Pacemakers Explained – Heart Rate Histograms

Pacemakers Explained – Heart Rate Histograms!

You have good Pacemaker histos!

I just wanted to write a quick post on pacemaker heart rate histograms, as a patient you may hear your pacing technician say something along the lines of “you have good histos” what they really mean is that you have a good heart rate spread. Here I try to explain a bit about them, they are often overlooked as a diagnostic tool but can be very handy indeed! 
Heart Rate Histogram is a posh name for a graph produced by the pacemaker software that tells us what amount of time you have spent at a particular heart rate, it will also tell us what percentage of this rate was paced (initiated by the pacemaker) and which is intrinsic (initiated by the hearts natural processes).

This is what a heart rate histogram looks like:

Pacemaker Histogram
I would say this patient has “Good Histos”
As promised you can see the amount of time spent at each rate interval.

The graph on the left shows that the Atrium spends the majority of the time beating between 60-80 bpm. Whilst the majority of these contractions are intrinsic (the white sensed portion of the bar) there is a definite presence of pacemaker beats (the black paced portion of the bars).
If you look at the graph on the right the Ventricular heart rate spends the majority of its time between 60-80bpm. Of these beats the majority are sensed (white sections of the bars) compared to paced (black sections of the bars). As a physiologist I would be quite pleased with these histograms. We can see that we do not pace the Ventricle much which as we know is desirable when avoiding increased risk of Heart Failure and Atrial Fibrillation. Further more we can see they have a nice heart rate curve (which is what we like) which refers to the sequentially decreasing % of beats that occur in the higher rates.

Pacemaker Histogram
This picture shows what I mean by a nice Heart Rate Curve
We like to see this kind of shape….
Histograms like the one above tell the physiologist that the pacemaker and the heart are behaving appropriately.
To show exactly what I mean here are two examples of heart rate histograms that are not so pleasing to the eye.

Chronotropic Incompetence 

This histogram was taken from a 65 year old male who is still active and walks his dogs twice a day.

Pacemaker Histogram

Now if you look at this, he spends his life with his heart rate nearly always at 80-90 bpm. His pacemaker was programmed at 80 bpm which fits in with the histogram. His heart rate seldom increase above this which might explain why he complained of excessive shortness of breath when walking up hills.
When a persons heart rate doesn’t naturally increase on exertion, they are said to be chronotropically incompetent this can lead to breathlessness. These patients are helped using Rate Response which increases heart rate when the patient is active. This patient had his rate response switched on based on his lifestyle and his histogram. When I see him again I expect a much nicer heart rate curve.
NB. Lifestyle has to be considered when analysing a heart rate histogram – for example if the patient above was wheelchair bound then this histogram would be appropriate.


The next histogram shows you how useful they can be in detecting arrhythmias also,

Pacemaker Histogram

The patient above had no know history of Atrial Fibrillation (a fast heart rate in the atrium, top of the heart). Yet the histogram clearly shows that for a large percentage of the time, the patients Atria were contracting incredibly quickly – a sure fire sign of AF or Atrial Flutter. If you can’t see where I mean, look to the right of the Atrial Histogram and there is a tall white bar that is more out of place than a fish up a tree.

Why a Graph?

The reason that histograms are there is to help the physiologist interpret heart rate spread by making it a visual commodity. A picture is worth a thousand words after all!
Essentially if the heart rate histogram isn’t what we would expect or what we would like then we can investigate further.

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

I do apologise that this is a pretty dull subject and probably more of a boringly informative read than usual. In my defence however I was talking about a graph… try talking about graphs on your next date and see how it goes 🙂

Thanks for sticking with me on this one,

Time for a posh dinner.

Cardiac Technician

Image courtesy of KROMKRATHOG/

Comments 6

  1. Hi Kristian,

    First of all thank you so much for all the valuable information on your website – the clarity of your explanations is really outstanding.

    I have a question about histograms. I am 61 years old and had a Medtronic Advisa DR pacemaker implanted about 5 weeks ago for symptomatic sinus node dysfunction (sick sinus syndrome). At my first follow-up appointment I asked for a print-out of the histogram report. It showed 34% pacing with most of this happening at heart rates in the 60-70 range. I notice that there is a very small amount of pacing at HR <40, 40-50 and 50-60 and was wondering how this could be given my base rate is set at 60? There are also similar low levels of pacing at HRs 70-120, and I'm not sure I understand the nature of this pacing at higher heart rates – is this related to rate response settings?

    I'm feeling much better since the procedure, and the Medtronic technician seemed to imply that rate response would automatically adapt over time to my circumstances/activity levels. If this is correct can you explain how this works, as I don't think I've read anything about this.

    Thanks so much.

  2. Thank you. That was a really informative explanation. I’ve just returned from my annual check and I heard them say I have very good histograms, so I thought I would look it up!

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