A brief guide to troubleshooting the pacemaker circuit

This topic makes frequent appearances in the past papers. All the pacemaker questions ended up in the Cardiothoracic ICU section, by an arbitrary decision of the site author.

Potential problems with pacing:

Output failure: failure to produce a pacing spike

Failure to capture: pacing spikes do not produce QRS complexes

Undersensing: pacemaker paces asynchronously in spite of the presence of obvious P waves

Oversensing: failure to pace in spite of obvious bradycardia

Cross-talk: failure of DDD pacing; atrial pacing spike is mistaken for a QRS by the ventricular lead

Endless loop tachycardia: atrial lead mistaking ventricular depolarisation for atrial activity

Daily pacemaker maintenance

Care for the epicardial wires:

  • Pacing wires must be dressed at least every 72 hours
  • When handling epicardial pacing wires, gloves should always be worn to prevent microelectrocution
  • Wires not in use should be rolled up in sterile gauze
  • Wires in use should be securely taped to the patient's abdomen

Minimum daily box and wire assessment:

  • Wound site assesment
  • Pacing wire connection check
  • Measure and document the wire length every nursing shift
  • Note position of wires on daily CXR
  • Check the impulse generator battery

Minimum daily paced patient assessment:

  • 12-lead ECG
  • Underlying rhythm and rate check (turn off the pacemaker for a few seconds)
  • Sensitivity check
  • Pacing threshold check
  • Pacing mode review (is it appropriately selected?)

Occasional (non-daily) checks:

  • These are mentioned by the college in Question 2 from the first paper of 2016. According to the model answer, these do not need to be checked on a daily basis.
  • Maximum tracking rate:the maximum atrial rate at which a pacemaker will deliver a ventricular pacing stimulus following each sensed atrial beat; i.e. if the atria are going at 130bpm and the pacemaker's MTR is set at 120, it will not pace any more frequently than 120.
  • AV interval: the interval following a paced or sensed atrial beat allowed before a ventricular pacing impulse is delivered. I.e. how long the pacemaker waits until it decides that the beat was not conducted through the AV node.
  • Post ventricular atrial refractory period (PVARP) is a pacemarker refractory period, intended primarily to prevent sensing of retrograde P waves; it prevents the pacemaker from sending another impulse too close to the last QRS, so as not to produce an R-on-T phenomenon.

How to check the sensitivity threshold

In summary:

  • Put the pacemaker in a VVI, AAI or DDD mode (i.e. endogenous cardiac activity should inhibit the pacemaker.)
  • Set the output as low as possible; you don't want to have any R on T phenomena - you only need to see the pacing spikes.
  • Change the rate to one which is much lower than the patients native rate
  • Increase the sensitivity value until no cardiac activity is sensed
  • Now, keep decreasing the sensitivity until the pacemaker senses every p-wave or QRS interval.
  • This minimal sensitivity value is the sensitivity threshold.
  • most of the time, you tend to leave the sensitivity turned down to half of the sensitivity threshold to ensure that the cardiac electrical activity will be sensed even if the electrode tip overgrows with filth.
  • If you turn the sensitivity value down any more than that, you risk oversensing. Oversensing is described in greater detail elsewhere; briefly, it is an inappropriate inhibition of pacing in response to some sort of trivial non-cardiac signals, like the friendly hum of the nearby microwave.

In detail:

How to find the pacing threshold ("Capture threshold")

In order to find the minimum current required for capture, one can perform a simple manoeuvre:

  • Set the pacemaker well above the native rate, so that the chamber of interest is being paced continuously.
  • Start reducing the output until a QRS complex no longer follows each pacing spike.
  • The output at which there is incomplete capture is the capture threshold.
  • Typically, one might want to set the output to about double capture threshold.

A systematic approach to troubleshooting the temporary pacemaker

Start with the box.

  • Is it even on?
  • Is the battery dying?
  • Are the wires detached from the pulse generator?
  • Are the leads connected?
  • Was the temporary pacing wire pulled out in course of a recent pressure area care?
  • Are the epicardial electrodes displaced? Is the transvenous electrode tip wiggling uselessly in the venticle?
  • Is there any weird twitching in the chest wall muscles of the patient? Is the ventilator demonstrating some bizarre sawtooth pattern, suggesting that the diaphragm is being paced?

