Oversensing

Can anyone explain oversensing to me? I am so confused....

My latest check showed that I had six atrial high rate episodes. The longest was only 10 seconds, and the highest rate was 380 A and 116 V. The tech said (and doc verified) that the strips from these events showed that I was being oversensed. They then raised my atrial lead sensitivity to .50 mV

However, previous to this, I was actually undersensing, and so they dropped my atrial lead sensitivity to .35 mV

My question is, does how in the world does oversensing cause such a high rate? I am assuming it wasn't really THAT high, but still pretty high. Also, if I am being undersensed at one value and oversensed in another, where the heck do we go from here? I called the doc once I got home to try and understand what was going on, but I am still absolutely lost. I thought someone on here might be able to explain it to me in non medical terms. Thanks guys!

4 Comments

Oversensing oversimplified

by donr - 2016-04-16 02:04:37

https://www.uthsc.edu/cardiology/articles/trouble1.ppt

So, another engineer in the mix - that should prove interesting!

Athlete oversimplified the definition of oversensing by giving you only ONE specific case of it.

I went to he link in my first line for the definition of it & tried to put it in laywoman's language. I also defined "Noise" so I could use the term. It is ANY electrical event that isn't supposed to be there - like radio static. (Or, to get a bit esoteric - a Beethoven symphony playing in the background when you want to hear acid rock! Or the atrial lead sensing some sort of INTRINSIC ventricular activity.)

Call up my link & look at slides 5 - 9 for the definitions & examples of under & over sensing. In slide 9 it talks about a noise source that is either physiologic or non-physiologic. Physiologic is something generated by YOUR body - like a ventricular signal sensed by the atrial lead, or some sort of nerve noise that is coming from outside the heart. Non-physiologic is noise from OUTSIDE your body, like an electro-cautery device, an electric shock because you did something in the house wiring or suffered a blast of the infamous EMI like getting too close to a high tension power line or a welding machine output.

Your first question this AM about being in AAIR mode - the heart's electrical pathway is supposed to be one way conduction from the SA Node to the AV Node & thence down the length of the heart. (Sorta like the cables in your home entertainment system, where the satellite box receives a signal from the dish outside, processes it & transmits it to your TV set. Get a loose connection & there can be reflections in the cables where the signal travels BACKWARDS & you see garbage on the screen. Remember those "Blue Screens of Death" you sometimes get that tell you there is a weak or loss of signal & to go check all the connections to be sure they are tight????? Same thing in the heart's "One Way" path from SA to AV to rest of heart.

IF for some reason, you get an intrinsic Ventricle signal in the Atrial section because it travels backwards (Retro-conduction) Your PM's atrial lead will sense it & it really does NOT want to, causing confusion in the system. Your PM is a digital computer - just another "Dumb Box of Rocks" that does exactly what it is supposed to do - gets a signal & does something w/ it, nothing more, nothing less.)

Next question - what causes variation in voltages/signals? WOW!!! that is a great question - but a tough one to explain. I gave a single sentence explanation of that in last night's epic verbosity. Bryna, NOTHING, ABSOLUTELY NOTHING, in nature is perfect. Nothing is perfectly repeatable. Everything varies, if even a little bit. That's why the mathematical science of STATISTICS was born - to describe & understand this natural variation in things.

You've heard the term "Precision" used. It has a companion called "Accuracy;" people confuse the two all the time. Sit your engineer husband down & get him to discuss the two words w/ you, based on the following:

Suppose your heart beats at what you think is 60 BPM. that means that your beats come at ONE second intervals. Ya wanna bet????????? It all depends on HOW you measure that 60 BPM. First sit down & take your pulse by placing fingers on your wrist, sensing the pulse & counting beats while watching a second hand on a clock for a full sixty seconds. You get 60 beats. Perform that pulse measurement 5 separate times. Suppose you get HR's of 60, 59, 61, 60, 58. Notice that they are NOT all the same. That variation from what is expected is a measure of "ACCURACY" of your method of measurement - also called "REPEATABILITY. It is also a result of the "PRECISION" of your measuring method. Precision means the number of decimal places in your measurement device. You counted by whole numbers - 1,2,3,4,5,...Nothing after the decimal point, like 1.2453, 2.4536, 3.8564... Your counting method just did NOT allow any more precision.

