Heart rate stays low or drops during exercise

Hi im new to this group - thankyou.

FYI male 60+, 96kgs, regular road cyclist average 10,000km pa for some years now.. Im not elite, dont race anymore but enjoy strong, safe and fastish group rides.. I used to have strong powerful mid game on long rides, but not a climber or sprinter.. 

Two months ago I had a biotronik endora 8 dr-t implanted after 18mths of denial and tests.

My problem was that after cycling for approx 45min my heart rate would drop from a comfortable say 130 - 150 to below 100... after another 10 - 20 min my legs tired to the point i had to reduce intensity and speed altough had no real increase in respiration just a drop in HR.. Weekend rides were often 2-3+ hours and weekdays 1.5hours. It was frustrating that i could comfortably lead out then have to drop off the back after 45min or so... I never felt nauseous or lacked control, just no power and sometimes a few seconds of feeling faint before the HR drop... This was worse if there was any incline.. 

Tests say i have a constant 1st degree heart block with occasional second degree heart blocks (type 1 & 2)... (for as long as i can remember I have had missing heart beats particularly at rest..)  Before the pacer my resting HR was typically 50ish and in the months leading up to the implant was often unusaully in the low 40s.. Despite a number of stress tests and wearing a holter for a month nothing was detected apart from ECGs showing the heart blocks.. Valves, heart size, wall thickness & function all OK..

Since the implant i have noticed i dont have dizzy spells when i get up in the morning or after intense exercise, however my heart rate (that is now set to be 50low and 150max) is typically 50-51 low but rarely above 130 high, in fact some days despite effort it wont go above 100 for most of the ride.. Also it is rare now that i can have the luxury of a steady increase in HR to say 130 - 140 without it falling to 60-90 after 45min the back up to 110. I have found the way to get it back is if i stop, rest for 10min and start all over... Since the pacer chest straps dont seem to work anymore so i use my garmin fenix watch.. that i have checked manually when i stop and appears to be accurate. 

Since the implant i have seen my cardoligist once who reported that the pacer is working regularly and is doing its thing although had no answer for the exercise issue.. We agreed to give it a bit more time and i will see him again in about a month.. I as wondering if anyone knows of similar conditions or can suggest what i can ask the cardolgist.. 

In reading about the Biotronik - it has a "Closed Loop Simulation" function - ill ask if that is working..

Thanks again Q




by Tracey_E - 2023-09-19 12:01:58

First of all, if you cycle, Biotronik is an excellent choice. Most others depend on movement to get our rate up when it doesn't go up on its own, and with cycling there is minimal movement. 

With heart block, it is presumed that the atria is beating normally and the pacer is only needed to keep the ventricles in sync. What happened to me was I got the pacer so now my ventricles are keeping up with the atria, and suddenly we learned I had some minor sinus issues also. The block masked it so we had no idea until the block was fixed. So, that may be what's happening with you.

Ask if CLS is turned on. If you are only diagnosed with heart block, sometimes they don't turn rate response on because it can compete with our natural heart rate. 

CLS learns you, so it works better over time. If it is turned on, it may just need more time. If you do a search for CLS, this topic comes up fairly often. 

If they can't figure out what's going on, a stress test can be useful so they can watch what is happening.

Last, if your rate should be 130-140 but it's only 100, back off and don't push harder until you figure out what is going on. 

Good luck! Know that it's pretty normal to take a few tries to get the settings fine tuned for athletes. We aren't as simple as couch potatoes ;o)


by Gemita - 2023-09-19 12:31:57

Quinny, welcome and thank you for your post.  I think many of us can relate to denial and needing lots of tests to convince us that we needed a pacemaker.  I certainly couldn’t believe what I was being told and that I would also need anticoagulation medication for the rest of my life to prevent a stroke, but that is another matter (Atrial Fibrillation) and not fixed by a pacemaker.

I think you are getting closer to the problem when you mentioned closed loop system (CLS).  I know it can be difficult to adjust for exercise and you will need to find an experienced technician to work with to optimise this setting.

