Rate response & subpectoral placement

Hazarding a guess

by donr - 2013-06-15 07:06:24

Good mid-morning to you, Inga.

Let me give the world something to think about on this problem. (IIRC, the symbol for Berlin is a bear, isn't it? So there's a connection to my black bear post) When placed S-C, the PM can wiggle around in its pocket a bit, hence the mechanical coupling between the body & the PM is somewhat reduced. - kinda like it is connected to the body on a spring w/ a damping device also connected to it.

Now go to your sub-pectoral implant - I'd hazard a guess that now the PM is considerably more constrained by the mass of muscle over it & a tighter fit in a less elastic pocket. Essentially, it is now held to the body like it was nailed to a wooden post. Every little vibration of the body is now passed on to the accelerometer in the PM & sensed - hence the lack of differentiation between walking & jogging.

Since the accelerometer is so tiny, it does not take much to excite it, unlike the accelerometers in inertial navigation systems in planes & missiles.

Make sense?

Don

Trigger

by golden_snitch - 2013-06-15 09:06:57

Hi Don,

thanks for your input!

Sounds very logical, but I am not so sure that vibration is actually what triggers it. I have been told that old motion sensors using piezo crystals were indeed triggered by vibration, but the new accelerometers respond towards upper body movement forwards and backwards. And in fact, I cannot trigger this sensor for instance by jumping up and down a bit or by thumping on the device. I can trigger it by leaning forward a bit, then backwards, then forward again. If the sub-muscular placement really plays a role, I'd also expect that the accelerometer responds to arm movement, but it doesn't, at least not at rest.

Oh well, I guess it's complicated.

Inga

Mine

by billylittle - 2013-06-15 11:06:10

I got a brand new medtronic device. You can tap mine and it will trigger rate response.

Piezo crystals

by donr - 2013-06-15 11:06:48

"Snitch - a piezo crystal is just a sensor....." I had the rest of this comment deleted because it was factually WRONG!  Was left as a draft last night & somehow got added to the thread.  I looked at it this AM & realized it was going in the wrong direction  Will start over in a new comment box.

Don

Sensia

by golden_snitch - 2013-06-15 12:06:22

Hi!

Then it probably has a piezo crystal inside. In the feature description on the Medtronic website, they only speak of "activity sensor". The Sensia is not that new, it came out in 2005/06 already. I have tried to find out what exactly Medtronic means by "activity sensor", but wasn't successful, yet.

There is a really good explanation of the (history of) different sensors at:
https://wiki.engr.illinois.edu/display/BIOE414/Rate+Response+Sensors

It is stated in this article, under "accelerometer":
"Unlike the vibration sensor, this sensor can better differentiate anterior and posterior motion from external sources such as tapping or walking up stairs."

Best wishes

Inga

Don

by golden_snitch - 2013-06-16 01:06:44

Thanks a lot!
I still don't get it, but thanks for trying so hard to explain this to me. I did get that last part, piezo crystals cannot function as an accelerometer all by themselves, though. So, apparently some pacemakers use an accelerometer that includes a piezo, and others don't? There must be different accelerometers then, right?

I guess, what makes it so difficult to understand is that pacemaker manufacturers do not use the term accelerometer for all motion sensors. When I googled my old Kappa, they wrote it had a piezo + MV sensor (NOT accelerometer). When I google my Sorin, it says it works with accelerometer + MV. And when I ask my Sorin rep, he says, their pacemakers work with accelerometer and NOT with piezo.

Anyways, many thanks again!

Inga

Hm...

by golden_snitch - 2013-06-16 02:06:37

Hi Don!

But why do they then differentiate between piezocrystal and accelerometer in some articles, like the one I posted the link to?

I have definitely been told several times that my pacemaker has no piezo crystal, and it does not respond to thumping. My old Medtronic Kappa 401 worked with piezo and minute ventilation, and it responded to thumping and all kinds of vibrations.

I have send a message to Medtronic to ask what they are working with now. Will post the answer as soon as I get it.

Best
Inga

Another article...

by golden_snitch - 2013-06-16 03:06:12

Here's another interesting article on the history of rate response sensors:
http://eurheartjsupp.oxfordjournals.org/content/9/suppl_I/I11.full

It states under "Historical perspectives":
"Physical exercise is accompanied by body movement, and this is detected by using either a piezoelectric crystal or an accelerometer, the so-called ‘activity sensing’.[...] Body movements during exercise result in changes in acceleration forces that are transmitted to the pacemaker casing. Technically, activity sensing can be achieved using a piezoelectric crystal, an accelerometer, a tilt switch, or an inductive sensor."

That sounds like there is a difference between piezo and accelerometer, too, doesn't it?

Inga

Piezo Crystals

by donr - 2013-06-16 09:06:50

New try! Piezo crystals can do many things, depending on how they are mechanically connected to the devices. They can be the transducer for an accelerometer, for presssure sensing or many other mechanical devices. They are extremely precise in what they do. They are also directionally sensitive for sensing.

