Suppression of re-entrant and multifocal ventricular fibrillation by the late sodium current blocker ranolazine

Abstract

Objectives: The purpose of this study was to test the hypothesis that the late Na current blocker ranolazine suppresses re-entrant and multifocal ventricular fibrillation (VF).

Background: VF can be caused by either re-entrant or focal mechanism.

Methods: Simultaneous voltage and intracellular Ca(+)² optical mapping of the left ventricular epicardial surface along with microelectrode recordings was performed in 24 isolated-perfused aged rat hearts. Re-entrant VF was induced by rapid pacing and multifocal VF by exposure to oxidative stress with 0.1 mM hydrogen peroxide (H₂O₂).

Results: Rapid pacing induced sustained VF in 7 of 8 aged rat hearts, characterized by 2 to 4 broad propagating wavefronts. Ranolazine significantly (p < 0.05) reduced the maximum slope of action potential duration restitution curve and converted sustained to nonsustained VF lasting 24 ± 8 s in all 7 hearts. Exposure to H₂O₂ initiated early afterdepolarization (EAD)-mediated triggered activity that led to sustained VF in 8 out of 8 aged hearts. VF was characterized by multiple foci, appearing at an average of 6.8 ± 3.2 every 100 ms, which remained confined to a small area averaging 2.8 ± 0.85 mm² and became extinct after a mean of 43 ± 16 ms. Ranolazine prevented (when given before H₂O₂) and suppressed H₂O₂-mediated EADs by reducing the number of foci, causing VF to terminate in 8 out of 8 hearts. Simulations in 2-dimensional tissue with EAD-mediated multifocal VF showed progressive reduction in the number of foci and VF termination by blocking the late Na current.

Conclusions: Late Na current blockade with ranolazine is effective at suppressing both pacing-induced re-entrant VF and EAD-mediated multifocal VF.

Figure 1. Activation During Pacing-Induced Ventricular Fibrillation

Snapshots of activation during pacing-induced ventricular fibrillation (A) at baseline and (B) after ranolazine (10 μM) perfusion. (A) Multiple (2 to 4) propagating and colliding wavefronts appear to maintain the induced ventricular fibrillation (Online Video 1), requiring electrical shock for termination. (B) However, after ranolazine perfusion and while the ventricular fibrillation could still be induced, it self-terminated. D is a schematic drawing of optical action potential showing the different phases of the action potential in color. E shows snap shots taken just prior to termination of the ventricular fibrillation after ranolazine as indicated by the double-headed arrow in C. SR is sinus rhythm. Dep = depolarization; ECG = electrocardiogram; F = fluorescence; F bar = mean fluorescence; MElec = microelectrode; Rep = repolarization; SR = sinus rhythm.

Figure 2. Initiation of VF in an Aged Rat Heart Exposed to H₂O₂ (0.1 mM) by an EAD-Mediated Triggered Activity

(A) Simultaneous optical action potential (O-AP), calcium transient (O-Cai) pseudo-ECG, and left atrial bipolar electrograms (LA Beg) at the onset of early afterdepolarization (EAD)-mediated ventricular tachycardia (VT)/ventricular fibrillation (VF). (B) Snapshots during the last sinus beat (SR) and the first EAD-mediated triggered beat that arose from the base of the LV. The arrows point to the direction of wave propagation and the numbers under each snapshot are the activation time (ms) with onset time arbitrarily set at 0. The color code is shown in C. (D) Glass microelectrode recording from the base of the heart in the same heart at the onset of another episode of EAD-mediated triggered activity causing the VT. Note the isoelectric interval during the initiation of EAD indicating the absence of electrical activity elsewhere in the heart. (E) Multifocal activation during an episode of VF in the same heart (Online Video 2). Abbreviations as in Figure 1.

Figure 3. Suppression and Prevention of EAD-Mediated Triggered Activity and VF by Ranolazine in Hearts Exposed to 0.1 mM H₂O₂

In all parts, the top recordings are pseudo-ECG and the bottom panels are glass microelectrode recordings. (A) Within 8 min of H₂O₂ perfusion, EADs emerge that then progressively degenerate to triggered beats, causing VT and VF 15 min after H₂O₂. (B) Thirteen min after ranolazine (10 μM) perfusion and in the continued presence of H₂O₂, the VF remains suppressed for the entire 40 min of ranolazine perfusion. However, after ranolazine washout and in the continued presence of H₂O₂, EADs emerged progressively, causing triggered beats and VF 65 min after washout of the drug. (C) Pre-treatment with ranolazine (10 μM) for 30 min before H₂O₂ perfusion in a different heart, preventing the formation of EADs and VF during the entire 60 min of ranolazine perfusion. However, on 40 min of ranolazine washout and in the continued presence of H₂O₂, the EADs emerged progressively, causing VT/VF 40 min after ranolazine washout as shown in D. Abbreviations as in Figures 1 and 2.

Figure 4. Spontaneous VF 15 s After Its Onset and Its Termination With Ranolazine in a Heart Exposed to 0.1 mM H₂O₂

(A) Snapshots of spontaneous VF 15 s after its onset in a heart exposed to 0.1 mM H₂O₂ and (B) its termination with ranolazine (10 μM) in the continuous presence of H₂O₂. Multiple epicardial foci (red) during the VF separated by recovered tissue (blue) is evident. This pattern of VF persisted for more than 3 min as confirmed by periodic optical mapping of the VF. However, 1 min after ranolazine perfusion (B), the number of multiple foci progressively decreased, hastening the termination of VF and resumption of SR. (C) Time course of the reduction in the number of foci after ranolazine perfusion. Abbreviations as in Figures 1 and 2.

Figure 5. Effects of Ranolazine on Maximum Rate of Phase-Zero Action Potential Depolarization (dV/Dt_max) of Phase Zero Action Potential, Conduction Velocity, and Ca_i²⁺ Transient Decline Rate Constant (τ)

(A) Ranolazine causing rate-dependent significant decrease in dV/Dt_max at faster pacing cycle length (PCL) (<130 ms). (B, left) Isochronal activation maps with a color code (right) during pacing from the base at a PCL of 200 ms under different conditions. (C) H₂O₂ prolongs τ, whereas ranolazine (10 μM) has no effect on τ either before or after H₂O₂, as shown by the adjoining superimposed Ca_i²⁺ transients.

Figure 6. Simulation in 2-Dimensional Tissue of Multifocal VF Initiated With H₂O₂-Mediated Ionic Current Changes

(A) Voltage snapshots in the presence of the late component of the fast sodium inward current (I_Na-L) (first frame) with its subsequent block (upward arrow) causing progressive decrease in the number of foci, eventually ending up with a single re-entrant wavefront causing transient VT, followed by termination. (B) Pseudo-ECG showing fibrillation-like state just before block of the I_Na-L with its subsequent block converting the VF to VT (first arrow), followed by tissue quiescence (second arrow). (C) Plot of the number of foci per second versus time for control (black circles) and with I_Na-L blocked (blue circles). Abbreviations as in Figures 1 and 2.