I have a Medtronic pacemaker so cannot help you with the CLS.  I have heard if it is not sensitively adjusted it may cause the symptoms you describe and lead to sudden rate drops while exercising and then the rate refuses to increase.  Something is clearly happening when you hit your maximum tracking rate.  There are a few other things which could be happening too, so ask if possible for a manufacturer representative/technician to also be present while you are having your checks.

Missing heart beats, particularly at rest, sound like ectopic beats (either premature atrial or premature ventricular contractions).  They may be more noticeable while you are getting used to pacing.  I find that a higher lower heart rate setting can be very helpful at first to prevent these from occurring or using a setting like Rate Smoothing if you have this feature?  You could ask about both these possibilities if you are having too many ectopics.  I am also taking a low dose beta blocker to calm my heart rhythm disturbances.

Otherwise I hope you are doing well?

See if you can get your EP-team to organise a tuning session

by crustyg - 2023-09-19 12:52:41

Also a road cyclist, and although he's a Medtronic-man, my EP-doc implanted BostonSci for the MV to provide a feed into Rate Response.  Not perfect but pretty good.

My local big hospital EP-techs tell me that they've heard good things about CLS (in Biotronik PMs) but it does require careful tuning for each patient, and there are several contributions here from folk with Biotronik-CLS 'issues'.

Ideally you'll have a session on a static bike while they make adjustments, you ride for 10min and see what sort of HR you achieve, and then further adjustments etc.  My session took a full hour.

Best wishes.

Had similar issues - post implant due to Sick sinus

by Evan1510 - 2023-10-22 06:01:09

Hi Guys - first post here. 

Had a boston implant October 22 - after sinus node decline over 5 years. Competitive athlete/cylist averaging 12-15k per year. Was getting abrupt drops in heart rate from say 140 - 150 down to 80 or 90 a few times a month then started almost every ride which forced my Cardio's hand to do the implant. Now on shorter training rides of 90 minutes or less I have no problems - my upper limit is set at 160 which I frequently and and no problem - any rides I do over 2 hours when I coast and recover my rate drops to 80 or 90 but then it doesnt recover back up to my tempo rate of 120 - 130. The rate can hover below 100 for 40 minutes or so then it picks up again. When it drops I feel like crap - breathless and need to stop and rest. I've had a few meetings with the tech and we've tried a few things but it seems sometimes the PM won't pick up exercise - the tech told me to try tapping the unit to wake it up but this doesnt work. My shorter work outs the unit responds and I dont get drops normally - just I find the longer rides it drops after 2 hours or so - and won't kick in again. Key to my point is is there a residual time frame the unit works to in terms of bounce back and then once it hits a certain time frame - won't detect exercise like it does early in a work out. 

Follow up with the cardiologist

by Quinny - 2023-11-27 23:30:17

Fisrt of all thankyou for the comments and to anyone who shares their experiences with this group. I realise now my problem is not much at all..

Its now been 5mths since i had the pacer (biotronik endora 8 dr-t ) installed, and i have just seen the cardiogiliost.. The data says the pacer his works for 5% of beats in the atria and 98% in the ventrical.. 

Thinking back recovery from the installation took months not a few days as i was expecting,, I still get heart rate drops during prolonged exercise howerver i think im getting generally fitter, but still get the ocasional dizzy spell when i get up quickly or on the bike, otherwise OK. Been trying to walk more, done a few swims and doing some yoga to focus more on breathing and core (i have a quite a big core these days despite being active) everything i read says exercise is a great way to aid recovery.

I discussed CLS with the cardioligist who said its not turned on on my device as it works more towards failiure in the atria not the ventrical.. He believes my nervous system is working ok and has adjusted other settings in the pacer to hopefully address the heart rate drop. He says its better to use the bodies natural rythym if its there rather than invoking one. CLS can be turned on, but not for now

Interestingly he had turned off the pacer for a bit wile i awas in his rooms without me knowing and my HR went down to around 40 bpm.. (its set at min 50, I couldnt tell. he seemed suprised?

Back to see him again in March, fingers crossed these adjustments may fix the issues i am experiencing,

Thanks again 


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