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Background Syncope in a patient with a pacemaker is a serious event requiring urgent action to ascertain its cause. Around 5% of cases are due to a pacemaker system malfunction. Case summary An 82-year-old man underwent dual-chamber permanent pacemaker implantation due to intermittent high-degree atrioventricular block (AVB) in sinus rhythm. Nine months later, the patient reported episodes of syncope. The chest X-ray showed both leads to be at their expected positions. The electrocardiography (ECG) showed common atrial flutter. Ventricular capture during pacing in atrial demand pacing (AAI) mode confirmed cross-stimulation due to the switching of the atrial and ventricular leads at the pacemaker header. Discussion Cross-stimulation is a rare possibility in a differential diagnosis of causes of syncope. The diagnosis is frequently made during the procedure or a few hours later. The lack of symptoms during 9 months in this case was likely due to the patient having normal sinus rhythm with preserved AV conduction most of the time, as well as ventricular capture from the atrial lead related to non-sensed P waves. When atrial arrhythmias occurred, the sensing of the F waves inhibited ventricular pacing. In order to avoid this complication, in patients with intermittent bradycardia, pacing at a slightly higher heart rate during implantation of the device should be recommended to see the chamber paced with the surface ECG connected to the device interrogator. The ECG and electrogram (EGM) should correlate during device interrogation in order to identify this complication.)

Background: Determining the mechanism of supraventricular tachycardias with prolongedP ventriculoatrial (VA) intervals is sometimes a challenge. Our objective is to analyse the determinants, time course and diagnostic accuracy (atypical atrioventricular nodal reentrant tachycardias [AVNRT] versus orthodromic reentrant tachycardias through an accessory pathway [ORT]) of spontaneous VA intervals variation in patients with narrow QRS tachycardias and prolonged VA. Methods: A total of 156 induced tachycardias were studied (44 with atypical AVNRT and 112 with ORT). Two sets of 10 measurements were performed for each patient—after tachycardia induction and one minute later. VA and VV intervals were determined. Results: The difference between the longest and the shortest VA interval (Dif-VA) correlates significantly with the diagnosis of atypical AVNRT (C coefficient = 0.95 and 0.85 after induction and at one minute, respectively; p < 0.001). A Dif-VA ≥ 15 ms presents a sensitivity and specificity for atypical AVNRT of 50% and 99%, respectively after induction, and of 27% and 100% one minute later. We found a robust and significant correlation between the fluctuations of VV and VA intervals in atypical AVNRTs (Coefficient Rho: 0.56 and 0.76, after induction and at one minute, respectively; p < 0.001 for both) but not in ORTs. Conclusions: The analysis of VA interval variability after induction and one minute later correctly discriminates atypical AVNRT from ORT in almost all cases.