Strong defibrillation shocks induce DeltaV(m) in the intramural layers of LV. During action potential plateau, intramural DeltaV(m) are typically asymmetrical (DeltaV-(m)>DeltaV+(m)) and become globally negative during very strong shocks.

Defibrillation shocks induce complex nonlinear changes of transmembrane potential (DeltaV(m)). To elucidate the ionic mechanisms of nonlinear DeltaV(m), we studied the effects of ionic channel blockers on DeltaV(m) in geometrically defined myocyte cultures. Experiments were carried out in cell strands with widths of 0.2 mm (narrow strands) and 0.8 mm (wide strands) produced using a technique of directed cell growth. Uniform-field shocks were applied across strands during the action potential (AP) plateau, and the distribution of shock-induced DeltaV(m) was measured using an optical mapping technique. Nifedipine and 4-aminopyridine were applied to inhibit the L-type calcium current (I:(Ca)) and the transient outward current (I:(to)), respectively. In control conditions, the distribution of DeltaV(m) across cell strands was highly asymmetrical with a large ratio of negative to positive DeltaV(m) (DeltaV(-)(m)/DeltaV(+)(m)) measured at the opposite strand borders. Application of nifedipine caused a large increase of DeltaV(+)(m) and a decrease of DeltaV(-)(m)/DeltaV(+)(m), indicating involvement of I:(Ca) in the asymmetrical DeltaV(m), likely as a result of the outward flow of I:(Ca) when V(m) exceeded the I:(Ca) reversal potential. DeltaV(-)(m) decreased in the narrow strands but remained unchanged in the wide strands, indicating that the changes of DeltaV(-)(m) were caused by electrotonic interaction with an area of depolarization. 4-Aminopyridine did not change DeltaV(-)(m)/DeltaV(+)(m). These results provide evidence that (1) the asymmetry of shock-induced DeltaV(m) during the AP plateau is due to outward flow of I:(Ca) in the depolarized portions of the strands, (2) I:(to) is not involved in the mechanism of DeltaV(m) asymmetry, and (3) the effects of drugs on DeltaV(m) are modulated by the tissue geometry.

Abstract-Defibrillation shocks induce nonlinear changes of transmembrane potential (⌬V m ) that determine the outcome of defibrillation. As shown earlier, strong shocks applied during action potential plateau cause nonmonotonic negative ⌬V m , where an initial hyperpolarization is followed by V m shift to a more positive level. The biphasic negative ⌬V m can be attributable to (1) an inward ionic current or (2) membrane electroporation. These hypotheses were tested in cell cultures by measuring the effects of ionic channel blockers on ⌬V m and measuring uptake of membrane-impermeable dye. Experiments were performed in cell strands (width Ϸ0.8 mm) produced using a technique of patterned cell growth. Uniform-field shocks were applied during the action potential plateau, and ⌬V m was measured by optical mapping. Shock-induced negative ⌬V m exhibited a biphasic shape starting at a shock strength of Ϸ15 V/cm when estimated peak ⌬V Ϫ m was ϷϪ180 mV; positive ⌬V m remained monophasic. Application of a series of shocks with a strength of 23Ϯ1 V/cm resulted in uptake of membrane-impermeable dye propidium iodide. Dye uptake was restricted to the anodal side of strands with the largest negative ⌬V m , indicating the occurrence of membrane electroporation at these locations. The occurrence of biphasic negative ⌬V m was also paralleled with after-shock elevation of diastolic V m . Inhibition of I f and I K1 currents that are active at large negative potentials by CsCl and BaCl 2 , respectively, did not affect ⌬V m , indicating that these currents were not responsible for biphasic ⌬V m . These results provide evidence that the biphasic shape of ⌬V m at sites of shock-induced hyperpolarization is caused by membrane electroporation. Key Words: defibrillation Ⅲ fluorescent imaging Ⅲ membrane electroporation Ⅲ virtual electrodes Ⅲ secondary sources T he success or failure of defibrillation is determined by the magnitudes and the distribution patterns of shockinduced changes of transmembrane potential (⌬V m ), but the mechanisms governing the ⌬V m dynamics are not well understood. Experiments in cardiac tissue have shown that unlike in mathematical models, shocks produce strongly nonlinear V m responses. Shocks applied in the plateau phase of the action potential (AP) typically produce two basic types of nonlinear ⌬V m . The first type is characterized by a monotonic ⌬V m shape and an asymmetric distribution of ⌬V m magnitude, with the negative ⌬V m being much larger than positive ⌬V m . [1][2][3][4][5][6][7] Stronger shocks induce ⌬V m of the second type, which is characterized by a nonmonotonic behavior of negative ⌬V m when strong hyperpolarization is followed by a positive V m shift. 4,5 In addition, the amplitudes of both positive and negative ⌬V m do not increase proportionally with increasing shock strength but reach saturation levels and then decrease. 1,5 Several studies investigated cellular and ionic mechanisms of nonlinear ⌬V m . It was found that the asymmetry of V m response was reduced by the application of nifedipi...

