Despite significant improvements in clinical care over the past decades, patients with Tetralogy of Fallot (TOF) remain at increased risk for ventricular arrhythmias and sudden cardiac death. While for many years the surgical scars introduced during repair operation were assumed to be responsible for ventricular arrhythmias, more recent data gives evidence of pro-arrhythmic perturbations originating from right ventricular (RV) myocardium unaffected by surgical interventions. The aim of this study was therefore to investigate possible electrophysiological remodelling of the action potential (AP) and changes in arrhythmia susceptibility in RV myocardium of repaired and unrepaired TOF patients, compared to patients with a less severe congenital heart defect, the atrial septal defect (ASD).

During operative repair or re-operation, RV outflow tract (RVOT) myectomies from 22 TOF patients (age 5 months – 56 years, median 11 months) and RVOT biopsies from 3 ASD patients (age 10 months – 10 years, median 10 months) were collected intraoperatively. The study has been approved by the Ethics Committee of the Albert-Ludwigs University Freiburg, and informed consent was given by all patients or their legal guardian. Intracellular AP were recorded from the live tissue samples at physiological temperature with different stimulation frequencies (0.5 – 4 Hz), using the sharp electrode technique. Thereafter, arrhythmias were provoked by perfusion first with a solution containing reduced potassium (from 4.7 to 2.0 mM) and 60 µM barium chloride, and then with a solution containing 1 µM isoprenaline. Association of electrophysiological characteristics (resting membrane potential, maximum upstroke velocity, AP amplitude and duration [APD] at 20%, 50%, and 90% repolarisation, area under the curve [AUC] at 90% repolarisation, occurrence of arrhythmias) and clinical parameters was analysed by a mixed linear effects model and binomial regression.

Arrhythmias in the form of early afterdepolarisations, stimulation-independent extrasystoles and couplets, as well as APD alternans and/or impaired APD restitution manifested frequently (17/22 patients) in TOF tissue, while they were rare or absent in ASD samples. Longer APD and larger AUC were positively associated with higher disease severity, i.e. patients requiring re-operation, TOF patients, prolonged QRS duration, proBNP elevation, pre-operative ß-blocker treatment, normal and severely elevated (but not moderate) echocardiographic RV to main pulmonary artery (PA) pressure gradient, and occurrence of tissue arrhythmias. In contrast, shorter AP with smaller amplitude and slower AP upstroke were positively related to pre-operative cyanosis and moderate echocardiographic RV to PA gradient. Age, and occurrence of clinical arrhythmias, which were of supraventricular or junctional but not ventricular origin in our patients, did not show relevant association with specific AP changes.

In conclusion, we were able to identify two distinct AP morphologies in our patients: severely diseased acyanotic patients with a more “heart failure-like” phenotype and arrhythmic tissue showed longer and larger AP, while cyanotic patients and those with moderate RVOT obstruction presented smaller and shorter AP. TOF patients exhibited relevant arrhythmic activity in RVOT tissue already at a very young age and before clinical manifestation of ventricular arrhythmias, calling for further investigations into possible underlying mechanisms.