What is a Pacing Threshold? What is a Cardiac Pacemaker Threshold and How does it Work?

What are technicians babbling on about when they say “your pacing threshold is 0.75V”
Cardiac Pacemaker ECG/EKG
Now if you didn’t already know that the heart has its own conduction system then I would recommend reading my post on it available HERE.
So we now know that the Sino Atrial Node is the hearts natural pacemaker, this lets of an electrical stimulus that starts the hearts contraction. To use the domino analogy the energy produced by the Sino Atrial Node is enough to push over the dominoes and start the chain reaction that is a heart beat.
If due to disease the Sino Atrial Node Fails to start the chain reaction/heart beat then you will have a pause in your heart rhythm. The pacemaker recognises this pause and sends its own electrical stimulus down the lead to start a heart beat. This is the most simple function of a pacemaker to recognise a pause and initiate a beat.
So what is a Pacing Threshold?
The threshold is the minimum amount of energy the pacemaker sends down the lead to initiate a heart beat. Imagine a patient who had a pacing threshold of 0.75V (Volts). If the pacemaker sent a pulse of 0.75V down the lead this would be enough to make the heart beat as would any pulse larger/stronger than this. If the pacemaker in the same patient sent a pulse of 0.5V down the lead, this would not be enough and the pacemaker would fail to initiate a heart beat (This would not be enough energy to push over the first domino).
How do we test it?
Firstly for want of a better word we need to hijack the heart, we want the heart to be ‘Paced’ that is all the heart beats during the test are being initiated by the pacemaker. To do this we normally pace at a quicker rate so any natural beats can’t get in the way. The patient may feel their heart rate increase normally to the region of 80-100bpm. Now we are pacing the heart we do something quite simple, we decrease the amount of energy being sent down the lead until we fail to make the heart beat. The lowest amount of energy that successfully made the heart contract is the threshold.
Why do we test the threshold?
There are a few main reasons why we check your pacing thresholds.
  1. We want to figure out our safety margin – now obviously in some patients that don’t have any of there own heart beat stimulation, failure to make the heart beat would be a big problem, so we have a more than adequate safety margin. We find the threshold and multiply it by 2! Because of a physics equation that I am far too bored by to go into, this actually Quadruples your safety margin. Its four times safe – Thats one big dollop of safe!
  2. Battery – If we just wanted to be safe we would put your pacing voltage up to 8V – but we have one eye on the battery minimising the safe amount of energy to use everytime we initiate a beat saves battery, makes your battery last longer and reduces the amount of pacemakers the patient has in a lifetime.
  3. Indication of Change – Sudden or gradual changes in pacing thresholds can be a sign that something isn’t right with the pacemaker or patient. Interestingly medications can also affect pacing Thresholds.
Before I sign off I just want to mention there is another aspect to pacing threshold and that is something called pulse width, I will do a small post on pulse width but essentially it is the time of which the electrical pulse is delivered. A pulse of 0.5V @ 0.4ms is 0.5V delivered for the duration of 0.4s. The longer the pulse width the more chance you have of initiating a heart beat.
I also want to take this opportunity to make something clear, when people say that there pacemaker does 5% of the work what they are actually saying is the pacemaker is having to initiate a heart beat 5% of the time. 95% of the time their heart is initiating its own beat. I think this is an important point:-
Your heart is still doing the beat, the organ itself does the work but the pacemaker is just giving it a gentle kick up the ass reminding it of its job!
P.S. You can have an Atrial and Ventricular Threshold as there are often leads in the Atrium and the Ventricle – They can have differing pacing thresholds.

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

Time to watch some garbage on TV

Thanks Again
Cardiac Technician

Comments 5

  1. Thank you. Recently my PM technician of my mother who is 91 years old, reduced Voltage her PM from 3 V to 2.5 V informing me that her threshold is good. I was wondering what is threshold and it’s connection with battery voltage. He in his report mentioned threshold as 0.9 v @ 0.4. I was worried whether it is safe to reduce the voltage. Thanks and I am relived now.

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