Ok, so the hardware is intact. If there is output failure, its not because of the leads or the battery. Move on to the software.

First check the sensing.

  • Put the pacemaker in a VVI, AAI or DDD mode.
  • Change the rate to one which is much lower than the patients native rate.
  • Observe the sense indicator.
  • Keep decreasing the sensitivity (increasing the mV value)
  • Find the maximum value- where the pacemaker is picking up NONE of the endogenous electrical activity.
  • Now keep increasing the sensitivity (decreasing the mV value)
  • Find the sensitivity threshold - where the sensor picks up EVERY endogenous electical event (i.e. no pacing spikes are visible)

The sensitivity setting should be half the sensitivity threshold (i.e the pacemaker should be twice as sensitive as the sensitivity threshold).

This will not take care of oversensing as a cause of pacing failure, but it should defeat undersensing.

Now, check the output threshold.

  • Set the pacemaker well above the native rate.
  • Start reducing the output.
  • Find the capture threshold - where a QRS complex no longer follows each pacing spike.

Crank the output to double the capture threshold.

Still not working?

  • Roll the patient to one side, and then another. Sometimes this influences the position of the transvenous pacing wire tip just enough to get you some capture.
  • Reverse the leads. Sometimes this works, but logically - it shouldnt.
  • Convert to unipolar pacing. Attach the negative lead to the positive electrode, and the negative lead to the subcutaneous tissue of the chest.
  • Give up. Time to pace externally while waiting for another wire to be floated, or the epicardial leads t be resited.

What happens when you put the magnet on?

Generally speaking, most pacemakers will respond to magnet exposure by becoming asynchronous, i.e. they will start pacing without sensing. AICDs will stop defibrillating. This is good to know if you are palliating a patient with an implanted device, and you don't want to have the defib firing randomly in the last few minutes of their life.

Of course, there is no standard among the manufacturers regarding what precisely should happen when the magnet is applied. Medtronic and Boston Scientific models will pace asynchronously; St Jude will cycle through some pre-programmed protocol which typically involves recording and storing an ECG, and Biotronic devices will do something completely random, depending on model and battery life. A good article about these idosyncratic behaviours is available on Medscape.

Specific temporary pacemaker problems

Output failure

Its just what it sounds like.

No output means no output. Somehow, the electrodes are dead- no current is flowing though them. This doesnt present you with any sort of characteristic rhythm to give you a clue. The pacemaker simply fails to work. This might be because there is something wrong with the battery or the leads, or it might be because the pacemaker is oversensing every little twitch of the surrounding muscles (though this would usually be a unipolar problem).

Lets say you have excluded oversensing by fiddling with the sensor threshold, or (less elegantly) by changing to an asynchronous mode.

Now, try increasing the power to maximum (20mA in most atrial modes, 25mA for the ventricles).

If at maximum power the pacemaker still cannot capture, you could try one last thing - attaching the pacing wires directly to the pacemaker, without the lead. This is usually not possible with the transvenous models. In epicardial pacing, this excludes the lead as a source of pacing failure.

Failure to capture

This is easily diagnosed. It is the dissociation between pacing spikes and QRS complexes (or P waves, for that matter).

failure to capture

Failure to capture means either you are using too little current, or your lead is in a stupid position (i.e. not anywhere near the ventricular wall). The latter you can do nothing about.
The former remains within your control.

Thus, the first step is to increase the pacemaker output.

If you are already using the maximum amount of current, there are several possibilities:

  • The wire tips have overgrown with fibrinous filth, and need to be replaced
  • The myocardium is resistant to pacing because of electrolyte derangement, recent defibrillation, or antiarrhytmic drugs interfering in the process.
  • The area of myocardium which the electrode is touching has infarcted and no longer does anything for anybody.

One can try to reverse the polarity of the electrodes in this situation to see if this helps. Alternativly, one can try to convert the bipolar circuit to a unipolar circuit.

