Closed Loop Simulation (CLS)

Hi there.  I've read quite a few other posts on this subject but (as everyone always thinks ;-)) none quite match my situation.

I'm a pretty fit and active 62-y-o in England who walks lots and runs at least one and often 2 or more 5km runs each week.  

But I'd been really struggling since the Spring.  After tests, I was fitted with a pacemaker for total heart block in August.  It's a Bitronik.  Accoring to the certificate it's model number 407147, but I don't think that is very meaningful?

Felt better almost right away, but noticed that sometimes on long walks I'd sporadically get really out of breath, and yet my heart rate per monitor remained at only 75.   The next day I'd do exactly the same walk and in the same sections heart rate would be up to 120-130 and no breathlessness.

I took mountains of stats to my first check-up last month, but to be honest they weren't really interested in them.  They said everything was absolutely fine per the data they downloaded from the pacemaker.   I asked about CLS and they asked why I was interested.   I said that I had read that it could be beneficial for very active people.

They said this is something that they could enable if I wished, but suggested that it would probably be best if I got back to my normal running pattern before deciding.  This seemed a reasonable argument.

Actually I am finding it very difficult to get back to full fitness.  Until the summer I could easily run 5km, now I am struggling to run for more than a few hundred metres, so I am mixing up the 5km "runs" by alternately walking and running.

Have I misunderstood what CLS might do for me?   Where could I read more: the Bitronik papers are very technical and I am not a doctor.


(PS I am a Grade A hypochondriac - is there any simple way to check that the pacemaker is working at all?  Apart from not dying, obvs ;-)).



by Penguin - 2023-11-03 19:59:49

Hi Valiant, 

There is a lot of information about CLS on this forum. If you place 'CLS' in the search box (tab at the top of this page) there are lots of posts on the subject.  Posts by a member named 'Golden Snitch' - who was / is a runner / very active and who has / had a Biotronik device with CLS enabled - may be useful to you. I think Golden Snitch was / is European.

Note: A lot of the other posts are from US recipients of Biotronik devices.  It seems that the specialist knowledge needed to programme Biotronik devices can be 'patchy' in the US and some people report inadequate technical support outside of specialist centres.  Keep this in mind when reading non-European / US posts! 

 In the UK you could ask to speak to the local Biotronik Rep should you decide to have CLS programmed. You can request that they help programme your device as they are the expert advisors.  

From what I understand CLS is super responsive and responds not only to demands when exercising but can also be stimulated by emotion. It is a type of rate response e.g. it increases your h/rate to facilitate exercise in people diagnosed with chronotropic incompetence / people whose h/rate doesn't increase normally when exercising.

I have read on this forum that Biotronik devices need to 'learn' your activity patterns. Therefore, the advice from your own techs may be deferring to this (?) I can't comment with any certainty as I don't have a Biotronik device and my own knowledge is limited to what I have read about them.  Check with an owner! 

Re: Is your pacemaker working?  

If you have CHB and no longer have the symptoms that you had pre-implant that is a fairly good indication that your PM is working. If you'd like proof 'in writing' request a copy of your interrogation print outs. Pacemakers report on the % of ventricular pacing used to keep your h'rate steady and effective.  This % VP statistic should reassure you that the pacemaker is working and pacing your ventricles to prevent the symptoms of CHB. 





Sounds like more than CHB

by crustyg - 2023-11-04 16:44:50

In theory, simple CHB is fixed with a dual chamber device - the atrial lead senses the normal SA-activation and bypasses the CHB by sending the activation (with a slight delay) to the RV.

It sounds as though your SA-node isn't responding to your exertions with an appropriate increase in HR: the quickest way to prove that to your EP-team is a Bruce protocol treadmill test - but be warned, if you have chronotropic incompetence, you won't enjoy it!

With only the original CHB diagnosis, your EP-doc won't be keen on enabling CLS.

Penguin is quite correct that CLS takes time to learn your patterns of exertion and heart contractions - but IIRC it has to be enabled first - see above.

I think you need to go back to your EP-doc and explain your symptoms.  Focus on what you felt, not the numbers. Many of us here have heart conditions that progress and change over time, so it's entirely possible that while CHB was the only electrophysiology issue that you had, that may no longer be true.

Take Crusty's Advice

by Penguin - 2023-11-04 18:29:15

Hi again, 

CI is a mystery to me! I have this diagnosis and there are others who have CHB on this forum who also have it. It helps to fill in your bio or list all conditions please to avoid confusion. 

To explain very basically CI means that your heart will not raise it's rate appropriately when you exercise. E.g. Pre pacing my own h/rate wouldn't budge above 50-60 bpm when exercising and this showed up on several holter monitoring tests. Did you get any of these tests pre-PM and did they show inappropriate h/rates when exercising?  If not, as Crusty points out, CLS will not get programmed for you. 

If you have no CI diagnosis what you're describing 'could' be something else like a decrease in your fitness levels since having your device implanted. 

