Question 28

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.


  • What problem is demonstrated?
  • Outline the steps that you would take to address the problem.

The problem is resolved and normal DDD pacing resumes. One hour later another ECG trace is taken.


  • What new problem is demonstrated?
  • Briefly outline the underlying pathophysiological mechanism.

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College Answer


Intermittent failure of ventricular capture.


  • Increase the ventricular output
  • Check the connections to the pacemaker and pacing connector leads
  • Reverse the polarity of the pacing to the ventricle
  • Replace pacemaker box and/or pacing connector leads
  • Unipolar pacing with cutaneous pacing stitch
  • Pharmacological therapy eg isoprenaline
  • Alternative pacing method – transcutaneous, transvenous
  • Open chest and replace epicardial wires


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:

  • Put external pacing pads on the patient. You want an alternative pacing method to be immediately available if everything goes tits-up.
  • Check the box.
    • Is the battery running out? (Typically, a pacing box can last 40 days or so on one 9V alkaline battery)
  • Check the circuit
    • is the patient still "plugged in" to the pacing box?
  • Check the pacing wires
    • Is the CXR available to check whether their position has changed?
    • Are the wires attached properly?
  • Check the patient
    • Correct electrolytes
  • Increase the ventricular output
    • Max is about 25mA
  • Reverse the polarity of the leads
    • this might improve the situation - for some reason the negative electrode develops fibrosis first. Swapping them around can sometimes improve conduction.
  • Connect the previously positive epicardial lead to the negative TPM cable, and use an external pacing stitch as the positive lead. This way, you use the (hopefully) still fresh lead to pace the patient in a "unipolar" fashion.
  • Start isoprenaline. Hopefully this makes the myocardium more responsive to pacing, or at least the native rate might improve.
  • Pace externally or transvenously - float a temporary pacing wire into the patient; abandon the epicardial leads as useless.
  • Open the chest and resite the epicardial wires - this is an extreme solution.

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.