Commentary
The ECG shows sinus tachycardia at a heart rate of 100 bpm. There are atrial sensed ventricular paced complexes with a sensed atrioventricular delay of 120 ms. There is no pacing spike or QRS complex after the 12th P wave in the rhythm strip lead II (arrow), raising the possibility of atrial undersensing. The following beat is a paced P wave at the lower rate of 60 bpm from the last tracked P wave, confirming that the preceding P wave was not sensed. There is subsequent resumption of atrial sensed ventricular paced complexes. Another point of concern is the RBBB morphology of paced QRS complexes. This contrasts with the expected LBBB morphology of paced QRS complexes when the lead is implanted inside the right ventricle. Although RBBB morphology of paced beats may point towards left ventricular pacing due to fallacious lead placement, it may still be seen in up to 8% of patients with uncomplicated RV pacing.1 Right ventricular pacing can be identified in patients with RBBB pattern on pacing, by the presence of left superior axis deviation and precordial transition at V3, with good sensitivity and specificity.1 One hypothesis suggests that portions of interventricular septum that are anatomically RV may behave functionally and electrically as LV and thus activating LV first.2Another possible explanation is early penetration of the electrical impulse to LV and RV activation delay due to baseline disease of RV conduction system,3 which may be the reason in our case. Appropriate lead positions were confirmed with fluoroscopy, echocardiography and a CT scan. Further, both the leads had normal sensing and pacing parameters.
Peculiarly, under sensing of the P wave was noticed on telemetry to be happening at regular intervals of around 90 seconds. The answer lied in understanding the pacemaker algorithms designed to detect atrial tachyarrhythmias. Multiple algorithms (Table 1) effect an appropriate pacemaker mode switch during episodes of atrial tachyarrhythmias to prevent tracking of high atrial rates. Atrial tachycardia with 2:1 atrioventricular conduction poses a special challenge for tachycardia detection and mode switch. Due to alternate tachycardia P waves lying in the post ventricular atrial blanking period (Figure 2A), the pacemaker is unable to distinguish this from 1:1 conduction of sinus tachycardia.
Blanked flutter search is an algorithm designed to unmask atrial tachyarrhythmia in such a situation. The algorithm is activated when (1) cycle lengths of eight consecutive tracked atrial sensed events are less than twice the sum of atrioventricular delay and post-ventricular atrial blanking period, and (2) twice the sensed atrial rate is higher than the programmed tachycardia detection rate.4 The algorithm then prolongs the post-ventricular atrial refractory period for next cycle, so that the next atrial event lies in refractory period and is not tracked. During an episode of atrial tachyarrhythmia with 2:1 conduction, the formerly blanked atrial activation will now no longer coincide with a blanking period, thereby unmasking atrial arrhythmia and initiating mode switch. If no atrial tachyarrhythmia is detected by the pacemaker, normal pacing resumes (Figure 2B). The algorithm checks again for atrial tachyarrhythmia after a set interval. The algorithm however, may operate during sinus tachycardia, leading to undersensing of a P wave at regular intervals, as in the present case. Another disadvantage is that mode switch can be triggered erroneously if an atrial premature beat occurs by chance during this period.4 Thus, for interpreting abnormal electrocardiograms after pacemaker implant, knowledge of various pacemaker algorithms is essential to avoid misinterpretation as pacemaker malfunction.
Acknowledgements None