Optical Transmembrane Potential Measurements During Defibrillation-Strength Shocks in Perfused Rabbit Hearts

Abstract

To study the optical transmembrane potential change (delta F) induced during shocks, optical recordings were obtained in 15 isolated perfused rabbit hearts treated with the potentiometric dye di-4-ANEPPS and diacetyl monoxime. Shock electrodes were sutured on the right and left ventricles. A laser beam 30 microns in diameter was used to optically excite di-4-ANEPPS. Fluorescence from a region 150 microns in diameter was recorded during a shock. In the macroscopic study (six animals), there were nine recording spots that were 3 mm apart between the two shock electrodes. In the microscopic study, there were three recording regions that were 3 mm away from either shock electrode and midway between them, with nine recording spots that were 30 microns (three animals), 100 microns (three animals), and 300 microns (three animals) apart in each region. After 20 S1 stimuli, a 10-ms truncated exponential S2 shock of defibrillation-threshold strength was given during the plateau of the last S1 action potential. In the microscopic study, shocks were also given during diastole, with delta F recordings at the three recording regions. Shocks of both polarities were tested. delta F during the shock was expressed as a percentage of the fluorescence change during the S1 upstroke action potential amplitude (the S1 Fapa), ie, delta F/Fapa%. In the macroscopic study, the magnitudes of delta F/Fapa% from recording spots 1 to 9, numbered from the left to the right ventricular electrodes, were 77 +/- 41%, 46 +/- 32%, 32 +/- 27%, 28 +/- 20%, 37 +/- 25%, 24 +/- 20%, 33 +/- 22%, 37 +/- 25%, and 59 +/- 29%, respectively (P < .05 among the nine spots). Depolarization or hyperpolarization could occur near either shock electrode with both shock polarities, but the magnitude of hyperpolarization was 1.8 +/- 0.9 times that of depolarization at the same recording spot when the shock polarity was reversed (P < .01). In the microscopic study, the change in delta F/Fapa% varied significantly over the microscopic regions examined. The maximum values of delta F/Fapa% for hyperpolarizing shocks during diastole reached only 7 +/- 10% of those for shocks during the plateau (P < .01). During diastole, the time until a new action potential occurred after the beginning of the shock was shorter when the membrane was depolarized (1.1 +/- 0.5 ms) than when it was hyperpolarized (12.8 +/- 9.1 ms, P < .01). Conclusions are as follows: (1) A shock can induce either hyperpolarization or depolarization. (2) Hyperpolarization or depolarization during a shock can occur near either the anodal or cathodal shock electrode. (3) Variation of delta F/Fapa% exists within a microscopic region.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial Changes in the Transmembrane Potential During Extracellular Electric Stimulation

“…For example, our previous studies have indicated that the relationship between the shock potential gradient and the ⌬V m caused by that gradient is much more complicated than the relationship predicted by mathematical models. 2,12,13 The present study uses papillary muscles from the left ventricle, which have a unidirectional fiber orientation. That geometry simplified the complex relationship between the response of the membrane during a shock and fiber orientation for this study.…”

“…The Tyrode's solution was of the following formula (in mmol/L): NaCl 129, CaCl 2 1.8, MgCl 2 1.1, KCl 4.5, Na 2 HPO 4 1, NaHCO 3 20, and glucose 11. The left ventricular anterior papillary muscle, 4.5Ϯ0.4 mm long and 1.5Ϯ0.1 mm wide, was removed and pinned on silicon rubber in the center of a 2ϫ2-cm 2 tissue bath. The base of the papillary muscle faced the right side of the tissue bath, and the apex faced the left side of the bath (Figure 1).…”

“…This is because the cellular excitation and alteration in action potentials that occur after a defibrillation shock are thought to result from the changes in the transmembrane potential caused by the shock. 1,2 Several studies have been performed to obtain such information. Some studies used optical recording techniques to evaluate the ⌬V m caused by the shock field in either isolated myocytes 1,[3][4][5] or perfused hearts.…”

“…2,6 -10 With a few exceptions, 1,[3][4][5]11 the optical recording represents the weighted average of the ⌬V m changes in many cells. 2,7,9 Recordings of the transmembrane potential by a doublebarrel microelectrode can supply information about the ⌬V m caused by the shock from an individual cell within the myocardial syncytium. 12,13 A range of modeling studies completed over the past 15 years suggests that shock-induced ⌬V m can change markedly over the length of an individual cell.…”

“…Either possibility might explain the fact that the sawtooth pattern, while evident in modeling studies assuming resistive discontinuities of intracellular coupling in both monodomain 14 -16 and bidomain 17 representations of tissue structure, has not been observed experimentally. 2,5 For example, a recent study by Gillis et al 5 used optical recording techniques to record the ⌬V m caused by electric shocks in individual cells of monolayers and strands of cultured neonatal rat myocytes. In that study, a local hyperpolarization and depolarization of the sawtooth pattern was not observed at the borders between cells when electric field stimulation was applied across the entire strand of cells.…”