What is Pacemaker Sensing? How does Pacemaker Sensing Work?

Sensing see's the hearts beats

What is the meaning of life? Well for us humans does any body truly know? Religions and cultures disagree, Larry down the pub thinks its Whisky and my nephew thinks its crying and milk.

Pacemakers don’t have this issue they have one fundamental purpose of life that comes before all others.
If the heart isn’t beating when it should be…. Make it!
Or for those philosophers out there an old Chinese Proverb,
Pace and the heart will beat with you, don’t pace when it beats on its own”.

So what does all this mean and when am I going to get to the point!?
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. If it doesn’t see a heart beat it will pace (make the heart beat with an electrical impulse).
How does it do this? Well the lead/s in your heart aren’t one way, they don’t simply put out electric charges they can detect electrical energy too.
The sensing is a measure of how much electrical energy the pacemaker is seeing when your heart beats!
If you have a dual chamber (two lead peacemaker) you will have two results for this. The sensing from the top of the heart (Atrial Sensing) and the sensing from the bottom part (Ventricular Sensing).
The bottom of the heart is larger, has more electrical energy and therefor the figures here are usually larger.
Some typical figures would be an Atrial Sensing of 3mV and Ventricular Sensing of 15mV…
Every patients sensing is different, and we find out your sensing when we are putting the pacemaker in and during a pacemaker check up. To do this we have to detect one of your true heart beats one that isn’t paced… one that your heart has done all by itself. So what do we do? we slow the pacemaker down and give your heart the chance to do some work. If we slow your pacemaker down to 30bpm this is giving your heart a whole 2s between beats to come in! COME ON, PULL YOUR FINGER OUT!
Bosh!! we get some intrinsic beats (this is the posh name for heart beats that have occurred naturally) coming through and the pacemaker measures them.
There is however, an annoyance for us pacemaker technicians and manufacturers alike! The heart isn’t the only muscle that produces and electrical energy that can be picked up by the pacemaker lead. Also some electrical equipment being used outside of the person can be picked up by the pacemaker.
This causes an issue! If the pacemaker thinks this interference is the heart beating, when in actual fact the heart is being lazy and not doing its job, then the pacemaker will not pace when it should be and you may well end up feeling faint…… or fainting.
So we tell the pacemaker that the signals need to be of a certain size.
Picture yourself (the pacemaker) looking out over your neighbors garden without a fence in the way! In your neighbours garden there are a two things going on, there is a child on a trampoline (Your heart beats) and his dad is on a space hopper (Electrical interference)! Now, aside from asking yourself where your old fence has gone and why the dad is on a spacehopper,  you need to do something to hide the dad from your view (Electrical interference) but allow you to still see the child on the trampoline (Heart Beats).
Luckily for you the dad is only bouncing up to 2mV high, where the child is jumping high up to 15mV high.

So this is easy you put a new fence up in the garden that is anywhere between 2mV and 15mV high! Perfect now you can see the child bouncing (heart beats) but you can’t see the dad on the space hopper (Electrical Interference)
This would work in theory, however there is an issue! Occasionally the dad gets a burst of energy and can jump higher and unfortunately every now and again the child doesn’t jump quite as high! So we need a safety margin! We say that the dad (electrical interference) will never jump that much higher but more importantly we know that even on his weakest jump the child (Heart Beat) will still be way above 1/3rd of his normal jump. So we put the fence at 5mV high, you know the dad will never jump over that and the child will always jump above it!

This is what we do to the pacemaker, we give it a fence (Sensitivity setting). We measure the electrical energy given off by the heart and we give it a safety margin of 1/3rd. The heart beat is never going to be smaller than 1/3rd of what it was measured at during the check and this should be a lot higher than the electrical interference would ever be!
If that child doesn’t appear from the other side of the fence then he hasn’t beat and the pacemaker has to kick in.
Of course in reality it is not always this simple and that is when the technician comes in and starts changing different settings and performing the juggling act that is settings and parameters! but like most things on this blog I am explaining the basic principles.

A complete explanation of these topics and more is available in the book Pacemakers Made Easy by Carl Robinson.

Got it? Good!
Time to put the kettle on!
Cardiac Technician

Image by http://sarabbit.openphoto.net”

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