Abnormal repolarization as the basis for late potentials and fractionated electrograms recorded from epicardium in experimental models of Brugada syndrome

Abstract

Objectives: The aim of this study was to test the hypothesis that late potentials and fractionated electrogram activity are due to delayed depolarization within the anterior aspects of right ventricular (RV) epicardium in experimental models of Brugada syndrome (BrS).

Background: Clinical reports have demonstrated late potentials on signal-averaged electrocardiography (ECG) recorded in patients with BrS. Recent studies report the appearance of late potentials and fractionated activity on bipolar electrograms recorded in the epicardium of the RV outflow tract in patients with BrS.

Methods: Action potential and bipolar electrograms were recorded at epicardial and endocardial sites of coronary-perfused canine RV wedge preparations, together with a pseudo-ECG. The transient outward potassium current agonist NS5806 (5 μM) and the Ca(2+)-channel blocker verapamil (2 μM) were used to pharmacologically mimic the BrS genetic defect.

Results: Fractionated electrical activity was observed in RV epicardium, but not in endocardium, as a consequence of heterogeneities in the appearance of the second upstroke of the epicardial action potential, and discrete high-frequency spikes developed as a result of concealed phase 2 re-entry. In no case did we observe primary conduction delay as the cause of the BrS ECG phenotype or of late potential or fractionated electrogram activity. Quinidine (10 μM) and the phosphodiesterase-3 inhibitors cilostazol (10 μM) and milrinone (2.5 μM) restored electrical homogeneity, thus abolishing all late potentials and fractionated electrical activity.

Conclusions: These data point to an alternative pathophysiological basis for late potentials and fractionated electrical activity recorded in the right ventricle in the setting of BrS. We demonstrate an association of such activity with abnormal repolarization and not with abnormal depolarization or structural abnormalities.

Keywords: cardiac arrhythmias; electrophysiology; pharmacology.

Figure 1. Heterogeneities in the appearance of the epicardial action potential second upstroke gives rise to fractionated epicardial electrogram (EG) activity in the setting of Brugada syndrome (BrS)

Left panel: Shown are right precordial lead recordings, unipolar and bipolar EGs recorded form the right ventricular outflow tract of a BrS patient (from Nademanee et al. (11), with permission). Right panel: ECG, action potentials from endocardium (Endo) and two epicardial (Epi) sites, and a bipolar epicardial EG (Bipolar EG) all simultaneously recorded from a coronary-perfused right ventricular wedge preparation treated with NS5806 (5 μM) and verapamil (2 μM) to induce the Brugada phenotype. Basic cycle length=1000 ms.

Figure 2. Concealed phase 2 reentry as the basis for late potential and fractionated bipolar epicardial (Epi) electrogram (Bipolar EG) activity in an experimental model of Brugada syndrome

Each panel shows (from top to bottom) a Bipolar EG, action potentials recorded from endocardium (Endo) and two Epi sites and an ECG all simultaneously recorded from a coronary-perfused right ventricular wedge preparation exposed to NS5806 (5 μM) and verapamil (2 μM) to induce the Brugada phenotype. Heterogeneous loss of the dome at epicardium caused local re-excitation via a ‘concealed’ phase 2 re-entry mechanism, leading to the development of late potentials and fractionated bipolar epicardial EG activity. No major delays in conduction of the primary beat were ever observed. Each panel shows results from a different preparation. Basic cycle length=1000 ms.

Figure 3

A: Concealed phase 2 reentry gives rise to late potentials and fractionated bipolar electrogram (Bipolar EG) activity recorded from epicardium but not endocardium (Endo) in an experimental model of Brugada syndrome. Each panel shows (from top to bottom) a bipolar epicardial (Epi) EG, action potentials recorded from Endo and two Epi sites and an ECG all simultaneously recorded from a coronary-perfused right ventricular (RV) wedge preparation exposed to NS5806 (5 μM) and verapamil (2 μM) to induce the Brugada phenotype. Heterogeneous loss of the dome at epicardium caused local re-excitation via a concealed phase 2 re-entry mechanism, leading to the development of late potentials and fractionated bipolar epicardial EGs. Basic cycle length = 1000 ms. B: Phase 2 Reentry-induced ventricular fibrillation. All traces were simultaneously recorded from a coronary-perfused RV wedge preparation exposed to NS5806 (5 μM) and verapamil (2 μM). The phase 2 reentrant beat produced a closely coupled extrasystole that precipitated an episode of polymorphic tachycardia.

Figure 4. Alternans of concealed phase 2 reentry gives rise to alternans of late potential and T wave

Traces are as in the previous figures. All traces were simultaneously recorded from a coronary-perfused right ventricular wedge preparation exposed to verapamil (3 μM) and hypothermia (30 °C) to induce a Brugada phenotype. EG: electrogram; Endo: endocardium; Epi: epicardial.

Figure 5. Stimulated premature beat (S2) restores homogeneity of action potentials and abolishes fractionated bipolar epicardial (Epi) electrogram (Bipolar EG) in an experimental model of Brugada syndrome

Traces are as in the previous figures. All traces were simultaneously recorded from a coronary-perfused right ventricular wedge preparation exposed to NS5806 (5 μM) and verapamil (2 μM) to induce a Brugada phenotype. BCL=1000 ms. S1–S2=240 ms. Endo: endocardium.

Figure 6. Effect of quinidine (10 μM) to reverse the repolarization defects responsible for phase 2 reentry and associated late potentials and fractionated epicardial (Epi) electrogram (EG) activity

Traces are as in the previous figures. All traces were simultaneously recorded from a coronary-perfused right ventricular wedge preparation exposed to NS5806 (5 μM) and verapamil (2 μM) to induce a Brugada phenotype. The addition of quinidine (10 μM) to the coronary perfusate reversed the repolarization defects, restored action potential homogeneity, normalized the ECG and abolished phase 2 reentry and associated late potentials on the bipolar electrogram (Bipolar EG). Endo: endocardium

Figure 7. Effect of milrinone (2.5 μM) and cilostazol (10 μM) to reverse repolarization defects responsible for phase 2 reentry and associated late potential and fractionated bipolar epicardial (Epi) electrogram (EG) activity

Traces are as in the previous figures. All traces were simultaneously recorded from a coronary-perfused right ventricular wedge preparation exposed to NS5806 (5 μM) and verapamil (2 μM) to induce a Brugada phenotype. Milrinone and cilostazol reversed the repolarization defects induced by NS5806 and verapamil, thus restoring the Epi action potential dome throughout, normalizing the ECG and abolishing phase 2 reentrant activity and associated late potential and fractionated bipolar Epi EG. Bipolar EG: bipolar electrogram; Endo: endocardium.

Figure 8. Comparison of the late potential and fractionated bipolar electrogram (EG) activity recorded form the epicardial (Epi) surface of the right ventricular outflow tract (RVOT) of a patient with Brugada syndrome (BrS) with similar activity recorded from coronary perfused right ventricular (RV) wedge models of BrS

Left: Bipolar RV Epi and endocardial (Endo) EGs recorded from the RVOT of BrS patient from the study of Nademanee et al. (11). Right: Bipolar EGs recorded from the Epi surface of coronary-perfused canine RV wedge models of BrS. All recordings were obtained from preparations displaying concealed phase 2 reentry. In all cases late potentials and fractionated EG activity was the result of repolarization defects created by a inward shift in the balance currents active during the early phases of the Epi action potential. In no case did we observe primary impulse conduction delays. In both clinical and experimental models, late potentials or fractionated activity was not observed in the Endo EGs.