Abstract-Strong electrical shocks can induce arrhythmias, which might explain why shocks fail to defibrillate. In this work, the localization of arrhythmia source and the relationship with local changes of transmembrane potential (V m ) were determined in geometrically defined cell cultures using optical mapping technique. Uniform-field shocks with strength (E) of 10 to 50 V/cm were applied across cell strands with width of 0.2 and 0.8 mm. The threshold for arrhythmia induction was dependent on the strand width: in the 0.8-and 0.2-mm strands, arrhythmias were induced at EՆ20.6Ϯ1.8 V/cm (nϭ8) and EՆ30.3Ϯ1.8 V/cm (nϭ8), respectively. At the same shock strength, the arrhythmia rate and duration were larger in the wider strands. During shocks that induced arrhythmias, the V m waveforms on the anodal side revealed a positive V m shift that followed the initial large hyperpolarization and postshock elevation of the diastolic V m . These V m changes were absent during failed shocks. To determine the localization of the arrhythmia source, arrhythmias were induced in narrow cell strands containing regions of local expansion. Optical mapping of the first extrabeat with a coupling interval of 315Ϯ60 ms revealed that in the majority of cases (9 out of 13) the source of arrhythmias was localized in the areas of shock-induced hyperpolarization. Thus, (1) induction of postshock arrhythmias, their rate, and their duration strongly depend on the tissue structure; (2) T he dependence of defibrillation success on the shock strength follows a bell-shaped curve whereby the likelihood of defibrillation first increases and then decreases with increasing shock strength. 1 The failure of defibrillation at higher shock intensities may be related to induction of arrhythmias. 2-4 Such postshock arrhythmias were demonstrated and studied in dog, 5,6 pig, 7 and rabbit hearts, 8,9 as well as in isolated cells and cell cultures, 4,10 -14 but important properties of these arrhythmias, such as localization of the arrhythmia source and relationship to shock-induced transmembrane potential (V m ) changes (⌬V m ), remain unknown. Recent studies utilizing the optical mapping technique revealed that the effects of shocks on V m in the heart are highly nonuniform [15][16][17] and strongly dependent on the tissue structure. 18 -21 Shocks produce areas of both positive and negative V m changes in different parts of the heart. Because the effects of shocks on myocardium are due to shock-induced V m changes, it can be suggested that initiation of postshock arrhythmias is also dependent on the tissue structure. From this, 2 questions follow: (1) what is the localization of the arrhythmia source in multicellular cardiac tissue? And, (2) what type of shock-induced ⌬V m cause arrhythmias? If initiation of postshock arrhythmias is restricted to certain areas of the heart and associated with a specific type of ⌬V m , this might help to design defibrillation electrodes in such a way as to minimize detrimental effects of shocks. To address these questions, postsh...

Abstract-Changes in intracellular calcium concentration (⌬Ca i 2ϩ ) induced by electrical shocks may play an important role in defibrillation, but high-resolution ⌬Ca i 2ϩ measurements in a multicellular cardiac tissue and their relationship to corresponding V m changes (⌬V m ) are lacking. Here, we measured shock-induced ⌬Ca i 2ϩ and ⌬V m in geometrically defined myocyte cultures. Cell strands (widthϭ0.8 mm) were double-stained with V m -sensitive dye RH-237 and a low-affinity Ca i 2ϩ -sensitive dye Fluo-4FF. Shocks (EϷ5 to 40 V/cm) were applied during the action potential plateau. Shocks caused transient Ca i 2ϩ decrease at sites of both negative and positive ⌬V m . Similar Ca i 2ϩ changes were observed in an ionic model of adult rat myocytes. Simulations showed that the Ca i 2ϩ decrease at sites of ⌬V Key Words: defibrillation Ⅲ fluorescent imaging Ⅲ membrane potential Ⅲ intracellular calcium C alcium ions play crucial roles in regulation of cardiac excitation and contractility, and they may be an important determinant of the tissue response to defibrillation shocks. The interaction between electrical field and Ca i 2ϩ may affect the outcome of a defibrillation attempt in several ways. First, it was suggested that very strong shocks cause calcium overload, which can lead to abnormal impulse generation, re-induction of rapid arrhythmias, 1 and defibrillation failure. 2 Second, it was reported that relatively weak shocks with an energy below the defibrillation threshold applied during fibrillation can prevent the loss of cardiac contractility often observed after successful defibrillation, so-called pulseless electrical activity syndrome. 3,4 In a related study, it was reported that cardiac contractility is enhanced when shocks are applied during the absolute refractory period. 5 Two alternative mechanisms were proposed to explain these effects: stimulation of intracardiac sympathetic nerves by the shock 4 or an increase of peak Ca i 2ϩ concentration caused by a direct effect of the shock on myocytes. 6 The direct assessment of the mechanisms of shock-Ca i 2ϩ interaction and its role in defibrillation requires measurements of shock-induced Ca i 2ϩ and V m changes with high spatial and temporal resolution. Previously, shock-induced Ca i 2ϩ changes were measured in single myocytes 7,8 and at a single point in whole hearts. 6 No spatially resolved data on shock-induced Ca i 2ϩ changes and colocalized V m changes in multicellular cardiac tissue are currently available. Such data are especially important because of the known complexity of shock-induced ⌬V m in the heart. It is well established that shocks produce highly nonuniform patterns of ⌬V m with areas of positive, negative, or negligible polarizations present in different parts of cardiac tissue, 9,10 suggesting that Ca i 2ϩ changes may also be nonuniform. In addition, shocks produce different types of V m responses that depend on the shock strength and the tissue geometry. With the exception of very weak shocks, shocks applied during the action potential ...