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