Sensing / Sensitivity / Sensor Rate & Sensability Settings

Settings

by AgentX86 - 2023-05-11 13:35:36

Let me give it a shot

Atrial vs. Ventricular sensing: A two-lead pacemaker is essentially two pacemakers with a connection between the atrial pacemaker and ventricular pacemaker. The purpose of a pacemaker is to insert a pacing beat if it doesn't sense the "natural" pacing beat. In the case of the atrial sense, it's looking for the SI node to do its thing.  If it doesn't, within some time, it triggers a bear.  if the pacemaker does sense a beat, it just sits there watching the heart operate normally.  The atrial pacemaker takes care of problems like SSS, pauses, and such. Sometimes the natural pacemaker works and sometimes it doesn't.  If both activate at the wrong time, bad things can happen so the pacemaker has to be inhibited if there is a natural pacing signal.

On the ventricular side, the pacemaker senses an atrial beat (from the atrial lead), either natural or paced, and starts a timer. If that timer expires, it supplies the ventricular pacing signal.  This is designed to mimic the AV node.  A second degree block, or in some cases a third degree, can be intermittent.  When the AV node is working, the pacemaker needs to get out of the way.  If the AV node is not performing as it should, like atrial pacing does with the SI node, the pacemaker inserts the pacing pulse, or inhibits the (artificial) pulse and lets the heart do its thing uninterrupted.

This gets us to a number of settings called "capture" settings.  There are both "pacing capture" and "sensing capture" settings and with each of these, is a threshold and a margin. Each of these would also have both an atrial setting and ventricular setting.

Pacing "capture threshold" is the voltage needed from the generator for the heart to "capture" the pulse and start the beat.  Pacing "capture margin" is the amont over this the pacemaker actually supplies, just to make sure the heart beats every time.

Sensing is more or less the same, except instead of the voltage for the heart to get triggered to beat, it's the voltage the pacemaker has to "see" before it recognizes the signal as a legitimate natural pace. Like the pacing parameter, this would have a threshold and margin, except that the margin is how much less than the threshold the pacemaker will count as a pulse.

Each of  these can be set manually or automatically.  EPs have different opinions about these (my EP doesn't think they would work for me, anyway). Some have problems putting up with the daily self test that goes on to set this.  It can keep people awake at night, when it's usually set to do its thing.

Now, if a pacing threshold is set too low, the pacemaker won't generate enough voltage to start the pulse. If the pacing margin is set too high, battery longevity is reduced.  I think a pacing margin of twice the threshold is common.

Conversely if the sensing threshold is set too high, the natural pace won't be seen so the pacemaker won't inhibit when a natural beat happens. If the threshold is set too low, extraneous signals can be seen by the pacemaker as natural beats so the pacemaker inhibits and no pacing signal  (either natural or from the pacemaker) will be seen at all.  Perhaps counterintuitively this is known as "over-sensing".

There are (at least) two rates.  One defines the lowest rate the heart is going to go.  The maximum tracking rate is the fatest the vntricles will be allowed to go before they uncouple from the atria.  Atrial tachycardia isn't generally a dangerous thing but ventricular is exceedingly dangerous.  This rate guarantees that the ventricles won't be over-driven because they're not following the atria anymore.

I have no idea what "sensability" settings are.  I expect that it's a communication problem somewhere here and it's really 'sensativity", or some such.

Under-sensing might cause EKG abnormalities because the pacemaker might not supply pacing signals so beats can be skipped.

Agent X - SenseAbility or AutoSense

by Penguin - 2023-05-11 15:08:12

SenseAbility or AutoSense is an algorithm on Abbott devices. So no its not a communication problem just yet another fancy name for an automatic way to adjust sensitivity. 

It's covered in this link: https://www.cardiocases.com/en/pacingdefibrillation/specificities/pm-pacing-sensing/abbott/pacing-sensing-abbott-pacemakers 

I've read the link but tbh I'm not much the wiser. I was after more of a layman's interpretation like your very helpful description of a how a pacemaker senses / paces etc.  

Do you know how sensitivity is programmed and what it actually does? I'm somewhat vague about the sensor rate too - how it's determined and programmed and how it interacts with the MTR. 

Thank you for your comment about Under sensing and EKG abnormalities. 

Sensitivity

by Selwyn - 2023-05-11 17:12:03

Sensitivity is the MINIMUM potential difference required between the terminals of a PM to suppress output. ( So as above, counterintuitively, a higher sensitivity means a lower potential is required!).

The sensing threshold is determined by programming the pacing rate lower than the intrinsic rate of your heart ( if any!) while the sensitivity is gradually reduced ( ie. a larger potential, measured in mV) until failure to sense is observed. Roughly, sensitivity is adjusted to at least half the threshold value.  The threshold value is the largest possible sensitivity measured in mV associated with regular sensing. 

Sensing varies with respiration as the chest impedance changes. ( This effect is used in the Boston Scientific respiratory rate/rhythm control for exercise).

Sensitivity settings

by Gemita - 2023-05-12 05:33:13

Penguin, thank you so much for your questions.  However difficult it is for me to answer questions like these, and to fully understand answers received, it is always helpful to see them because they help us to learn about our pacemakers and perhaps to appreciate what an incredible piece of engineering we have inside us.  

I can appreciate how complex our settings are and how our pacemakers are so finely controlled by these settings that even small changes can make a real difference to how we feel.  Both AgentX86 and Selwyn’s contributions are a study (!) but valuable ones nonetheless and since hopefully questions like these will come up time and time again, the more we see them and any responses, the easier they will be to understand.

Not so long ago, my husband had his Right Ventricle Voltage Threshold Output increased from 1.0 to 1.5 V, to give a better safety margin and more certainty of consistently capturing the heart and making it beat.  My husband has a single lead pacemaker to his right ventricle and this increase was to try to see if it would help with his near syncopal events that have been increasing.  We were told something along the lines that the threshold is the amount of energy it takes to make the heart contract and the output is how much energy the device is delivering to accomplish this.  If they set a fixed output they usually set it 2 or 3 times the threshold. 

Our thresholds can change a little during the day apparently and the safety margin will allow for that fluctuation.  I am not sure whether Michael has auto threshold on.  Many have it set to fixed output because they don't like the auto threshold daily or nightly tests.

I, on the other hand, had my atrial sensitivity setting recently changed because during a check, the technician picked up some under-sensing of my arrhythmia atrial fibrillation on one of my stored atrial electrograms. The technician therefore adjusted my "atrial sensitivity” to the highest level, from 0.30 mV to 0.15 mV (where the lower the number = the more sensitive the pacemaker).  I was told my Mode Switch (which is a feature on most pacemakers to prevent the tracking of a fast arrhythmia) should work better too with the atrial sensitivity level set higher (although probably means more Mode Switches).

While not specifically answering your questions Penguin, I hope my contribution with some practical examples of where sensitivity settings have been changed and why, will add generally to the discussion.

Thank you - Selwyn & Gemita

by Penguin - 2023-05-20 14:19:32

Sorry for the late reply to this thread, but many thanks both of you for your input. Both useful and well explained. 

Understanding how sensitivity may affect the detection of arrhythmia is a useful snippet.