Runaway Pacemaker

Report:

Pacemaker tachycardia 180/min 5

Runaway pacemaker 5

Comment:

Most modern pacemakers would have a ventricular rate-limiting mechanism to prevent this degree of tachycardia. The trace itself does not offer any clues as to the cause: all one can see is large spikes followed by paced QRSs of the expected LBBB/LAD variety.

This is not an endless loop tachycardia (like in Case 13), predicated on retrograde conduction of the paced beats. The upper rate limit would have been set unusually high and some VA block would also be expected. This tachycardia was due to some undefined malfunction in the pulse generator. Sometimes this is due to battery depletion. The patient was treated with adenosine, then xylocaine, then shocked! The ECG below (Fig 19a) shows sinus rhythm with RBBB; the pacemaker spikes are now completely dissociated from any myocardial activity. This is due to the countershocks raising the pacing threshold (or even making the electrode lose contact with the endocardium).

The initial therapy with adenosine for “SVT” is less silly than the subsequent one with xylocaine for “VT”: adenosine could have interrupted the retrograde conduction of the paced beats or (another possible mechanism) pacemaker tracking of an actual SVT. Another possible line of therapy would have been flecainide, a drug known to make patients “unpaceable”.

The first step would, of course, be to place a magnet over the offending unit and try to make it behave as a fixed-rate (asynchronous) one at its pre-programmed rate. Curiously, a magnet was not available in Casualty and they did not think of getting one from the CCU! This was tried later, however, and did not work; all attempts to “interrogate” the pacemaker or alter its settings also failed. The entire unit had to be replaced.