August 28, 2018

Interesting Pacemaker Behavior

A 30 yo with history of congenital heart disease, pulmonary HTN, heart failure with reduce ejection fraction, s/p ICD is admitted s/p cardiac arrest.

Cardiac monitoring showed something in Image # 1. Is this a PM malfunction?

Image # 1 – ECG case

Upper Rate Behavior 

Image # 2 – ECG case with P waves marked

The underlying rhythm is sinus tachycardia vs atrial tachycardia with atrial rate of about 136-150 bpm (sinus P waves marked with blue and green arrows). The P waves with red arrows are atrial paced complexes. The P waves with green arrows are tracked by the pacemaker (PM) with an AV interval of 160 ms. The P waves with blue arrows are not tracked by the PM and not followed by a ventricular paced beat. The P waves (blue arrows) are ignored because those P waves are in the postventricular atrial refractory period (PVARP). As, the atrial rate increases, the P waves will eventually fall in the PVARP zone. This is one of the manifestations of upper rate behavior of pacemakers.

The Pacemaker Timing Cycles

Image # 3 - Pacemaker Timing Cycles and Refractory Period (AP – atrial pacing, AS – atrial sense, VP – ventricular pacing, VS – ventricular sensing, APC – atrial premature complex, AV – atrioventricular interval, VA – ventriculoatrial interval, LRI – lower rate interval, TARP – total atrial refractory period, PVARP – postventricular atrial refractory period, VRP – ventricular refractory period, URI – upper rate interval.

Image #3 is a screenshot from Ellenbogen K et al,2011, Cardiac Pacing, Defibrillation and Resynchronization Therapy,4th ed. Pacemakers (esp dual chamber pacemakers), have programmable parameters like LRI, URI, VA interval and PVARP. The “lowest rate limit that the pacemaker will allow” is the lowest rate limit which is expressed in “pulse per minute” (ppm) or if expressed in time (millisecond) will correspond to lowest rate interval (LRI). The “highest rate that the pacemaker will allow the heart to be paced” is the upper rate limit or if expresses in time is the upper rate interval (URI). The VA interval (also called atrial escape interval) is the period of time from a V-paced or V-sensed event that the PM will wait until it will pace the atrium.  
Also, like the biologic conducting system, the pacemaker has refractory periods. One of which is the PVARP.  PVARP is a programmable parameter which prevents a P wave shortly after a R wave from being sensed or a P wave occurring during a PVARP is "ignored". This will prevent the phenomenon called pacemaker mediated tachycardia (PMT). 

PMT is an undesired rapid pacing due to the pacemaker or interaction of the pacemaker with the patient. The classic example of PMT is endless loop tachycardia (Image # 4) where PMT is trigerred by a premature ventricular complex with retrograde conduction and atrial capture. This retrograde P wave will be sensed/tracked by the pacemaker creating the tachycardia cycle.

Image # 4 - Schematic Representation of PMT

Another common cause of rapid pacing is an atrial arrhythmia like atrial fibrillation, atrial flutter or ectopic atrial tachycardia. The pacemaker tracks the atrium to the upper rate limit which causes rapid pacing and may be undesirable. This is not a pacemaker malfunction.

Back to the case...

Going back to the case, at one point in the hospital stay of the patient, it was noted that the heart rate was about 140 bpm (Image # 5).

Image # 5 - V-pacing at 140 bmp, A-sensed

So, the PVARP was increased to “break” the PMT. The major limitation of a long PVARP is that it limits the upper tracking rate. As, the atrial rate increases, at one point the P wave (blue arrows) will fall on the PVARP (black box in Image # 6) which the pacemaker will ignore. This will look like a dropped P wave or the classic AV block we typically see.

Image # 6 - The ECG case with PVARP marked with a black box
So, what we are seeing on the ECG case is an upper rate tracking behavior of the pacemaker. 

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