Automatic threshold tests leadless MICRA PM

Hi everyone

Since having my leadless PM put in in July. It is set to 40BPM and it seems it hasn't had to pace me yet. I got it after a 13 second heart pause out of the blue (I had a loop recorder). I've had serious palpitations every 30-60 minutes since the PM op. I was taken to hospital recently whilst travelling as my heart had started racing (still unclear what that episode was as the loop recorder was taken out). But, while I was there I asked the technician about the palpitations, she did a little test whilst interrogating the PM and said that I was sensitive to the automatic threshold tests that the PM does. She said this cant be turned off as it wouldn't be safe! Something along the lines of "it needs to threshold test to know how much power will be needed".

I went for my first PM check a few weeks after that incident to my local hospital, armed with the information the other technician had given me about the automatic threshold tests. I mentioned the palpitations and the technicians there didn't suggest that it could be the automatic threshold tests. That kind of concerned me about their knowledge. I then told them that I was aware of that from the other technician and they then agreed to check this, they did the test and agreed that I'm feeling these threshold tests. They said they can turn the automatic threshold testing off and said it was safe. They said all pacemakers operated without automatic threshold testing in the past?

I agreed as the palpitations were bordering on painful and at least every hour without fail I was getting them. I'm no longer getting frequent palpitation like before so it was definitely sensitivity to the automatic threshold testing. (I have been getting some episodes of palpitations but I'm sure these are from missed beats as I've been checking my pulse in my neck and feel complete missed beats?) That's a separate issue I'm worried about.

I feel really concerned about the total differences in opinion of the PM technicians and couldn't find any info on the internet about this. 

Does anyone have any idea about this?

Thanks so much :)


Automatic threshold

by AgentX86 - 2019-09-16 15:10:24

First, let me state that I know little about the Micra and little has been said about it, so your information is very welcomed. Please keep us up to date with your findings.

I too would have your concerns with the different information you're being given. Very concerned. The Micra is very different than classical PMs in that it doesn't have leads hardwired into heart muscle. A normal pacemaker"s electrical circuit is pretty well fixed when the device and its are implanted in the body. It shouldn't change much, so the need for recalibration isn't great. I had mine turned off too and they do it manually during interrogations but it doesn't change.

A Micra is different,  I think, in that it's not fixed to the heart so the circuit impedance may change as you/it moves. Micras are new enough that I'd want this clarified by someone who really knows (read: factory guru) before shutting off the feature. Sure, it's a normal thing to do with a normal PM but the Micra is not a normal PM.

Truth is no-one *really* knows

by crustyg - 2019-09-17 11:00:19


Leadless PMs are sufficiently new, and there are so few patient-years of experience and follow up that I don't think anyone can actually claim that they *know* the best approach and show data to support their opinion.

It makes perfect sense for the device to automatically determine your pacing capture threshold quite regularly - arguably more often than a conventional lead+PM setup - as we know from very long experience that the effectiveness of the electrical connection between the PM's output conductors and the actual heart muscle that it is trying to activate varies over time.  AFAIK, all modern leads are steroid-eluting (they have a tiny dose of a very potent steroid, dexamethasone) built into the tip, and this reduces the build up of fibrous tissue around the tiny area where leads contact the heart, which has been shown to reduce the increased impedance of the junction.  The usual process of fibrous tissue growth at the leads anchors the leads even better than the active fixation screw, but the tip is helped by the steroid, and by the time that the steroid is all washed out, the leads are anchored and the fibrous tissue growth slows or stops.

For the leadless devices, there is a much greater chance of the device being dislodged, even a tiny amount (I'm not talking about becoming free-floating), so a much higher chance of the effective contact changing, so more frequent +/- aggressive pacing threshold testing is needed.

I *think* that you, together with your EP doc + manufacturer's rep have to choose: reduce the active threshold adjustment and risk losing capture (no pacing......) or stick with it and put up with the palpitations.  Your manufacturer rep might be able and willing to share the numbers that the company has and the duration of experience.  Unless your palpitations are actually painful, I suggest that you *can* learn to ignore them.  I had palps for months before each ablation, sometimes I was more aware of them than other times.  But if I were PM dependent (not yet) then I wouldn't want a significant risk of my PM losing capture.

Hope that helps!

Micra settings

by AgentX86 - 2019-09-19 10:43:40

It's a good idea to ask for a copy of your pacemaker settings after each interrogation. You'll quickly be able to see if anything changed and you can use it to learn more about your device. You've already found the web site so the information is there. If you don't understand something, ask (the device tech, your EP, us). At least you'll know what to ask.


by _Claire_ - 2019-09-23 07:34:06

Great advice, I'll do that! Thanks 

You know you're wired when...

Your device acts like a police scanner.

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