Mechanisms Underlying Epicardial Radiofrequency Ablation to Suppress Arrhythmogenesis in Experimental Models of Brugada Syndrome

Abstract

Objectives: This study sought to test the hypothesis that elimination of sites of abnormal repolarization, via epicardial RFA, suppresses the electrocardiographic and arrhythmic manifestations of BrS.

Background: Brugada syndrome (BrS) is associated with ventricular tachycardia and ventricular fibrillation leading to sudden cardiac death. Nademanee et al. reported that radiofrequency ablation (RFA) of right ventricular outflow tract epicardium significantly reduced the electrocardiogram and arrhythmic manifestations of BrS. These authors concluded that low-voltage fractionated electrogram activity and late potentials are caused by conduction delay within the right ventricular outflow tract and that the ameliorative effect of RFA is caused by elimination of this substrate. Szel et al. recently demonstrated that the abnormal electrogram activity is associated with repolarization defects rather than depolarization or conduction defects.

Methods: Action potentials (AP), electrograms, and pseudoelectrocardiogram were simultaneously recorded from coronary-perfused canine right ventricular wedge preparations. Two pharmacological models were used to mimic BrS genotype: combination of INa blocker ajmaline (1 to 10 μM) and IK-ATP agonist pinacidil (1 to 5 μM); or combination of Ito agonist NS5806 (4 to 10 μM) and ICa blocker verapamil (0.5 to 2 μM). After stable induction of abnormal electrograms and arrhythmic activity, the preparation was mapped and epicardial RFA was applied.

Results: Fractionated low-voltage electrical activity was observed in right ventricular epicardium but not endocardium as a consequence of heterogeneities in the appearance of the second upstroke of the epicardial AP. Discrete late potentials developed as a result of delay of the second upstroke of the AP and of concealed phase 2 re-entry. Epicardial RFA of these abnormalities normalized Brugada pattern and abolished arrhythmic activity, regardless of the pharmacological model used.

Conclusions: Our results suggest that epicardial RFA exerts its ameliorative effect in the setting of BrS by destroying the cells with the most prominent AP notch, thus eliminating sites of abnormal repolarization and the substrate for ventricular tachycardia ventricular fibrillation.

Keywords: ECG; J-wave syndrome; electrophysiology; sudden cardiac death; ventricular arrhythmias.

Figures

FIGURE 1. Effect of a Combination of Pinacidil and Ajmaline to Induce Arrhythmogenesis and Late Potentials Depends Exclusively on the Magnitude of the Epicardial Action Potential Notch and Consequentional J-Wave at Baseline

(A and B) Each column shows action potentials simultaneously recorded from an endocardial (Endo) and 2 epicardial (Epi1 and Epi2) sites together with a bipolar epicardial electrogram (Bipol. Epi EG) and an electrocardiogram (ECG) recorded accross the bath. (A) Preparation exhibiting a large spike and dome action potential morphology at baseline (control). The provocative agents induce a Brugada syndrome ECG and concealed phase 2 re-entry giving rise to distinct late potentials (Bipol. Epi EG). (B) Preparation exhibiting a relatively small spike and dome action potential morphology at baseline. The provocative agents do not induce a Brugada syndrome ECG, but diminish the J-wave. (C and D) Comparison of epicardial action potential notch arear and J-wave arear of preparations vulnerable (inducible) and nonvulnerable (noninducible) to the induction of Brugada syndrome pattern and arrhythmias. n = 6 for inducible and n = 5 for noninducible preparations. (C) Parameters at baseline. Inducible preparations showed an average 3.9-fold higher action potential notch and 4.3-fold higher J-wave area at baseline, compared with the noninducible ones (inducible vs. noninducible, p = 0.002 and p = 0.024 for notch arear and J-wave arear, respectively). (D) After the addition of provocative agents, inducible preparations showed a pronounced increase (p = 0.004 vs. baseline), whereas noninducible preparations showed a significant decrease (p = 0.017 vs. baseline) in both J-wave and action potential notch area. The provocative agents produced an average 60.5-fold higher notch area and 88.7-fold higher J-wave area in inducible compared with noninducible preparations (inducible vs. noninducible, p < 0.001 and p = 0.004 for notch area and J-wave arear, respectively).

FIGURE 2. Radiofrequency Ablation of Epi Suppresses the Electrocardiographic and Arrhythmic Manifestations of Brugada Syndrome in Coronary-Perfused Canine Right Ventricular Wedge Model Generated Using a Combination of Pinacidil + Ajmaline

Traces are as described in Figure 1. (A) Epi displays pronounced action potential notch at baseline. (B) Addition of pinacidil (2 μM) and ajmaline (3 μM) to the coronary perfusate induces the typical Brugada syndrome phenotype. The bipolar epicardial electrogram (Bip. Epi EG) shows fractionated electrogram activity and a late potential due to the development of concealed phase 2 re-entry. (C) Successful conduction of phase 2 re-entrant beat gives rise to ventricular tachycardia. (D) Recorded 40 min after Epi ablation and withdrawal of the provocative agents. Action potential recordings were obtained from midmyocardial (Mid) and subepicardial (Subepi) layers due to ablation of the epicardial cells. (E) Recorded 10 min after reintroduction of the provocative agents to the perfusate (in the same concentration as before). After Epi ablation, Brugada syndrome phenotype and arrhythmias were no longer inducible. Abbreviations as in Figure 1.