Take Crusty's advice and speak to your doctor and obviously, yes get it enabled first if CLS is thought to be necessary! 

Best Wishes




Re: CLS - thanks for the responses

by Valiant - 2023-11-05 18:19:02

Thank you both for your interest.

A few points:
Noted suggestions about asking Biotronik rep/clinic for advice and data.   The National Health Service in the UK is wonderful in many ways - my diagnosis and pacemaker insertion cost me not a penny - but it's very paternalistic and resource-limited.  There simply is no facility for getting attention to ask questions about things that aren't life threatening.  So I am not sure that this will be possible, but I'll look into it.

I guess CHB is complete heart block, but I don't know what is SA-node, EP-doc or CI.  Is there a glossary available somewhere?

I've filled in my Bio as suggested.   My only symptoms were that, when running or walking hard, I often had to stop and felt very faint.   I can't recall having this since the pacemaker, though I seem to be out of breath a lot.

I ran my first 5k without stopping on Saturday.  Took me ages (42 mins, pre-2023 I was around 33 and was as fast as 27 5 years ago), and ache like Hell but feel very pleased with myself.

Thanks again!!

NHS & Bio & Terminology

by Penguin - 2023-11-06 05:59:11

Hi Valiant, 

Firstly well done on your 5K run and significant achievement. I'm impressed that you did this.

Secondly apologies for the use of initials and terminology.  There is a glossary of terms under the 'Learn' tab above.  However, I agree that it's not helpful to bombard people with these things and I'm guilty of that so I'll try to hang the three misunderstood terms together for you. 

SA node = sinus node (sino-atrial node) - this is the hearts natural pacemaker / node at the top of the heart.  Your problem is with the AV node.  The SA node was mentioned because patients with Sinus Node disease (rather than CHB) often get diagnosed with CI / chronotropic incompetence.   

CI = Chronotropic Incompetence. CI means that your heart rate doesn't rise appropriately - often with exercise.  A CI diagnosis triggers the techs to programme a setting called rate response. 

RR = Rate response. Rate response is a pacemaker setting which increases heart rate (hopefully) when appropriate.  CLS is Biotronik's form of rate response. 

EP Doc = Electrophysiologist.  A cardiologist who specialises in electrical conduction issues.

Thank you for filling in your bio.  If CI has not been diagnosed it is unlikely that your techs will have programmed CLS for you. 

Hope this clarifies the responses a bit. 

Best Wishes

In a perfect world

by Gemita - 2023-11-06 06:17:10

Valiant, your response:  "There simply is no facility for getting attention to ask questions about things that aren't life threatening" made me chuckle.  In a perfect world we would all sit down with a list of our questions and only leave the consulting room when our questions had been answered, but in today's world, this just isn't possible. 

I feel it is important for the patient to focus on a few essential questions only that can be answered in an all too brief consultation and/or find out who to contact to get further information about their pacemaker or settings. 

What I am doing now is emailing my pacing clinic with any questions I have.  They explain everything to me and I will then have it in writing.  Remember electrical disturbances are so difficult for the majority of us to understand and so are pacemakers and their settings.  We cannot learn about our condition overnight.  It is a process that can take years.  What is important is that we deal with any symptoms that worry us and focus on getting to the bottom of these, rather than trying to understand what our pacemakers, settings and heart condition means.  Work with symptoms first.

Glossary of terms?  Yes there is a Glossary of Abbreviations.  Go to Learn, then Abbreviations and you will see a list of them.  They are in the process of being updated.  We are all guilty of using abbreviations which mean precious little to newcomers.  To answer your questions:-

What is an electrophysiologist?   They are heart specialists with a deep understanding of your heart’s electrical system. This is what makes your heart beat and circulate blood to your body.   A type of cardiologist, they can do testing to find which area in your heart is causing a problem with your heart rhythm (like heart block and bradyarrhythmias).  Many consultant cardiologists in my hospital in the UK work as both cardiologists and electrophysiologists, although some cardiologists specialise in the structural side of the heart looking after valves, coronary artery disease for example as opposed to looking after patients with electrical disturbances.

CI = Chronotropic incompetence is the inability of the heart to increase its rate commensurate with increased activity or demand.  This is common in patients with cardiovascular disease and produces exercise intolerance that can impair quality of life.

SA node:  the heartbeat begins at the natural pacemaker of the heart – called the sinoatrial node.  This is located in the upper back wall of the right atrium and is made up of cells known as pacemaker cells.  These cells can produce an electrical impulse known as a cardiac action potential that travels through the electrical conduction system of the heart, causing it to contract.  In a healthy heart, the SA node continuously produces action potentials, setting the rhythm of the heart (sinus rhythm), and so is known as the heart's natural pacemaker. The rate of action potentials produced (and therefore the heart rate) is influenced by the nerves that supply it.


Glad to hear that you had a good run on Saturday.  

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