Effects of electroporation on optically recorded transmembrane potential responses to high-intensity electrical shocks

Abstract

The outcome of defibrillation shocks is determined by the nonlinear transmembrane potential (DeltaVm) response induced by a strong external electrical field in cardiac cells. We investigated the contribution of electroporation to DeltaVm transients during high-intensity shocks using optical mapping. Rectangular and ramp stimuli (10-20 ms) of different polarities and intensities were applied to the rabbit heart epicardium during the plateau phase of the action potential (AP). DeltaVm were optically recorded under a custom 6-mm-diameter electrode using a voltage-sensitive dye. A gradual increase of cathodal and well as anodal stimulus strength was associated with 1) saturation and subsequent reduction of DeltaVm; 2) postshock diastolic resting potential (RP) elevation; and 3) postshock AP amplitude (APA) reduction. Weak stimuli induced a monotonic DeltaVm response and did not affect the RP level. Strong shocks produced a nonmonotonic DeltaVm response and caused RP elevation and a reduction of postshock APA. The maximum positive and maximum negative DeltaVm were recorded at 170 +/- 20 mA/cm2 for cathodal stimuli and at 240 +/- 30 mA/cm2 for anodal stimuli, respectively (means +/- SE, n = 8, P = 0.003). RP elevation reached 10% of APA at a stimulus strength of 320 +/- 40 mA/cm2 for both polarities. Strong ramp stimuli (20 ms, 600 mA/cm2) induced a nonmonotonic DeltaVm response, reaching the same largest positive and negative values as for rectangular shocks. The transition from monotonic to nonmonotonic morphology correlates with RP elevation and APA reduction, which is consistent with cell membrane electroporation. Strong shocks resulted in propidium iodide uptake, suggesting sarcolemma electroporation. In conclusion, electroporation is a likely explanation of the saturation and nonmonotonic nature of cellular responses reported for strong electric stimuli.

Electroporation induced by internal defibrillation shock with and without recovery in intact rabbit hearts

“…Shock energy has a significant impact on electroporation (35), so it was important to use clinically relevant shock intensities. Defibrillation threshold (DFT) was first measured in seven MI hearts, since it was expected that DFT would be higher in MI than in control hearts.…”

Atria are more susceptible to electroporation than ventricles: Implications for atrial stunning, shock-induced arrhythmia and defibrillation failure

“…(25) A transparent plastic box filled with Tyrode's solution with a 6 mm round hole was gently pressed against the left or right atria or ventricular free walls to seal the opening. A 16 × 16 photodiode array optical recording system with a sampling rate of 5 kHz was focused on the 11 × 11 mm area of epicardial surface with the hole in the center.…”

“…Electrical field values in the Tyrode's solution near the heart were calculated using the equation E = Jρ as done previously, (25) with ρ (resistivity) of the Tyrode's solution estimated to be 50 cm, the value found by Weidmann for extracellular Tyrode's solution. (29) We did not directly measure field gradient values within the heart tissue because Woods et al (30) have shown that measuring devices, such as plunge electrodes, cause unintended experimental artifacts and we did not attempt to calculate them because virtual electrode polarization, (28) and tissue heterogeneity make it impossible without exceedingly sophisticated modeling using real anatomical measurements for each preparation.…”

“…(21) We did not investigate the amount of transmembrane potential change (ΔV m ) that was caused by the different shock strengths as in our previous study. (25) The absence of a plateau phase and shock-induced motion artifacts in the atria did not allow us to determine if the same strength shock caused more transmembrane polarization in the atria than in the ventricles.…”

“…To estimate the electroporation threshold, many electrophysiological parameters have been used: membrane conductance, depolarization of the cellular membrane during diastolic interval resulting in depression of excitability,(1;10;11;36) reduction of APA (25) and dV/dt max (11) , and elevation of intracellular calcium concentration. (36) Additionally, information about electroporation can be indirectly inferred by staining tissue with fluorescent dyes such as Propidium Iodide (37) or Ethidium bromide,(38) which penetrate cells only through the electropores, and subsequent histological imaging of intracellular space.…”