Effects of Antegrade Accessory Pathway Conduction on QRS Terminal Vector in Patients with Preexcitation Syndrome

Background Ventricle preexcitation through accessory pathway changes QRS initial vector, and manifests as delta wave on electrocardiogram (ECG). However, QRS terminal vector can also be affected. Methods A total of 158 patients who had single accessory pathway (AP) with antegrade conduction capacity were included and divided into two groups according to the ECG with or without delta wave. Note that 150 patients had delta wave (overt AP group) on ECG; classical preexcitation syndrome was diagnosed before radiofrequency ablation. Eight patients had no delta wave on ECG (unapparent AP group); preexcitation was induced by transesophageal atrial pacing. ECGs and intracardiac electrogram (IEGM) before and after ablation and during atrioventricular reentrant tachycardia were analyzed. Results (1) In the overt AP group: QRS terminal vector amplitude and polarity changes were observed in all the 150 patients, and were related to AP location and delta wave polarity. (2) In the unapparent AP group: QRS terminal vector changes were found in two out of eight patients, and the initial activation of ventricle myocardium via AP on IEGM was almost simultaneous with the onset of QRS complex on ECG. Conclusions It is not only the QRS initial vector, but also the QRS terminal vector that can be changed by the antegrade accessory pathway conduction in patients with preexcitation syndrome. The change of QRS terminal vector is valuable for the diagnosis of atypical preexcitation.