Now go hook yourself up to an electronic device that can count with great precision - like measuring counted in "Milliseconds" like an ECG machine or an ER monitor. That means the device can count time in little bitty intervals of THREE decimal places - .001 seconds per count. Much more precise than you counting. In one second on that device, it counts one thousand times! So you watch this sucker display your HR & it gives you numbers like 60, 60, 60, 60, 59, 60, 61, 60, 58.....Not only that, it does it for EVERY single beat instead of after every 60 beats like your counting while watching the clock. This system is much more precise in its ability to count (3 decimal places compared to your single one; & much more accurate -more repeatability in its output.)

No matter how precisely your counter counts while measuring time between beats, you will ALWAYS get a variation in the HR as an output. That's because the heart's timer is just NOT that accurate (Remember - repeatable).

What science (Especially physics) has discovered over the last century is that the greater the precision you get in your measuring devices, the worse the accuracy (Repeatability) of that which is measured. Turns out that you cannot describe some phenomena in absolutes (Like 60 BPM for HR or 1.5 Volts for the amplitude of an ECG wave) but you have to describe them in statistical terms to account for their natural variability.

Have you ever heard anyone talk about HOW the SA node measures time in order to generate it's P wave outputs? I haven't. It is doing it on an extremely small physical scale, meaning that things are not explainable in absolute terms, but in statistical terms only - meaning there are variations in everything! Let something get just a bit out of whack down there in the statistical weeds & all heck breaks loose up where we can sense or understand what is going on.

Notice that Cardios only talk in terms of Milliseconds when discussing times of events & delays in the heart. There is a good reason for this - humans cannot yet understand the functioning of a heart on any more of a precise (Number of decimal points in measurement) level.

We had a member of the PMC named ElectricFrank (He died about two yrs ago) who was an EE, also. Frank went out & took a measuring device that could measure time much more precisely - to billionths of a second (.000000001) & found unusual things in an ECG displayed at that level of precision. He went to a conference & talked w/ cardios about it & they admitted that he was generating information at a level not yet understood.

Understanding the electrical functioning of the heart is still in its infancy. PM's have only been around during my adulthood. You are asking questions that even those on the cutting edge of research cannot yet answer.

Enjoy Japan - we spent over 4 yrs on Okinawa in Uncle Sam's Army back in the 1970's. Had a fun time.

Donr




Good Question

by donr - 2016-04-16 03:04:46

Let's start w/ UNDERSENSING - Undersensing is the PM failing to sense an intrinsic voltage wave form that actually exists. Consider a heart that is functioning normally, w/ all the little humps & bumps & squiggles in it's electrical output from the two nodes. Stick a dual chamber PM into the act. It has a lead in each set of chambers looking for electrical activity. In your case, you asked about the Atrial signal being UNDERSENSED. The sensitivity setting of the PM to reading & acting on the voltages determines how efficiently the device functions. Let's assume that the max amplitude of the P wave is 1 volt (for simplicity). The Cardio sets the sensitivity at .1 volts for a first cut at functioning. In short, the PM will function any time it senses any voltage over .1 Volt Since the actual P wave gets up to 1 Volt, the PM will react to it. Raise the sensitivity to .5 volts; the P wave is still higher, so the PM functions. Now raise the sensitivity to 1.1 volts. WHOOPSIE - nothing happens - the sensitivity is set higher than the P wave goes (1 Volt). THAT is undersensing.

That was just to illustrate the definition of undersensing. Now look at the REAL world. The P wave is not always 1 Volt. Nothing in the real world is always the same amplitude. Perhaps your heart puts out a P wave that varies from .5 Volts to 1.5 Volts. Now your Cardio has a challenge - to select a sensitivity setting that will just about ALWAYS sense the P wave. If he sets it at .3 volts, it should always sense it, since the lower end of the voltage range is .5 Volts & the sensitivity is set LOWER than that. Supposing that he sets the sensitivity at 1 Volt. That's another big WHOOPSIE - the PM will fail to sense all those P waves that happen to be BELOW 1 Volt - hence it will be undersensing part of the time.

What can cause UNDERSENSING?
1) Sensitivity set too high (like in my example).
2) Lead is dislodged
3) Lead fails (Breaks)
4) Lead maturation (Gets old)
5) Intrinsic Voltage generated by node changes

That's the easy one. Now for OVERSENSING. Oversensing is the PM sensing inappropriate electrical activity. That's a fancy way of saying it senses NOISE. Noise is any activity that is NOT supposed to be there - like static on a radio.

