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

Abstract

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, dsigma*/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 (sigma* = sigma ( theta )/P, where sigma ( theta ) 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 dsigma*/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 dsigma*/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 dsigma*/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 dsigma*/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, dsigma*/dt (max) was significantly correlated with decreasing EDV (p < 0.05). There was also a significant correlation between dsigma*/dt (max) and dP/dt (max). With a strong correlation between the values of dsigma*/dt (max), dP/dt (max), and EDV in all five subjects, dsigma*/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 dsigma*/dt (max) can be accurately applied.

Extent of load-independence of pressure-normalized stress in swine

“…The variations in preload and afterload in the present study were equal to or significantly greater than those reported in the literature. Reports of preload increase from 6% to 24% by saline infusion and preload decrease from 10% to 32% by IVCO have been made 14,34,36,38,39 as compared to 49% and 30% in our study, respectively. Similarly, afterload increases varied from 16% to 70% by aorta occlusion (as compared to 70% in this study), while afterload decreases varied from 13% to 29% by nitroprusside or nitroglycerin (NTG) in the literature.…”

“…Black et al 34 reported a preload-dependence of d*/ dt max in canines based on IVCO. This disagreement is likely multi-factorial.…”

“…Similarly, afterload increases varied from 16% to 70% by aorta occlusion (as compared to 70% in this study), while afterload decreases varied from 13% to 29% by nitroprusside or nitroglycerin (NTG) in the literature. 14,34,36,38,39 Although the loading conditions were varied very widely, the load-independence was found for a physiologically significant subset of the range (as listed in Tables 1–4).…”

Intra-left ventricular blood flow analysis to determine cardiac performance and dysfunction indices

“…The detailed study is shown in the Appendix. However, its loading dependence is still debatable [60]. The heart can also be viewed as a mechatronic organ.…”