Good pacemaker gone bad

PACEMAKER SYNDROME:
The symptoms of pacemaker syndrome included dyspnea on exertion, paroxysmal nocturnal dyspnea, orthopnea, hypotension, pre-syncope, and even syncope. Heart failure signs include elevated neck veins, rales, and pedal edema. Physical exam can often reveal cannon A-waves. This sign occurs secondary to ventricular-atrial (V-A) conduction and the contraction of the atria against closed A-V valves. Although relatively uncommon, syncope has been attributed to pacemaker syndrome. Syncope is usually associated with systolic blood pressure declines of greater than 20 mm Hg that can occur with the onset of pacing. Additional symptoms attributed to pacemaker syndrome include easy fatigability, malaise, headache, and the sensation of fullness and pulsations in the head and neck. Pacemaker syndrome is most severe when intact V-A conduction is present⁶. The elevated venous pressures associated with the contraction against closed A-V valves causes a vagal afferent response resulting in peripheral vasodilation and hypotension.

So, it occurs if have retorgrade conduction of the paced ventricular signal, or if there is bidirectional block AND both the atrium and ventricle happen to also have the same rate of contraction!

PMT:
This is re-entry using the pacemaker lead.

So, what happens is that each paced beat always, in every pacemaker in the world, creates a retrograde atrial wave. This atrial wave is ignored by virtue of all pacemakers being programmed with a PVARP (postventricular atrial refractory period). But sometimes the VA conduction time is so slow that it outlasts the PVARP, and therefore reentry occurs.
How quick will the pacemaker go? Well, all pacemakers are programmed with an upper rate limit, so it will not exceed that.
The solution for this that companies have come up with and programmed some pacemakers with, is that the pacemaker, when it sees that it is consistently running at the upper rate limit, will intentionally drop a beat/intentionally fail to pace the ventricle for a beat. This ends the tachycardia :)