Increase in right ventricle voltage threshold

Hello everyone,

Today my husband (83 yrs) had his annual pacemaker check at Kings College Hospital, London (UK).  His single lead pacemaker was implanted in March 2018 for bradycardia, heart block (Mobitz II/Left Bundle Branch Block) and occasional loss of consciousness due permanent Atrial Fibrillation (AF) with a very slow ventricular response rate.

We were told he has 5.5 years remaining battery life, although this of course is only an estimate.  He had no events (high heart rates) recorded.  He has rate response on.  He has been getting periods recently of near syncope, but nothing was picked up by his pacemaker as a potential cause.  He is pacing in the right ventricle 80% of the time at 60 bpm.

The technician told us she had increased his Right Ventricle Voltage Threshold Output from 1.0 to 1.5, to give a better safety margin and more certainty of consistently capturing the heart and making it beat.  The downside is that it might use up a little more battery.  Is this small increase in threshold likely to cause any other problems, do you know, or is a 0.5 increase insignificant?  Many thanks.


12 Comments

Increased capture margin

by AgentX86 - 2022-04-20 01:35:30

It shouldn't cause any problems but there is a chance that the higher voltage will stimulate the diaphragm.  If that doesn't happen he should be good.

It's a matter of "is the voltage high enough to cause the heart to "constrict" , or capture threshold.  And how much safety margin?

Raising the capture margin will reduce battery life but it doesn't really matter if the pacemaker isn't doing its job because of a negative capture margin.

Increased capture margin

by Gemita - 2022-04-20 06:01:54

Hi AgentX86, thank you for your comments.  They are helpful.  Yes I believe the technician only increased this setting to give it a bit more of a safety margin to see if it helps with the pre-syncope events.  She mentioned if I recall correctly “if the device is pacing the heart and we don’t give enough energy to capture the myocardium, it will pace but may not consistently make the heart beat”.

The technician was very thorough in her checks.  We both liked her and she was always willing to answer our questions although time was limited.  Of course she mentioned as we often hear on this site, that “the pacemaker is working well and there are no obvious technical problems to account for these intermittent symptoms”.  A full cardiology opinion is well overdue.  Last in clinic appointment with his cardiologist was over two years ago

Pulse width is the other big factor and probably more important

by crustyg - 2022-04-20 09:38:59

What really controls battery life is the amount of charge that each pulse output consumes from the battery.  Each battery has <x> many milli-C (it's a big unit) so accumulated pulse outputs plus internal self-discharge => battery exhaustion.

But increasing pulse voltage output if there's evidence of increased impedance won't take a great deal more charge per output than before.  Just think about it in DC-terms: 50% increased voltage into 30% higher resistance is only a 15% increase in current flowing so 15% more charge per pulse.

However, increasing pulse *width* will make a big difference: charge is flowing from the battery to the heart muscle for the entire time, just as leaving the oven on after baking consumes more electricity on the meter.  Increase pulse-width by 50% and that's 50% more charge for every paced output.

Pulse Width

by Gemita - 2022-04-20 16:15:12

crustyg, thank you for your help.  I am not sure whether there was evidence of increased impedance yesterday since I don't have the settings.  I am somewhat reassured by your explanation that we are not talking about pulse width here (unless increasing pulse voltage output automatically changes/affects pulse width)?

I hope the new setting helps but if not at least I will know what was changed and by how much.  A slow learning process but we came away very happy with the help we received yesterday.  What a difference it made.   Even my husband got involved and started asking questions about his battery.  A thoroughly useful appointment

Pulse width

by AgentX86 - 2022-04-20 23:45:42

Crusty, I'm not sure why you're assuming a 30% increase in impedance but even so, using your numbers:

1.5/1.3 is indeed 15%

but 5.5Y * (100% -15%) = 4.7Y

Seems pretty significant to me.

Battery Life

by Terry - 2022-04-22 15:26:25

Can your pacemaker give you a projected battery life remaining, 1.0 V vs. 1.5 V output?

Battery life

by AgentX86 - 2022-04-22 19:31:39

I can't imagine any pacemaker manufacturer would make that information public.  They know but it would almost assuredly be proprietary information. As Crusty indicated, the area under the voltage waveform (voltage times duration) divided by the impedance (times the heart rate) gives a SWAG at the increase in usage.

 

.50 is small

by PacedNRunning - 2022-04-23 05:10:10

It shouldn't effect his battery that much at .5 Incresse. It's very minimal  It will take a full 30 days to get a accurate battery life. 

I am probably worrying unnecessarily again

by Gemita - 2022-04-23 05:55:06

I can see this could open up a much wider discussion than I would be able to host with my limited knowledge on pulse width, pulse voltage output and other lead measurements, but I am happy to follow your interesting comments.  

Although keeping the battery functioning for as long as possible is obviously an important goal for an 83 year old because of the potential risk of getting an infection with a change of device, my question was also about whether an increase of 0.5 V in threshold output could lead to other potential problems too?  I am assuming it will increase his single lead right ventricular pacing which is already at 80% with a 60 bpm base rate.  My main concern would be whether this small change in threshold output could in any way adversely affect my husband's right sided heart failure (due pulmonary hypertension).  Our doctors are still trying to find the right balance of therapies and it has proved difficult.

PacedNRunning, AgentX86, Terry, crustyg, thank you all for your help.  Your comments, as always, have reassured me

Shouldn’t change anything.

by PacedNRunning - 2022-04-24 00:31:50

The increase in output is different than threshold. Threshold is amount of energy it takes to make the heart contract. The output is how much energy the device is delivering to accomplish this. If they set a fixed output they usually set it 2x or 3x the threshold.  If he was at 1.0 to 1.5 my guess is his threshold is .5V  The tech decided fo change the safety margin to 3x  probably to keep him safer since it's a one lead and he relies on it quite a bit.

 Our thresholds can change a little during the day and the safety margin will allow that fluctuation.  If he has auto threshold on that can save battery because  the device will set it .5v above the threshold.  Did he have auto threshold turned off? Or Perhaps it wasn't working as it should So they set it to fixed output? I have mine fixed because I don't like the auto threshold daily test. I'm at 1.8v. Anything under 2.5v is pretty standard ideal would be 2.0v or less. So he's in a good range. It shouldn't affect his heart or health but keep in safe. 

hope that helps! :).

PacedNRunning

by Gemita - 2022-04-24 12:14:39

PacedNRunning, oh yes, thank you, this is useful EP info!  As I mentioned to crustyg, I don’t have access to my husband’s PM lead settings or indeed any settings.  I don’t think the EP felt she needed to change anything actually until we mentioned about the pre syncope and she then double checked the lead/other settings.   All she said initially was the usual stuff “the pacemaker is working well”.  

I cannot answer your specific question on whether auto threshold was off or whether they set it to fixed output but your helpful answer has made me curious and wanting to learn more, but I did trust the EP.  She was so patient and helpful.   

You have explained the subtleties of lead output versus threshold exceedingly well.  I can remember specifically asking what setting the EP changed and she mentioned “thresh output - the minimum amount of energy given to pace the heart” and that was as much as I could take in.

I am completely reassured by all the comments I have received in this thread and my questions have been answered

 

You’re welcome

by PacedNRunning - 2022-04-25 01:06:48

You're welcome. It helps when we get a variety of answers. 
 

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