The following ECG trace was taken from a 68-year-old male, one hour following aortic valve replacement for aortic stenosis.
Atrial and ventricular epicardial pacing wires are in place, and the pacing mode is DDD.
The problem is resolved and normal DDD pacing resumes. One hour later another ECG trace is taken.
Intermittent failure of ventricular capture.
Pacemaker mediated tachycardia.
The dual chamber pacemaker is forming part of a re-entry circuit. A ventricular ectopic has triggered retrograde conduction along the patient’s conducting system. The resulting P-wave has been sensed by the atrial lead of the pacemaker, and this has triggered ventricular pacing. The paced ventricular impulse has triggered retrograde conduction along the patient’s normal conducting system, and the cycle continues.
There are two pacing spikes. DDD mode pacing is clearly detecting an absence of either atrial or ventricular spontaneous activity. Indeed if you look at what happens when it fails to capture, the patient native rhythm looks eerily like asystole. The atrium seems to respond (there is a little bump of atrial contraction after each atrial spike) but there is a failure of AV conduction.
To troubleshoot this problem, one ought to use a systematic approach. LITFL again come to the rescue with an excellent summary of troubleshooting steps.
One could summarise it thus:
c) and d) refer to the phenomenon of "endless loop tachycardia" where the pacemaker triggers its own atrial sensor by having the ventricular lead's signal conducted into the atria by a reentry circuit. These are hilarious. The rate is dictated by the size of the circuit, so the closer the atrial sensing lead to the reentry point, the more rapid the tachycardia. You can usually terminate these by putting a magnet on the pacemaker (forcing it to pace asychronously, totally ignoring the atria).
How do you recognise this? Well, the rate is going to be about 125-50 (judging by the rhythm strip) so one can assume that if the pacemaker seems to be initiating it, it must be malfunctioning in some way.
Reade, M. C. "Temporary epicardial pacing after cardiac surgery: a practical review: Part 2: Selection of epicardial pacing modes and troubleshooting."ANAESTHESIA-LONDON- 62.4 (2007): 364.
FURMAN, SEYMOUR, and JOHN D. FISHER. "Endless loop tachycardia in an AV universal (DDD) pacemaker." Pacing and Clinical Electrophysiology 5.4 (1982): 486-489.