F'rinstance - the electrical trace shows a Blip that is not supposed to be there or you're undergoing surgery & the surgeon uses electro-cautery that causes electrical activity to occur in the heart area & the PM senses it. There can be other stray electrical activity caused by the nervous system that may occur.

Here it gets a bit messy. The Cardio has to set the sensitivity HIGH enough that the PM will NOT react to electrical noise. If the NOISE just happens to be a higher
voltage than the actual voltages the heart's nodes produce, you have a real problem. Generally speaking it is not, so they pick some sensitivity that is high enough that the noise is a lower voltage & will not be sensed. If the noise is at .2 Volts, they set sensitivity at .3 Volts - the PM will ignore the noise. Any time the PM acts on the noise, it is called OVERSENSING, because the sensitivity is set too LOW.

Sounds like you have a noise problem such that the Cardio is trying to find a sensitivity setting that will be higher than the noise, but still low enough to sense the real P wave.

The same definitions apply to the Ventricle voltages & their sensing - except that they are MUCH higher than the Atrial voltages.

The Cardio does not have the capability of seeing the noise that the PM is sensing, so he has to find a safe sensitivity by trial & error. He's done that for your Atria. First he set it at .35 Volts & discovered that was too low - down in the noise & sensing that, so he raised it to .5, hoping that would miss all the noise but still be low enough to catch the P wave.

So, how does oversensing cause a higher rate than the intrinsic rate of the heart? Easy-peasy! F'rinstance: supposing that your heart id just pumping away at 60 BPM & the PM starts OVERSENSING by picking up NOISE that it thinks is a P wave sooner than the next P wave is supposed to occur. That phony P wave triggers the PM into action & it starts looking for a QRS complex for the ventricles beating. It's not there, so it triggers a QRS ventricular wave sooner than it should. Get a whole string of those & you have a high rate beat in one set of chambers.

How does the strip tell them that you are oversensed? Also easy. there is a marker channel on the strip that shows where each PM output pulse is located. They find one for the Atria (or Ventricles) & look up to the ECG trace & look for the corresponding squiggle on it. If it's not there, you have been oversensed.

Hope this helps.

Donr





Simpler version

by athlete735516 - 2016-04-16 07:04:34

In basic terms, over sensing is when the lead in your atrium senses the electrical signal coming from your ventricular lead and thinks it is the atrium beating again. It is pretty much sensing your heart is beating twice as fast or more. It isn't a big deal but whoever is getting alerts about your pm will probably get a couple messages!

Hope this clears things up!!

Even if...

by BrynaR - 2016-04-16 11:04:19

... My ventricular lead is turned off? I am in AAIR mode, so theoretically the v lead should only be sensing. Could the a lead be picking up the intrinsic ventricular activity?

Donr, I would expect nothing less from an engineer ;-) Thank you for your detailed response. My husband is also an engineer (though not electrical so he claims to be useless here) and when asked for his opinion he has a tendency to be equally verbose.

And thank you, athlete as well. I'm still bothered that they raised my sensitivity back to the point which caused undersensing a few weeks ago.

Another question for anyone-- what causes the amplitude of the various waves to change? What an interesting thing, this heart of ours and all its wiring.

We are planning on moving to Japan this summer (we lived there for six years before moving to Jacksonville, Florida two years ago) and I hope to have things straightened out before we move. Of course there are fantastic EPs in Japan, but the language barrier with something like this could prove to be a problem. My EP here has said that the next step would be an ablation of my ectopic focus so that I can be paced at a lower rate (lower limit is 90 to outpace it right now, and I still go into junctional tach, however it is for a few beats here and there only-- no one is even thinking about an ablation for that!). He says he isn't advocating an ablation, given all I've been through with the SVC syndrome, but it is the next step. Before this last check I was inclined to say 'forget about the ablation, I'll manage at this rate for a few more years' but seeing the new wrench (oversensing) thrown in the works has me feeling very frustrated. I just want to get to a point of 'easy maintenance' where I'm only seeing the EP and pacer techs twice a year instead of twice a week! I'd also love to hear anyone's thoughts on an ablation for ectopic atrial rhythms.

Thank you both again for your comments :)

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