LV wall volume (V _m) and maximum time rate of change of the ventricular volume, dV/dt _max. The index dσ*/dt _max has been studied in patients with echocardiogram-derived volume, but up until this point its characteristics in canines have remained unknown. Validating this index in the canine will allow for a more intensive and wide-range investigation of the index that is not available with humans. The purpose of this study was to validate dσ*/dt _max as a load-independent measure of contractility in the canine heart with the hope that it was a noninvasive assessment of contractile function. To assess the load independence of dσ*/dt _max, the index was estimated over a range of preloads (end diastolic volume, EDV) during a vena caval occlusion (VCO). The study was conducted in five canines under various pacing modes [right atrial (RA), right ventricular (RV), left ventricular (LV), and biventricular (BV)] at rates of 90 or 100, and 160 bpm. The animals’ ventricular volume measurements were assessed by conductance catheter, calibrated with echocardiography. A 50 Hz filter was applied to the volume signal before differentiation to obtain dV/dt _max. Echocardiography was used to calculate left ventricle mass and V _m. In eight of ten cases, dσ*/dt _max was significantly correlated with decreasing EDV (p < 0.05). There was also a significant correlation between dσ*/dt _max and dP/dt _max. With a strong correlation between the values of dσ*/dt _max, dP/dt _max, and EDV in all five subjects, dσ*/dt _max is not load independent in the canine heart when preload is altered by a VCO. Further evaluation of this index is required to delineate the situations where dσ*/dt _max can be accurately applied.