Pacemaker self-test and voltage

Hello everyone! 

I have a question about the self-test voltage vs. manually programmed voltage. 

I have an Astra Sure Scan Medtronic pacemaker (Model X2DR01) to manage my LongQT (Type 2). I'm paced at 80 bpm all the time. I've had this particular pacemaker for 5 years (but have had a pacemaker for nearly 25 years). For the first 4 years of this pacemaker I couldn't feel the self-test and only in the past year started feeling it. This seems weird to me, but my cardiologist reassured me that my pacemaker is working fine. 

During my last pacemaker appointment 6 months ago, I explained the new self-test sensation to another cardiologist in the clinic and he changed my pacemaker to the lowest threshold and I stopped feeling the self-test. Yesterday, after my 6-month checkup in pacemaker clinic, I came home and again started feeling the self-test. 

I called the clinic and they looked into my file and told me no parameters were changed so it's just a "coincidence" that I can now feel the self-test again. Although this sounds weird, I accepted their answer, but asked if we could turn the self-test off. They agreed. 

Today, when I went into clinic, they explained that when the self-test runs, it's testing for the best threshold/voltage for me to be paced and in order for them to turn off the self-test they would have to program the pacemaker to the voltage "reported" by the self-test. In my case, this meant they had to double the voltage of the current threshold programmed. In other words, let's say my pacemaker is programmed to 0.5V and during the self-test it automatically adjusts to 1.0V, then 1.0V is the new setting that would need to be programmed in order to turn off the self-test. 

This all makes sense, right? But, what happened is after they did this, upon me going to leave clinic only a couple minutes later, I felt faint (palpitations, shortness of breath, weakness). They told me that this was only a "coincidence" and that it had nothing to do with them changing my pacemaker settings. I understand that in principle this adjustment shouldn't change how I feel, but it did! After 45 minutes of the sensation not going away, I finally convinced them to just change it back and I'd see how I feel. Funny enough, after they changed the settings back, I felt fiine again!

My question is about the device itself, is it possible that my pacemaker during the self-test is more precisely calibrating the voltage my heart needs vs programming the threshold manually? Could there be a slight descrepancy in what the self-test "reports" as the voltage and the actual voltage it's adjusts to? Perhaps this is a question for Medtronic, but I thougth I'd see if anyone here has run into this issue too? 

Thanks for reading,
Leigh

4 Comments

Automatic adjustment

by crustyg - 2024-02-23 08:11:08

IIRC the logic is this: find the lowest voltage and pulse-width that achieves 100% capture and then double it (as a safety margin) or use the lowest voltage that the device can manage above that selected value.  My PM can only be set down to 2.0V, even though that's more than x2 my capture voltage.  Pulse-width is also important - you want the narrowest pulse as this is the biggest factor (in device settings) that determines battery life.

However, capture voltage is also dependent on rate: what might achieve capture at 80BPM won't necessarily achieve reliable capture at 130BPM.  And the quality of the lead=>heart muscle connection can change over time, hence the built-in self check.  For BostonSci devices, automatic adjustment is done slightly differently to normal pacing, so Medtronic may do something similar.

If your lead impedance and pacing voltage is stable over time, I think it would be reasonable to ask for the auto-calibration to be turned off.

Whether any patient can feel the auto-calibration depends on a number of factors and can change over time.

Being aware of device auto-calibrating is a common thread on this forum, and it's especially troublesome late at night.  Lots of folk have had it turned off for that reason.

Best wishes.

 

Hi Leigh

by PacedNRunning - 2024-02-24 01:10:10

That is odd.  I can't explain. From what I know about the self test.

It determines the lowest amount of voltage needed to cause the heart to contract consistently. This is called threshold test. They then add a safety margin. LIke he explained above the safety margin is because our threshold or amount of energy can change based on HR or movement. So there is a safety margin to account for this. 

The safety margin for my device (Boston) is 0.5V over what ever the test determined the voltage to be.

So for example, if my test was successful at 0.7V, the device would set my ouput to 1.2V  for the ventricle lead. For the A lead the lowest it will allow for output is 2.0V, so even if the test showed 0.4V, it would set it at 2.0V.  If I had the self test turned off, which I do because I do not like the testing at all. They set a fixed threshold at 2-3x your results of the testing when the daily threshold testing is off.  

So for me, if my test is .7V, they can set me at 1.4V or 2.1V fixed.  The fixed voltage uses more battery than a lower set output.  So it doesn't make sense to me why you would feel more symptomatic with it off versus on. There should be no discrepency in the testing.  It should have the same results.  I do know when I have changes,depending on the type of change, it takes me a few hours to feel normal again.  I don't see that change causing any symptoms unless they set you at a much higher output than what the device determines and you are use to the higher out put?  I can honestly say, Ive had alot of PM changes in my settings and I have a pretty good understanding of what the device can and cannot take away as far as symptoms or what symptoms it can bring on.  

Thanks for posting

by piglet22 - 2024-02-24 05:10:30

Something else the medics don't mention.

I guess it's the need to know principle.

I haven't got a clue whether my old Medtronic device is set to recalibrate or even if it's capable.

Time for Dr Google.

Automatic adjustment may not use identical settings

by crustyg - 2024-02-24 12:05:41

The Boston Sci Ref guide for PMs states that automatic atrial threshold operation uses unipolar pacing (tip=>PM case, or ring=>PM case), whereas normal pacing is bipolar (tip=>ring) so a) the thresholds determined may not match exactly pacing thresholds determined during an in-person adjustment and b) unipolar pacing may well produce more sensation from the pacing impulses.

If the device is sending 3-5V from the device can to the tip or ring of a pacing lead there's a significant chance that this will activate other muscles or be felt by the patient.  Normal bipolar pacing has the output applied over 2-4mm inside the heart - very little chance that any other tissues will be affected by the ouput.

It may be that Medtronic devices also use unipolar pacing for automatic threshold determination.

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