One drawback to piezos is that they do not care where physical force comes from - they respond to it. So if yu have a pressure sensor w/ a piezo transducer, it will respond to an acceleration force if one is applied in the direction in which the piezo is sensitive. That's why your old Kappa responded to thumping as well as pressure changes.

I'm sorry that wrong start got posted. I realized something was going astray, so just shut down & went to bed, leaving an open draft. This is my daughter's computer & she told me a few minutes ago that it restarted itself last night & did me a favor by posting a half-baked comment.

The point I was aiming for is that a piezo crystal can be used as the sensor (transducer) in an accelerometer, which senses inertial forces, indicating changes in velocity of something.

We are getting into some esoteric distinctions among definitions. Acceleration is something that affects MASS only. So a MASS is stuck out there to sense the fact that an acceleration is taking place. Now we have to somehow convert (transduce) the mechanical affect on the mass into an electrical signal that we can process in a computer. A piezo crystal can be the transducer - as can a tilt switch, inductive sensor or a variable resistor. A piezo crystal sitting there all by itself CANNOT act as an accelerometer - it needs a free MASS attached to it on one side & the other side must be fixed.

Let me give you a simple experiment to illustrate what I mean. Put a slick covered magazine on the dashboard of your car. Set something moderately heavy atop the magazine - like a book. Now go out to the local Autobahn & slam the gas pedal to the floor suddenly. As the car accelerates forward the book will slide to the rear off the magazine & fall to the floor. If there is a passenger sitting behind the book, they will act as your transducer - yelling in agony when the book hits them in the knees as it falls.

In this sense, the discussion is wrong in the second reference - a piezo crystal cannot sense acceleration by itself, only as a component of another system.

Don

Rate response & subpectoral plecement

by philip.thecyclist - 2013-06-17 05:06:50

Hell Inga - again!

My first pacemaker was a Medtronic accelerometer device, subcutaneously placed, so did not match your circumstances. However, it did respond in the way you describe when walking - even to the extent that shoes themselves made a difference. It would also respond inapropriately, such as high pacing rates when driving over certain road conditions, or travelling on buses. Thsi is why my last two pacemakers have been minute ventilation devices, which have made a big difference - but these are not for you. To come to the point I want to make: prior to the decision to change to MV blended sensing, I was assured by the medics that the algorithms now used for accelerometer devices were far more sophisticated, so I am daring to suggest that this aspect may in fact be more important than the actual dynamics of the sensor. Of course, the algorithms would be proprietary intellectual property of the manufacturers but perhaps Medtronic are able to provide performance indicators of their later devices. Hope this makes sense.

Best wishes,

Philip.

Sensors

by golden_snitch - 2013-06-18 01:06:17

Thanks a lot, Philip!

Are you on MV only? Because usually when you have an MV sensor it's combined with an accelerometer since the MV alone would provide a pretty slow rate response (it cannot speed your heart rate up within seconds when you run to catch a bus, for instance).

I got an answer from Medtronic yesterday saying that their new accelerometers even provide you with an adaquate rate when you go swimming or walk the stairs; right now my heart rate drops when I walk up the stairs and when I swim not much happens to my rest rate, hardly any increase. So, what Medtronic reports sounds good to me :) Also, you can program more rate response parameters manually in Medtronic pacers. I think they still work with a piezo crystall, but where it's placed in the pacer unit is different than in the past. They said because it's now placed on the circuit board, it no longer reacts to pressure from the tissue surrounding the pacemaker. I remember my old Kappa even increased my heart rate when I lay on my stomach.

Best wishes
Inga

Sensors

by philip.thecyclist - 2013-06-18 05:06:58

Hello Inga,

My last and present pacemakers have blended sensors, both from Guidant/Boston Scientific. It's too early to say just how fine-tuned the present one is, but the last one took a while to optimise the rate responses and interaction of the two sensors. Normally this might not have been necessary, but I am especially active, running (up to two years ago) and cycling - I'm a bit demanding :-).

Much of what you say about your Medtronic I recognise from my experiences my old Medtronic. Plus, my pulse raced when I played boogie-woogie! The stair problem has been long recognised in the literature, which has no doubt helped the development of accelerometers and/or crystals and their algorithms. The literature now acknowledges the benefits of MV sensors, but also stresses the need for blending with accelerometers to cover the immediate demand of sudden movements which MV sensors are slow to react to.

What Medtronic say about the new devices is again supported by the literature, which is why the medics advised me that the newer accelerometer-based devices would overcome the disadvantages of my old Medtronic, but I was sold on the reported benefits of MV sensing for cycling.

I have quite a lot of stuff on file which I will dig out to see if there is anything else worth adding.

Best wsihes,

Philip