Pacemaker Dysrhythmias

Pacemaker-Mediated Tachycardia

Pacemaker-mediated tachycardia (PMT) is a reentrant tachycardia that is comprised of the pacemaker with retrograde conduction via the AV node. The retrograde P waves from retrograde conduction via the P waves are sensed by the pacemaker, which stimulates it to depolarize the ventricles (leading to retrograde conduction through the AV node). The maximum rate of this reentrant tachycardia is determined by the pacemaker settings (they have upper limits of rate of firing). This tachycardia is not as common as new pacemakers have settings to terminate PMT. PMT can be terminated by breaking the cycle, usually either with adenosine or magnet application.

Sensor-Induced Tachycardia

Sensor-induced tachycardia is when the pacemaker intentionally increases the heart rate to perceived stimuli that should increase heart rate. This occurs as newer pacemakers have functions to allow for increased heart rate to physiologic stimuli that should increase heart rate (ie exercise). However, stimuli that should not increase heart rate may be misinterpreted by the pacemaker, causing it to increase heart rate. Loud noises, fever, limb movements, vibrations, or electrocautery are all examples of stimuli that may induce this. Because pacemakers have an upper rate limit, this tachycardia cannot exceed this (usually 160-180bpm). If this process is continuous, it can be terminated by applying a magnet.

Runaway Pacemaker

Runaway pacemaker is a very dangerous tachycardia in patients with older pacemakers and not commonly seen anymore. It is when the pacemaker fires at extremely high rates (ie 2000bpm!) when the battery gets low. If these capture, there is obviously a significant lack of time for ventricular filling with development of cardiogenic shock. Clinically, this would be very similar to VT except ridiculously higher rates (may be 10x as fast). Although these runs of tachycardia can be terminated with placement of a magnet, the device commonly needs replaced as a long-term solution.

Examples:

Further Reading:

https://litfl.com/pacemaker-malfunction-ecg-library/

https://rebelem.com/pacemaker-basics/

http://www.emdocs.net/ecg-pointers-pacemakers-and-when-they-malfunction/