FIGURE 3. The Opposite Effects of Ajmaline to Mask or Accentuate the J-Wave (Jw) Depend on the Basal Level of Ito-Mediated Action Potential Notch

Traces are as described in Figure 1. The bipolar electrograms (Bip.-EG-Epi) were recorded from the epicardium using 3 different low cut filter settings (10 Hz, 30 Hz, and 100 Hz) and 25 0Hz “high cut” filter. When action potential notch was small (A), 10-μM ajmaline produced a decrease in J-wave and action potential notch area (B). The effect was reversible on wash-out (C). However, when the action potential notch was amplified using the Ito agonist NS5806 (D), the same concentration of ajmaline caused a marked accentuation of the JW appearing as an ST-segment elevation (E to G). Fragmented electrogram activity developed progressively as the repolarization defects became more pronounced and heterogeneous (D to F). Pronounced action potential notch (without re-entry) produced delayed potentials in a lower frequency range (D), whereas phase 2 re-entry depicted as “high-frequency” spike (E and F). After 15 min of ajmaline, loss of the action potential dome occurred throughout the preparation, which led to disappearance of the late potentials (G). Subendo/Mid = action potential from the subendocardium/midmyocardium; other abbreviations as in Figure 1.

FIGURE 4. Radiofrequency Ablation of Epi Suppresses the Electrocardiographic and Arrhythmic Manifestations of Brugada Syndrome in Coronary-Perfused Canine Right Ventricle Wedge Model Generated Using a Combination of NS5806 + Verapamil

Traces are as described in Figure 1. (A) Control. (B to D) The addition of NS5806 7 μM and verapamil 2 μM to the perfusate induced pronounced J-waves and phase 2 re-entry depicting as abnormal electrogram activity when concealed, and giving rise to ventricular fibrillation when it succeeds in propagating out. (E and F) Recovery period of the preparation after epicardial ablation. Note the normalization of ST-segment elevation after 70 min. Action potential recordings were obtained from midmyocardial (Mid) and subepicardial layers due to inactivation of the epicardium. (G) With superficially ablated epicardium, the readministration of the provocative agents (in the same concentration as before) did not produce pronounced J-waves or arrhythmic activity. (H) Photograph of wedge preparation after epicardial ablation to a depth of 1 to 2 mm, taken at the end of the experiment. Abbreviations as in Figure 1.

FIGURE 5. Radiofrequency Ablation of Endo Fails to Suppress Brugada Syndrome Phenotype

Traces are as described in Figure 1. (A) Control. (B) Recorded 25 min after addition of 8 mM NS5806 and 2 μM verapamil to the coronary perfusate. Homogeneous delay of the second upstroke of the epicardial action potentials gives rise to a late potential on the bipolar electrogram (EG). (C) Recorded 50 min after addition of provocative agents. Concealed phase 2 re-entry gives rise to a high-frequency late potential in the bipolar electrogram. (D) Recorded 10 seconds later. Successful propagation of phase 2 re-entrant extrasystole initiates polymorphic ventricular tachycardia. (E) Recorded 80 min after Endo ablation and withdrawal of the provocative agents. (F) Recorded 25 min after reintroduction of the provocative agents. Endo ablation failed to exert any beneficial effect: the reintroduction of provocative agents induced pronounced Brugada syndrome phenotype with sustained polymorphic tachycardia. Subendo = subendocardium; other abbreviations as in Figure 1.

FIGURE 6. J-Wave and Maximal Action Potential Notch Area Recorded at Each Step of the Epicardial Ablation Experiments in the 2 Models

Addition of the provocative agents, NS5806 + verapamil (NS + Ver; n + 6) or ajmaline + pinacidil (Ajm + Pin; n + 4), significantly increased, whereas epicardial ablation significantly decreased J-wave area (A) and maximal action potential notch area (B). *p < 0.001 versus control. †p < 0.001 versus pre-ablation. ‡p = 0.027 versus control. §p = 0.017 versus pre-ablation.

FIGURE 7. Tpeak-Tend Interval and Dispersion of Repolarization Recorded at Each Step of Epicardial Ablation Experiments in the 2 Models

(A) Addition of the provocative agents, either NS5806 + verapamil (NS + Ver; n + 6) or pinacidil + ajmaline (Pin + Ajm; n = 4), significantly increased, whereas radiofrequency ablation of epicardium significantly reduced Tpeak-Tend intervals.*p < 0.01 versus control, †p < 0.01 versus pre-ablation. (B) Addition of the provocative agents significantly increased but epicardial ablation significantly reduced both TDR and EDR. Dark colors represent EDR, pale colors represent TDR. ‡p < 0.001 versus control. §p < 0.001 versus pre-ablation. ∥p ≤ 0.015 versus control. ¶p ≤ 0.028 versus pre-ablation. The highest Tpeak-Tend values were associated with the most pronounced delay of the second action potential upstroke giving rise to negative T waves and appeared just before the start of arrhythmic activity. EDR = epicardial dispersion of repolarization; TDR = transmural dispersion of repolarization; Tpeak-Tend = interval between the peak and the end of the T wave.