A potential pitfall of the modified 12 lead electrocardiogram (Mason–Likar modification) in catheter ablation of idiopathic ventricular arrhythmias originating from the outflow tract

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Abstract

Aims

The Mason–Likar modified electrocardiogram (ML-ECG) can be interchanged with standard 12 lead ECG electrode positions (standard ECG) without affecting the diagnostic interpretation during sinus rhythm, but the morphological differences during ventricular arrhythmias have not been sufficiently evaluated. This study aimed to elucidate the morphological changes in the ML-ECG precordial leads.

Methods and results

In 53 consecutive patients with premature ventricular contractions predicted to originate from the outflow tract (OT-PVCs), the arrhythmias were analysed by those two ECG methods. The OT-PVC origin sites, which were predicted by currently published criteria with the respective ECG methods prior to catheter ablation, were compared with the successful ablation sites. Compared with the standard-ECG, S-waves in the ML-ECG became shallower in leads V1-4 (P < 0.05 in lead V1; P < 0.001 in leads V2-4), and pseudo-R-waves in lead V1 appeared in seven patients. The precordial leads transition zone shifted counter-clockwise in 18 patients in the ML-ECG. In leads I and aVL, the negative deflection amplitudes of the ML-ECG were greater than those of the standard ECG (P < 0.001), and polarity reversals in lead I appeared in 18 patients. The R-wave amplitudes in all ML-ECG inferior leads were greater than those in the standard-ECG leads (all for P < 0.001). Those changes had an effect on the diagnostic indexes for the localization, and the specificity of the criteria for the ML-ECG was poorer than that for the standard-ECG.

Conclusion

Great differences were found between those two ECG methods. Predicting OT-PVC origins by diagnostic criteria with the ML-ECG might result in a misdiagnosis and inefficient ablation.

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