Failure to sense - "undersensing"

This is a situation where the pacemaker, instructed to inhibit itself whenever a viable P or QRS comes along, instead paces irresponsibly, ignoring normal cardiac activity. This pacemaker is in essence asynchronous.

The solution to this problem is to reduce the sensor threshold until something happens. If nothing happens, there is something cardinally wrong with the electrode. It may not be possible to rescue this situation with polarity reversal or conversion to unipolar pacing, but one should still try.

Inappropriately high sensitivity - "oversensing"

This is a situation where something other than genuine myocardial electrical activity is mistaken for an inhibitory signal. The pacemaker in AAI VVI or DDD mode will dutifully switch off, while it should in fact be pacing.


This typically looks like a bradycardia. No pacing spikes are visible (or, there are too few of them).

One sometimes sees this in awake patients; when they are exercising their chest muscles (eg. trying to sit up in bed) the pacemaker is eerily silent, suggesting that skeletal muscle contraction is being interpreted as an inhibitory signal.

Alternatively, these potentials can be misintepreted as triggers for pulses by a VAT or DDD pacemaker. The result is a pacing spike each time your muscles twitch. This results in a horrendous sensor-induced tachycardia.

This problem, thankfully, is easily solved. One can increase the sensitivity of the pulse generator; or one can simply switch it to an asynchronous mode.


This is a phenomenon of dual-chamber DDD pacing.

Essentially, the atrial pacing spike is sensed by the ventricular wire, and interpreted as a ventricular contraction. Thus inhibited, the ventricular pacing lead fails to issue a pulse, and the ventricle doesnt get paced.

This is all fine and good if you have intact AV conduction, but - lets face it - you have dual chamber pacing wires, so you probably have some sort of serious problem with your AV conduction. In this case, you will have no ventricular activity and crosstalk will probably kill you rather quickly.


With independent control of atrial and ventricular pacing, one can decrease the sensitivity of the ventricular pacing wire to prevent this from happening.

At the same time, one can reduce the output current of the atrial wire, to make the pacing spike less obvious, and thus less likely to trigger the ventriular sensing.

These days, dual chamber pulse generators have a default "blanking period" which disables sensing in the ventricular lead precisely at the moment of atrial pacing.

Thus, in a modern box, the other possibility is that this is occurring because the ventricular pacing wire has migrated into the atrium.

"Endless loop" tachycardia

This is in some ways the reciprocal of crosstalk. In DDD mode, the atrial pacing wire would intepret the ventricular pacing spike as an atrial depolarisation, and trigger another ventricular impulse, and so on, and so on. LITFL have an excellent example on their pacemaker malfunction page, which is reproduced here with neither their  permission, nor shame nor guilt:
pacemaker mediated tachycardia from LITFL

Alternatively, the retrograde conduction of ventricular depolarisation could fool the atrial lead, presenting itself as native atrial activity. The atrial lead would dutifully discharge the ventricular pulse, and this would also be conducted in a retrograde fashion, and so on and so forth.

Both examples would have no appreciable p-waves, looking like a broad-complex SVT. The tachycardia would be at the maximal rate allowed by the pacemaker.

Again, this is a failure of sensing. One can easily remedy this by changing to an asynchronous mode.


Reade, M. C. "Temporary epicardial pacing after cardiac surgery: a practical review." Anaesthesia 62.3 (2007): 264-271.

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.

Gammage, Michael D. "Temporary cardiac pacing." Heart 83.6 (2000): 715-720.

Sanders, Richard S. "The Pulse Generator." Cardiac Pacing for the Clinician. Springer US, 2008. 47-71.

Kirk, Malcolm. "Basic principles of pacing." All You Wanted to Know (2008): 1.

Hayes, David L., and Paul A. Levine. "Pacemaker timing cycles." Cardiac pacing and ICDs. Blackwell Publishing Malden (MA), 2002. 265-321.

Medtronic have a .pdf operation manual for their model 5388 external pacing pulse generator.

Reade, M. C. "Temporary epicardial pacing after cardiac surgery: a practical review." Anaesthesia 62.3 (2007): 264-271.