HRV and SPO2 accuracy on Sense?

[hawaiichica]

Just got Sense. The resting heart rate measurement seems accurate at rest, however I have concerns about the accuracy of the HRV and SPO2. For SPO2, I have a separate Pulse Oximeter. When compared, the Sense reading is always a bit lower. So far, the HRV result I've gotten is very low. Considering the averages for my age and my overall health, this seems very off, and I will be testing HRV on another device. Has anyone else noticed these issues?

Moderator edit: updated subject for clarity and label.

[Laly04]

I’m first user of Fitbit devices.
Yesterday i got the new Sense, and also the subscription for Premium. 
I am not getting the reading for HRV nor for breathing rate in the Health Metrics section.
I’m not sure if there are settings I need to do in order to get those. Also, not sure if this has to do with the area I’m in (Canada)

[DrJWo]

It needs to collect 3 days/nights worth of data to establish a baseline. 

[Laly04]

Thanks for confirming!

[SunsetRunner]

Hi @hawaiichica - Good question. Please keep in mind that when you use the Pulse Oximeter, you are taking a snapshot of the SPo2 at that moment. The SPo2 clock face uses sensors to estimate your blood oxygen saturation (SpO2) while you sleep and reports on that. From what I’ve read, our oxygen saturation levels are lower when we sleep so this would explain the difference in your readings. 

[mfs800]

I've read that SpO2 readings on the wrist is typically lower than the fingers.  It's good at detecting changes, but will always differ from fingertip readings.

[SunsetRunner]

Mine has been on the 20s and now today it's at 18. I can tell you that it is definitely not working or I would be dead.  I'm really glad I pay extra for this watch to scare me to death and send me into a full-blown panic thinking I could possibly be dying. 🙄

[N8teGee]

@SunsetRunner I was under the impression (and looking at the user manual - here) that Fitbit wouldn't report an spO2 reading below 80%.

"Fitbit SpO2 does not measure or display
SpO2 values lower than 80%. Any measurement that is lower than 80% will be displayed as
“<80%”." 

Could you please post a screenshot of where it shows your spO2 being in the 20s and 18%.

Thanks. 

[jinks]

The HRV is extremely simple to calculate (assuming their heartbeat timing is accurate to begin with) so I'm assuming Fitbit's data is accurate. Bear in mind also that HRV can be low in some people and they can be perfectly healthy, it's not a 100% reliable statistic.

[hawaiichica]

They may be talking about the HRV reading. These numbers (18 and 20 ms) are very low for HRV and similar to the readings I've been getting. They're likely inaccurate considering that the average HRV for younger people is between 50 and 100 ms... 

[wookietim]

When it comes to HRV, I don't know. But I can say that the SPO2 sensor seems to come up with numbers that every other option I have to measure that agrees with. So I will take that it is within the good range of accurate - certainly not as good as medical instruments that would cost me multiple thousands of dollars but good enough to get a decent idea of that health metric.

[wookietim]

I think it takes a couple days to start measuring the HRV stuff. And remember - to get measurements of the SPO2 you have to be using the SPO2 watchface while sleeping for that. For whatever reason Fitbit decided to make the watch measure that only while that watchface is in use and you are sleeping to get that measurement. I suspect it might be to save a bit of battery life.

[Giampi71]

For sake of clarification @SunsetRunner is speaking about HRV, not SpO2 (see here), as correctly guessed by @hawaiichica 

[SunsetRunner]

I'm definitely talking about HRV. Those reading's haven't gotten above 24 since it started calculating it over a week ago. 

[imjrode]

I've definitely thought the HRV numbers are off, as well. I'm thinking this is an algorithm issue of some sort, because I've found the HRM to be pretty accurate while resting.

My HRV was apparently 25ms last night. That's super low. I have a RHR of 60 bpm, for the record.

[Pegomas1]

I think the HRV data is complete nonsense on the fitbit. The versa 2 actually gives me fairly accurate Heart rate even during exercise, so I can only think it is the algorithm that seems to be the issue. The day after having a few drinks I actually got a slightly better hrv score which is the opposite of what would normally happen. 

[fjcruzer]

I believe HRV will vary based on how they calculate it.  With a chest strap HRM using EliteHRV, i'm typically 45-60.  But with the sense and my oura ring, it actually has them at similar which is typically around 26-32.

[mfs800]

My SpO2 readings on the Sense are consistently about 4 points lower than readings taken with a finger pulse oximeter.  

[SunsetRunner]

Same here. The sp02 is off and so is the hrv. 

[t.parker · Accepted solution]

HRV and HR are two different things in terms of calculation. The sensor must deliver R-R intervals which require not just good accuracy but excellent if the data is going to be useful. I'm not sure how much data Sense provides (just number?) but without knowing the raw data it's same accurate like the sleep score and any other made-up metric. I've been using EliteHRV app which shows all the cards to the user and so far the app uses only two tested sensors which have been made with measuring HRV in mind. Here's how much data can be obtained for HRV (it's not everything):

It is important whether the algorithm uses RMSDD or SDNN to compute the HRV (each parameter works different and it depends on the context which one suits better, it's important whether it's a long measurement, morning measurement etc.). Some watches don't provide R-R intervals and as much as their sensors are good for calculating BPM and RHR, they are useless when it comes to HRV. Some sensors provide R-R intervals but smooth them trying to filter out the noise coming from the readings (optical sensors gather lots of noise). Without knowing which path Fitbit decided to take and how they calculate HRV I wouldn't use it for anything serious. This goes to any other wristwatch with claims of calculating HRV without exposing data and methodology 🙂 HR is easy to capture and measure, HRV isn't. Without Fitbit being open on how they compute it and how the data is obtained, HRV on Sense is going to be just a gimmick for rather casual use.