Spatiotemporal Relationship Between Intracellular Ca ²⁺ Dynamics and Wave Fragmentation During Ventricular Fibrillation in Isolated Blood-Perfused Pig Hearts

Abstract

Abstract-Normal "master-slave" relationship between the action potential (AP) and intracellular Ca 2ϩ transient (Ca i T) is sometimes altered during ventricular fibrillation (VF). The nature of AP/Ca i T dissociation during VF and its role in inducing wavebreaks (WBs) remain unclear. We simultaneously mapped AP (RH237) and Ca i T (Rhod-2) during VF in blood-perfused pig hearts. We computed AP and Ca i T dominant frequency (DF) and Ca i T delay in each AP cycle. We identified WBs as singularity points in AP phase movies and sites of conduction block (CB) as sites where an AP wavefront failed to propagate. We analyzed spatiotemporal relationship between abnormal AP/Ca i T sequences and CB sites. We used a calcium chelator (BAPTA-AM) to abolish Ca i T and test its involvement in WB formation. During VF, the DF difference between AP and Ca i T was Ͻ10% of the respective values in 95% of pixels, and 80% of all Ca i T upstrokes occurred during the initial 25% of the excitation cycle. Aberrant sequences of AP and Ca i T occurred almost exclusively near CB sites but could be traced to normal wavefront sequences away from CB sites. Thus, apparent AP/Ca i T dissociation was largely attributable to spatial uncertainty of the absolute position of block of each wave. BAPTA-AM reduced Ca i T amplitude to 30.5Ϯ12.9% of control and the DF of AP from 12.2Ϯ1.6 to 10.4Ϯ1.3Hz (PϽ0.01), but did not significantly alter WB incidence (0.76Ϯ0.19 versus 0.72Ϯ0.19SP/mm 2 ). These results do not support presence of spontaneous, non-voltage-gated Ca i Ts during VF and suggest that AP/Ca i T dissociation is a consequence rather than a cause of wave fragmentation. (Circ Res. 2007;101:e90-e101.) Key Words: ventricular fibrillation Ⅲ pig heart Ⅲ action potential Ⅲ calcium transient Ⅲ wavebreak R ecurrent wave fragmentation, or wavebreak (WB), is a hallmark of ventricular fibrillation (VF). 1,2 Wavebreak occurs when a propagating wavefront encounters an obstacle which may result from electrophysiological heterogeneities, [3][4][5][6] intrinsic repolarization instabilities, 7,8 anatomic structures, 9 or regional ischemia. 10 Repolarization instabilities in the form of action potential duration (APD) alternans are at least in part mediated by beat-to-beat changes in L-type Ca 2ϩ current (I Ca,L ) and Na ϩ -Ca 2ϩ exchanger (NCX). 2,11 These currents are bidirectionally coupled to intracellular Ca 2ϩ (Ca i ) cycling, so that Ca 2ϩ influx via I Ca,L triggers Ca 2ϩ release from the sarcoplasmic reticulum (SR), whereas an increase in Ca i modulates inactivation of I Ca,L and the transsarcolemmal current generated by NCX. 12 SR Ca 2ϩ cycling can exhibit intrinsic dynamics in the form of Ca i transient (Ca i T) alternans (even when the AP waveform is fixed under voltage clamp) 13 and in the form of spontaneous, non-voltage-gated Ca 2ϩ releases. 12 It was hypothesized that such intrinsic dynamics of Ca i T can promote APD fluctuations at high excitation rates and thus contribute to mechanisms of WB during VF. 2,13 To test this hypothesis, Omichi et al 14...

Calcium Dynamics and Ventricular Fibrillation

“…However, after 60 minutes of BAPTA-AM infusion, neither Ca i alternans nor VF was inducible. The only arrhythmia inducible was sustained monomorphic ventricular tachycardia.In the present study, we confirmed that a short duration of BAPTA-AM infusion did not prevent VF in rabbit hearts in vitro, consistent with that reported by Warren et al 1 However, alternans and VF were no longer inducible after 70 minutes of BAPTA-AM infusion. These latter findings confirmed the antifibrillatory effects of BAPTA-AM as reported by Billman and colleagues.…”

A technical review of optical mapping of intracellular calcium within myocardial tissue

“…Little difference in the fundamental rates of transmembrane potential and Ca i 2ϩ transients was observed, and in most of the area that was imaged 80% of all transient upstrokes occurred during the initial 25% of sarcolemmal depolarization, indicating substantial coupling. However, decoupled Ca i 2ϩ transients and sarcolemmal potentials were observed, but exclusively near sites of conduction block, leading to the conclusion that decoupling was a consequence rather than a cause of block (112). Overall, these previous studies demonstrate the unique insights into complex spatiotemporal activity that can be developed using simultaneous imaging of a potentiometric and Ca 2ϩ -sensitive probe.…”

“…Elimination of the motion artifact requires suppression of contraction using electromechanical uncoupling agents (7,28,77) or, less frequently, mechanical constraint (112). Blebbistatin is a popular uncoupling agent for optical mapping, especially when imaging Ca i 2ϩ because it specifically inhibits the myosin ATPase without altering sarcolemmal ion currents or SR Ca 2ϩ kinetics (28).…”

“…5E) distinct relationships between fluorescence maps and each process having significantly different fundamental rates (73). Later, excised blood-perfused swine hearts were studied using simultaneous imaging to determine the relationship between conduction block and Ca i 2ϩ cycling during fibrillation (112). Little difference in the fundamental rates of transmembrane potential and Ca i 2ϩ transients was observed, and in most of the area that was imaged 80% of all transient upstrokes occurred during the initial 25% of sarcolemmal depolarization, indicating substantial coupling.…”

Evidence Against the Role of Intracellular Calcium Dynamics in Ventricular Fibrillation

“…In our recent publication, 1 we argued that intracellular calcium (Ca i ) dynamics is not an essential mechanism of the maintenance of ventricular fibrillation (VF). Our argument was based on 2 major observations: (1) Ca i transient passively followed the optical action potential along the entire wavelet span, except at the very wavelet tip; and (2) chelating Ca i with BAPTA-AM did not change the incidence of wave break (WB) during VF.…”