title: Assessment of dsigma*/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.
authors.affiliation: Therapy Delivery, Medtronic, Minneapolis, MN, USA.
authors.forename: Adam
authors.initials: A
authors.lastname: Black
authors.forename: Nathan
authors.initials: N
authors.lastname: Grenz
authors.forename: Schaible
authors.initials: S
authors.lastname: Niccole
authors.forename: Peter
authors.initials: P
authors.lastname: Arndt
authors.forename: Jordan
authors.initials: J
authors.lastname: Lucht
authors.forename: Kellen
authors.initials: K
authors.lastname: Nesvig
authors.forename: Dan
authors.initials: D
authors.lastname: Ewert
authors.forename: Lawrence
authors.initials: L
authors.lastname: Mulligan
mesh_terms: D000818:Animals; D001794:Blood Pressure / Q000502:physiology*; D003198:Computer Simulation; D004285:Dogs; D006339:Heart Rate / Q000502:physiology*; D008955:Models, Cardiovascular; D009200:Myocardial Contraction / Q000502:physiology*; D013318:Stroke Volume / Q000502:physiology*; D016277:Ventricular Function, Left / Q000502:physiology*