Recently I came across a relatively new product that has the potential to be a huge advance in both heart rate rhythm and heart rate variability metric investigation. If I had a wish list of features for a device encompassing the above purposes what would they be?
Most importantly it would be precision of RR interval detection which is essential to accurate HRV. I have been very frustrated with the high relative artifact occurrence (usually missed beats) that the Polar H10 reports. This is also present in HRM belts from Garmin and Wahoo which I have also tested. Missed beats can affect several HRV indexes especially the non linear ones such as DFA a1.
Another function which relates to understanding artifact type is the ability to visualize the ECG tracing. Visualization is helpful not only from the standpoint of manually correcting a tracing for more accurate HRV, but knowing if the aberrant beats are dangerous or not. You may see large amounts of "artifact" in the RR series, not from noise but from a potentially serious cardiac arrhythmia.
Lastly, it would be nice to have a device that is easy to put on, lightweight and inexpensive. Although I love the Hexoskin, it is costly, each shirt has a finite lifespan (in washes) and takes some effort to gear up.
Well, the wish list is no longer a wish.
The Suunto team have put together a fantastic, tiny device that does all the above and more. It is officially know as the Movesense Medical Sensor:
(Edit 6/25 - see the bottom of the page for features of the Movesense HR+)
The price is about 180$ USD.
My unit arrived about a week ago and this is a short review (part 1) of it's ECG functionality so far.
The module mounts on a standard Suunto chest belt strap and is controlled by either an android or IOS smartphone. I use a Samsung S20 to collect data. The IOS app is in the app store, the android versions are online here.
Install the app, put the sensor on your chest in the standard location.
Let's go through the steps to acquire data.
First - after opening the app, connect to the module:
This may come up straight away and gives you the choice of which "sensor field" you want to record.
The one below is the main menu if you hit the back button of the above screen. To get to the sensor list choose the Movesense button option. Ignore multi connection, DFU (firmware flash) for now. Saved data is usually in the Movesense folder on the root level of storage, but your phone variant may be different.
You will want to scroll down the sensor list to ECG:
Choose the ECG option, then you will see the screen below:
There is only two functions - Subscribe and Rate. The Subscribe switch starts and ends the recording. Before doing this you can change the sample rate of the ECG from 128 up to 512 Hz. The higher rates will tax your Bluetooth bandwidth so your success may vary. My S20 has no trouble at the 512 option.
Once you start subscribe the following real time display occurs:
This is not a random animation - it truly is your single lead ECG.
Once you are done with your measurement session, just switch the subscribe button off.
The files are in the Movesense folder as noted above.
There is a problem (as of this date/version) where the file name contains ":" and "." which can confuse the Windows OS. The files can copy to your Dropbox or a PC, but they won't show up (they are invisible) unless you change the file name in android first. So get the extra characters out in android before the transfer.
The files are remarkably simple:
This data stream is going to turn into a waveform with the timestamp as the X axis and the Count as the Y axis (shape and amplitude of the wave):
This is a section during an indoor trainer session at about 155 watts:
The red arrow on top is an APC, with the yellow marker possibly showing an inverted P wave from a more junctional area of conduction. The red arrow lower down is the "artifact" in the RR series. If we did not have the tracing, it could have been noise, APC, VPC etc.
Having the visualization is amazingly valuable.
So yes, a neat little single lead ECG system - but how does
it perform outside on a bumpy road at high intensity? Below is a section of a ride I did yesterday on a bumpy hill with a power between 300 and 400 watts over about 2 minutes.
One is the lack of any "drop out" artifact - a similar hill using a Polar H10 looks like this:
Notice the 7% artifact rate (yellow) with the Polar H10. Each one of those vertical lines represent a missed beat (so a doubling of the RR interval).
The Movesense had none of this.
The second observation is the near perfect waveform despite substantial body motion. This seems to rival the Hexoskin in both RR tracking and ability to visualize the ECG.
Relevance to HRV monitoring?
The following is an example of how a visualized waveform can help correct artifact and noise in the RR sequence. In this case, the pre corrected series had a misplaced R peak due to some noise (yellow marker). Kubios correctly determined that the RR sequence was out of sync (the red arrow on the bottom with the blue vertical line).
