Electromagnetic Interference and Pacemakers/ICD’s (Implantable Cardiovertor Defibrillators)
fter my post on Pacemakers/ICD’s and Airports
, I received a question asking about the science behind Electromagnetic Interference and Pacemakers or ICD’s. The two don’t tend to mix very well and with additional stories in the news suggesting Ipads
and Swimming pools
could affect the functioning of your device… It seems like a good time to explain the interaction and why it is an issue.
The most common problem lies with the sensing function of the pacemaker
. If we recap – sensing is the eyes of the pacemaker that ‘see’ the heart beat. If it see’s a heart beat it will sit there happy in the knowledge everything is going OK. I used a fence analogy to show how we eliminate day to day interference and muscular interference (we call this artefact) from the pacemakers decision making.
Below is a picture representation of what the pacemaker ‘Sees’ in technical circles we refer to this as an Electrogram (Egram). The bigger the deflection form the ‘baseline’ (Isoelectric point) the greater the electrical signal that has been ‘seen’ by the pacemaker. The pacemaker does not mind if the deflection is above the baseline (electrical energy moving towards the sensor) or below the baseline (electrical energy moving away from the sensor), it is only concerned with the size of the deflection. For that reason, the fence is in place both above and below the baseline. I have circled a few natural heart beats that are being seen by the device in green, we know the device is picking these up appropriately as they have been categorised as a VS (ventricular sense) by the device – I have circled this in heart colour.
You can see that there is some interference (and T waves for the smarty pants out there) and in the image below I have drawn on the sensing ‘fences’. The device ignores anything that doesn’t come above/below these fences and as a result they have eliminated the interference (artefact).
In this instance the sensitivity setting of the pacemaker (Fence) is set up correctly and appropriately eliminating the electrical interference. In the example below the interference is large and at times greater in size than the ‘sensitivity setting’ and is there for registered by the pacemaker (circled in green).
In these electrograms I would guess that the interference present is muscle artifact (electrical energy given off when muscles other than the heart are contracting) and the pacemaker thinks that this is heart beats.
Electromagnetic Interference (EMI) and Pacemakers
This may come as a surprise to you, but our bodies are also giant aerials that conduct external electrical energy (if only we could get Sky TV). When we are close to devices that give off a lot of Electromagnetic Interference (EMI) we are conducting a lot of this EMI and this can be picked up by the pacemaker. It has exactly the same potential to confuse the pacemaker as muscle interference but is normally more uniform and can be seen in the image below.
We know that when the pacemaker ‘see’s something’ it doesn’t initiate a heart beat, it is happy to sit there and watch. Now in this instance given earlier no harm was done because behind all the muscle interference there was a nice heart rate working well. However what would have happened if the patient had a very slow or no natural heart rate like the one below? but was being masked by electrical interference?
The sensing would see the interference and assume it was a heart beat. The device would then NOT pace… the patients heart would not be beating and they would feel dizzy, faint or worse. Not the time to be using a chainsaw :/
This is the primary reason why pacemakers and Electrical Interference do not really mix, it could stop the pacemaker from making the heart beat when it needs to – we call this inhibit pacing (something stopping the device from pacing).
The interaction between EMI and ICD’s
has all the concerns from the pacemaker side of things (if the patient requires the pacing feature of the ICD) and the added risk of inappropriate therapy.
Inappropriate therapy will at some point get its own post but in its simplest terms it is treatment delivered by the ICD (a shock or ATP)
that is actually not needed. That person is not in a potentially fatal arrhythmia
and therefor receives an unnecessary and painful electric shock.
This is explained better by the image below that shows the pacemaker getting confused by the interference and then eventually deciding that the heart is in Ventricular Tachycardia (VT Detected – circled) when it isn’t. In some cases, this can be enough to initiate a 35J shock being delivered to the patient, again… not what you want whilst you are using a chainsaw!
So there we have it, this is how EMI can affect pacemakers and ICD’s and this is why we like to exercise caution. Now obviously this isn’t something you need to be overly worried about, but it is something you should be aware of. If you are using equipment that you are unsure of then you should check with your pacing centre if it is ok for you to use.
ALSO!!! JUST INCASE YOU DO USE A CHAINSAW! – Check with your technician, doctor or other pacemaker/ICD professional first!
A complete explanation of pacemakers, EMI and more is available in the book Pacemakers Made Easy by Carl Robinson.
Time to work on my app!
Thanks for Reading,
Image courtesy of foto76 / FreeDigitalPhotos.net