Heart rate tracking
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Heart rate variability
HRV is defined as the changes in the time interval between successive heartbeats.
People are interested in HRV primarily because it is regulated by the autonomic nervous system (ANS), which also regulates heart rate and blood pressure. This means that HRV can potentially provide useful information about questions pertaining to our overall nervous system, including information about stress, rest, recovery, and immune response. The two components of the ANS, the sympathetic and the parasympathetic nervous system, both influence HRV.
Since HRV measures the change over time of the interval between heart beats, there are many options available for producing a value relevant to a specific question. The heartbeat itself is not instantaneous, of course, but for the purpose of HRV measurement, the time is measured between ECG peaks known as R-R intervals. However, many people have irregular heartbeats, and so HRV is better calculated using NN intervals; that is, the time between ECG peaks when the heart is in a normal rhythm. Four common ways of determining HRV values, with their commonly used associations, are:
- Standard Deviation of NN-Interval (SDNN)
- Risk of mortality in cardiac patients
- Standard Deviation of 5-minute averages of NN-Interval (SDANN)
- Predominant circadian rhythm
- The Root Mean Square of Successive NN-Interval Differences (rMSSD)
- Vagal modulation of heart rate.
- The Percentage of NN-Intervals that Are Greater than 50ms Different than Prior NN-Interval (pNN50)
Any instrument that reliably provides continuous heart rate data can be used as a basis for calculating HRV. Of course not all instruments are equally reliable and convenient, but the suitability for HRV is determined both by the instrument quality and by the situation of measurement. Less precise instruments can be used when the measurement context is well controlled.
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Tools to track heart rate
There are many tools that measure and record heart rate, aka pulse, either continuously, during an exercise session or periodically throughout the day. There are lots and lots of such devices and so not all have been evaluated. Heart rate is most common vital in Biofeedback.
Bands and Watches
Most smartwatches and fitness trackers that are worn as wrist-bands provide heart rate data in one form or another. Virtually all of them do so through Pulse Oximetry and try to avoid measurement artifacts through the use of accelerometer data. Cheap devices might actually not be able to reliably read heart rate from the pulse oximetry and rather "make up" heart rate "readings" solely based on the amount of movement as measured by the accelerometer. Widely used wearable devices that include heart rate data are:
- Fitbit's have many studies using them and Quantified Scientist says that they are one of the most accurate ones. Fitbit devices typically record continuously while worn.
- Apple Watch of all generations include a heart rate sensor. During normal wearing they only sample heart rate data every few minutes but provide continuous data during workout recordings. Additionally, more recent models also include the option to record ECG.
- The Xiaomi Mi Band are a cheaper option. But according to DG's (talk) experience with the miband6 they are still not perfect at measuring HR during exercise.
- The Polar Verity Sense sits on the upper arm.
There are devices that also use pulse oximetry but are worn on a finger, rather than on the wrist, in this sense they are similar to medical devices that use pulse oximetry.
- The Oura Ring mostly records heart rate data during rest and sleep. Since the 3rd generation of the device it also measures throughout the day and can additionally support workout recordings.
- Contec Medical System 50F/CMS50f. According to DG's (talk) experience, they are bulky to wear and not accurate during movement but accurate during rest.
- Wellue (not recommended by DG (talk))
- The Biostrap is rather expensive ($400) but includes two heart rate sensors and many others.
- Welltory  Can detect HRV when finger is placed in front of phone camera.
Chest straps are considered the gold standard for measuring heart rate during exercise but do not work well for continuous monitoring in particular during sleep as they are typically not comfortable to wear in bed.
Many manufacturers offer chest straps, including:
Few Lead ECG Patch
These devices make us of electrocardiograms and are based on electrical pulses. Some devices are designed to only record spot-measurements, while others are designed to collect longitudinal data. For these latter to work they have to be temporarily glued on to a patch of skin, typically on the chest.
Due to their design these are more comfortable to wear than chest straps and can be used for recording data during sleep, but as they require to be stuck onto the skin with temporary glue, these might be less appropriate for continuous longitudinal data collect for more than a few days at a time.
As such they might be less appropriate for long-term data collection, similar to chest straps.
There are a few options for few/single-lead ECGs:
- Omron KardiaMobile EKG, a small device to do spot-measurements, not continuous recordings
- uECG Open Source ($100), worn on the chest with sticky patches, records continuously
- Hexoskin ($600), includes many other sensors.
Medical Multi Lead Holster
Similar to the few-lead ECG patches above but these are typically tested medical devices that are used for long-term ECGs where the device is worn for up to 1-2 weeks. They are expensive and very bulky and less accessible for individuals outside the healthcare system.
- Contec makes one for about $400, can sometimes be found on ebay or alibaba
Tools to Track HRV
QS forum list of, likely deprecated, ECG devices.
|Company||Model or Series||Release Date||Metrics||Sensor||Still Produced?|
|Apple||Watch Series 4+||September 12, 2018|
|Fitbit||Versa||Optical heart rate monitor|
- Pasadyn SR, Soudan M, Gillinov M, Houghtaling P, Phelan D, Gillinov N, Bittel B, Desai MY. Accuracy of commercially available heart rate monitors in athletes: a prospective study. Cardiovasc Diagn Ther. 2019 Aug;9(4):379-385. doi: 10.21037/cdt.2019.06.05. PMID: 31555543; PMCID: PMC6732081.