Extracellular field required for excitation in three-dimensional anisotropic canine myocardium.

Abstract

It is not known how well potential gradient, current density, and energy correlate with excitation by extracellular stimulation in the in situ heart. Additionally, the influence of fiber orientation and stimulus polarity on the extracellular thresholds for stimulation expressed in terms of these factors has not been assessed. To answer these questions for myocardium in electrical diastole, extracellular excitation thresholds were determined from measurements of stimulus potentials and activation patterns recorded from 120 transmural electrodes in a 35 X 20 X 5-mm region of the right ventricular outflow tract in six open-chest dogs. Extracellular potential gradients, current densities, energies, and their components longitudinal and transverse to the local fiber orientation at each recording site were calculated from the stimulus potentials produced by 3-msec constant-current stimuli. The resulting values in regions directly excited by the stimulus field were compared with the values in regions not directly excited but activated by the spread of wavefronts conducting away from the directly excited region. Magnitudes of 3.66 mA/cm2 for current density, 9.7 microJ/cm3 for energy, and 804 mV/cm for potential gradient yielded minimum misclassifications of 8%, 13%, and 17%, respectively, of sites directly and not directly excited. A linear bivariate combination of the longitudinal (l) and transverse (t) components of the potential gradient yielded 7% misclassification (threshold ratio t/l of 2.88), and linear combination of corresponding current density components yielded 8% misclassification (threshold ratio t/l of 1.04). Anodal and cathodal thresholds were not significantly different (p = 0.39). Potential gradient, current density, and energy strength-duration curves were constructed for pulse durations (D) of 0.2-20 msec. The best fit hyperbolic curve for current density magnitude (Jm) was Jm = 3.97/D + 3.15, where Jm is in mA/cm2, and D is in msec. Thus, for stimulation during electrical diastole 1) both current density magnitude and longitudinal and transverse components of the potential gradient are closely correlated with excitation, 2) the extracellular potential gradient along cardiac cells has a lower threshold than across cells, while current density thresholds along and across cells are similar, 3) anodal and cathodal thresholds are approximately equal for stimuli greater than or equal to 5 mA, and 4) the extracellular potential gradient, current density, and energy excitation thresholds can be expressed by strength-duration equations.

Prolongation of Ventricular Refractoriness by Defibrillation Shocks May be Due to Additional Depolarization of the Action Potential

“…This heart showed the same phenomena characteristic of the remaining three but, in the absence of the voltage gradient 500 msec 11,17,24, and 26 msec, respectively. The shock voltage gradients for traces a through e were 2,5,9,13, and 20 V/cm. respectively.…”

‘The electrical spiral of the heart’: its role in the helical continuum.The hypothesis of the anisotropic conducting matrix

“…This assumption is not consistent with the well-demonstrated structural anisotropy of the heart [45][46][47]. Myocardial fiber orientation affects not only the sequences of activation, and repolarization, but also the gradient field distribution and the stimulation threshold [48][49][50][51].…”

Modelling induction of a rotor in cardiac muscle by perpendicular electric shocks

“…The electric field at the critical point Ecr was lower in the model. However, given the large errors involved in the experimental determination of field strength FRAZIER et al, 1988), the values can be considered comparable. In both the model and experiment, increasing $2 shocks from 100 to 150 V pushed the critical point ycr further from the $2 electrode.…”