Optimising a Charge 5 for Bradycardia

[WillPegg]

Hi Guys, 

I originally bought a fitbit to track my bradycardia (low heart rate). My heart rate often drops down into the 35 to 40 range, probably norms in the low 40s. Unfortunately fitbit cannot read a heart rate below 40 (during the day) as soon as it drops into 35 to 40 range it starts reading each beat as 2, so I see it record a rate of 78 I know it's actually 39. I've confirmed this with the ECG app which does seem accurate and complains it cannot run an ECG because my heart rate is too low, then tells me it's 39 while the main app is happily displaying 78. Interestingly when it thinks I'm sleeping this threshold for doubling drops to 30 which suggests there's a software kludge in place specifically to account for this and they had to drop it for night time. 

Ok, so fair enough it's not really what the fitbit is for and I cannot really complain given my HR is an outlier (I would note that it allows an alarm to go off when your HR drops below 40, but this can never actually happen except when sleeping - when it wakes me up to tell me). My question is how I can get accuracy out of the rest of fitbit. It seems to record zone minutes for fairly trivial exercise, I clocked up 94 today for a 50 minute brisk walk. It's recording my resting heart rate at 40 for the last week, so I wonder if this is lowering the threshold for what constitutes a zone minute. I also wonder if the 39/78 thing is distorting my heart rate variability but I'm not entirely sure what that really is or how it's calculated.

Does anyone have any advice how I should configure this in order to provide useful feedback, or is it actually doing so already? Any general advice on what configurations I should tweak would be appreciated.

[JohnnyRow]

It would have been interesting to me to know if your interest in tracking your bradycardia was from a primary care provider, a heart specialist, or just on your own.

Anyway, I challenge your initial conclusion that fitbit cannot read a heart rate below 40 awake, or below 30 when asleep, and your confirmation with the ECG app. The basic heart rate monitoring and the ECG app are measuring 2 different things.  The heart rate monitor is meant to measure the blood flow (i.e. pulse) through the wrist, though admittedly it is not always perfect at that.  The ECG however measures the electrical signals of your heart.  Normally with no heart rhythm abnormality, these are the same. But with some arrhythmias, they can be different.  One possibility could be that when your pulse (blood flow rate) gets that low, your heart is actually getting 2 electrical signals but only sending out blood once.

I am not a doctor, and certainly no medical expert, but I do know from personal experience, with some arrhythmias, the heart's electrical signal can circle one chamber, where it originates, a few times before passing into the chamber where it tells the heart to contract to pump blood.  So heart rate as measure by electrical signal might not concur with heart rate as measure with pulse as measured by blood flow.

As I said, I am no expert, and I might have gotten some details above wrong, but I suggest you see your doctor about this, if have not already.  But maybe you have been, and are just following up on doc's advice.

[WillPegg]

Hi Jonny,

My doctor (and private specialist follow up) have run all the tests they need (echocardiogram, 24hr tape) and concluded the bradycardia is a sinus bradycardia (normal rhythm, just slow) that I just have to live with, they have not encouraged me to follow this up in any way. I was originally keen for personal interest to use the fitbit both for a micro tracking of how low it goes at various points, and also a macro tracking of my average heart rate as I varied diet, exercise levels, etc. Unfortunately the issues tracking the rate during the day prevent me from getting this use from it.

Regarding my conclusion, it's not really contestable that one of the two is wrong, you just seem to be placing the blame on the ECG rather than the blood flow monitor. Both are reporting to me a heart rate (how they measure it is their business) and they don't agree on the answer so something is clearly incorrect. I can watch the monitor tick gradually down to 40 then spike up to 78 meanwhile my pulse (which is very easy to read when you've a bradycardia as each pulse is quite significant) is quite clearly not ticking at over 1 per second and the fact it's doing this while I'm just sitting down makes it all the more suspect. Your suggestion that there may by increased electrical signals would make the ECG higher yet that is consistently tracking inline with my own measurement, it's the blood flow measurement that is incorrectly high. 

[LZeeW]

@thomsas0   My background is physiology and biomedical engineering.  Your issue is the optical heart rate monitor (OHRM) and its limitations.  Fitbit used to quote an accuracy range for the OHRM, both awake and asleep.  I think it was removed from the Help article because there are different OHRM models in different Fitbit models.  This user sees the same thing in a Garmin model.  I use medical grade OHRMs and I see double heart rate counting from time to time.

With respect to your HRV question, double counting will interfere with your HRV calculation.  HRV is calculated from the R-R interval on an ECG.  An OHRM substitutes the flow peak interval as its R-R interval.  It can't calculate HRV correctly if it doesn't measure your heart rate correctly.  In the science world, we say garbage in = garbage out.

Fitbit uses your RHR in the zone minute heart rate calculation.  The equation is in this lengthy Help article. 

The ECG records a Lead I ECG.  My RHR isn't as low as yours, but I always get inconclusive because my heart rate is low, too.  "Trained eyes" can easily see a normal sinus rhythm.  I suspect Fitbit had to put in a range in order to get FDA approval and chose 50 bpm based on the accuracy of its algorithm.

I honestly don't have a magic answer for fixing your heart rate issue.  I guarantee that the usual troubleshooting recommendations won't work.

[TomSq]

Hi Will,

I found the thread you have generated very interesting. I am 3 days in with a Charge 5, and have sinus bradycardia with a heart rate often 30 to 35 in the day time and occasionally lower during the night. While awaiting further treatment to try and increase my nighttime heart rate i bought the Fitbit to give me peace of mind that I at least knew roughly what my heart was up to. I immediately noticed these daytime spikes, and figured it must be a simple doubling - but that just seems very lazy by whatever algorithm they use! In the night it allows a minimum of around 35 but you can see on my chart that it is clipping my low heart rates. The ECG function does exactly the same for me. Reading your post was as it i had written it for my issue. Question is, do other trackers perform better? Or will the fitbit software change in the future to accommodate us ‘outliers’ ?

Cheers! Tom

[WillPegg]

Hi TomSq, 

Thanks for the reply, it's useful to know that other people are also experiencing this issue. I also notice a 35 lower bound while sleeping, I see a 40 lower bound during the day. I'm afraid that I don't know about other trackers, my suspicion would be that the same problem will occur, one link in this thread was to a Garmin device user with the same issue. I can tell you that this problem will persist, and you can ignore Fitbit support's advice to give the device some time to get used to you. The Fitbit Premium features are all rendered useless by this problem, the sleep tracking will think you're awake for periods when you're actually asleep because the HR is doubled, the readiness score will be all over the place because of the inability to correctly assess your heart rate variability. I cancelled my premium subscription but stuck with my device on the decision it was only useful as an exercise tracker, and to roughly track trends in my resting heart rate (the number may not be correct, but I think the change in value is representative). So I'm sorry to say it has not improved and I've not seen any indication from Fitbit that they even acknowledge this problem, so it's doubtful we'll be seeing anything from them.

Best of luck with your further treatment, I hope things turn out well.

Will