A Brief Guide to Troubleshooting the Pacemaker Circuit
A more detailed summary of pacemaker troubleshooting, complete with rhythm strips, is available in a dedicated chapter within the section concerning mechanical haemodynamic support. This topic makes frequent appearances in the past papers:
- Question 2 from the first paper of 2016
- Question 29 from the second paper of 2013
- Question 28 from the first paper of 2013
- Question 8.2 from the second paper of 2010
- Question 10.1 from the second paper of 2009
- Question 3 from the second paper of 2004
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
- 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.
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 sensor threshold.
- 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 increasing the sensitivity.
- Find the sensitivity maximum - where the pacemaker is picking up NONE of the endogenous electrical activity.
- Now keep decreasing the sensitivity.
- Find the sensor threshold - where the sensor picks up EVERY endogenous electical event (i.e. no pacing spikes are visible)
Crank the sensitivity setting up to double the sensor threshold.
This should take care of oversensing as a cause of pacing failure.
Alternatively, as the college recommend in Question 2 from the first paper of 2016, you can simply set the pacemaker to VOO mode. No sensing = no oversensing.
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 to 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.