We then manually change the R peak over to the correct position as shown below (yellow, but no blue line on the bottom anymore). Now, the bottom pane no longer has a marker of an out of sync RR series (red arrow where it was before correction).
The result is a correction of the noise artifact and improvement in HRV metric precision/accuracy.
Usage in arrhythmia?
A recent article was published evaluating the Movesense module compared to a standard cardiac ECG recorder for the detection of atrial fibrillation. They found excellent concordance with chest belt module to the gold standard Holter device:
Importantly the quality of the P wave detection was excellent, with very few samples having a poor rating:
What about Premature Ventricular Beats.
Although way beyond the scope of this blog and post, if you do come across wide complex premature beats, a more through evaluation may be needed. Although they can occur in healthy subjects, increasing numbers during exercise can be cause for a cardiac evaluation.
Here is a flow chart from the link above:
Here is an example of 2 wide beats that I found in a tracing at rest (I've never seen any with exercise):
- There is a full compensatory pause, ST depression associated with the positive R wave, no P wave and a wide QRS complex.
- The above was recorded with the Movesense module illustrating it's potential for arrhythmia detection at rest as well as during exertion.
How does it track heart rate?
The module will also record to a Garmin watch (or other head unit) as a regular HRM. Simply, do not start the Movesense app, but put your Garmin watch in search mode and it will find the module as a HRM. Start recording on the watch and you are good to go. To switch recording back to the phone (for ECG), stop the Garmin watch and exit the activity menu, start the Movesense app and begin the "subscribe" process. Remember - we are still limited to one bluetooth connection at a time - you can't do both Garmin and a phone app recording simultaneously.
Here is what the Movesense module displays to the nRF connect app on android:
Therefore, you are able to use the module as you would a standard HRM, but with enhanced accuracy and potential waveform analysis.
Here is a comparison of the Movesense module recorded by a Garmin watch, with the Hexoskin worn concurrently:
- The tracking is pretty much identical. A scan through the RR intervals showed zero lost beats.
- This was far different than the H10 ever performed.
- Although the module is very usable at the moment, it is still a development device and there are some issues.
- There are occasional app crashes with loss of data. I believe stability will improve as the code is further debugged. If you need a section for analysis - keep the recording as short as possible. Or, split up the recording into separate files by start and ending the subscribing.
- File name forbidden characters as above. Edit before transfer to a PC
- Some files can't be opened by Kubios - an error is thrown. However, the file can be read by Excel and I am in contact with tech support for a fix.
- The Movesense Medical module is a lightweight, single lead ECG in a chest belt form factor.
- It has an amazing ability to accurately track both the RR series as a simple HRM as well as showing a stable ECG waveform despite body motion during intense exercise. This feature is very attractive to those interested in monitoring HRV metrics during exercise. Not only can it track without drop outs, it has a very readable waveform for both arrhythmia tracking and artifact correction.
- An advantage over the Hexoskin (besides cost, ease of use and longevity) is that the visualized waveform is shown in Kubios. Although the Hexoskin waveform can be inspected, you will need different software to do so and you will not be able to manually correct artifact (as shown above).
- The cost is competitive with standard chest belts given it's ability to track RR intervals. If compared to single lead ECG systems, I don't know of any that can compare in size, function and price.
- The development team has been very helpful and cooperative. One of the programmers went to the trouble to fix a bug in the android app within 24 hours of me asking (thank you, Petri).
- In a future post, dynamic HRV during exercise intensity will be evaluated.
- This is potentially "game changing" technology for both research groups, sports teams, amateur athletes and health care investigation.
The modules little brother (or sister):
The Movesense HR+ is another option on the Suunto website. It is advertised as a heart rate only sensor:
I decided to order one as well, on the chance it would record the ECG waveform (even though it was not in the specs). Well, guess what - it does. The sensor recording option appears similar to the Medical sensor (in the showcase app). Below are examples of the waveform from the HR+. The appearance is not optimal since I was wearing the other Movesense module next to it and there was possible chest strap interference.
Here is a normal series of QRS complexes:
- The HR+ is definitely capable of a good single ECG waveform.
- The device is a steal at the list price.