The Truth about Optical Heart Rate Monitors

Optical Heart Rate monitors

Most of you have heard about the new wave of wearables that measure your heart rate at the wrist, arm or finger. There are now tens of these devices on the market including the most well known one: the Apple Watch. But what they actually measure? Are they reliable? Are they accurate? Do we need these metrics on the road of the Quantified Self?

First we need to understand the technology behind this and we refer you to our Science corner here. 

So the first question is: But what they actually measure?

There smart health wearables measure the average time difference between two heart beats using the light absorbed or reflected by the blood vessels. The full concept is that when the heart contract to eject the blood into our vessels, a pressure wave is travelling from the heart to the location of the optical sensor. So the pressure wave takes some time to reach the sensor and this time is called the pulse arrival time or PAT. This pulse arrival time is further composed of the actual time the wave travel also called the pulse transit time or PTT, plus what clinicians call the heart pre-ejection period or PEP. So we have PAT=PEP+PTT.

Illustration of the difference between the true interbeat interval (RR), the optical interbeat interval (PP) and the PAT. Adapted from Chern-Pin (*)

The PAT is continuously changing from beat to beat due to local phenomena happening both at the heart level (PEP) and during the travelling of the wave in our vessels (PTT). Depending on our physical and mental condition, these PEP and PTT can change quite significantly.

When the heart contracts at time Th1, the sensor ‘reads’ the pressure wave at time Ts1. When a second beat is happening at time Th2, the sensor ‘reads’ the pressure wave at time Ts2. The real interval between two heart beats is Th2-Th1, which is commonly called the RR interval, and the inverse of the average of many of those during some time (typically between 5 and 20 seconds) is your true heart rate.

Now, from the optical sensor point of view, he sees a first beat at time Ts1 and a second at time Ts2, thus an interval between two beats of Ts2-Ts1 which is commonly called the pulse to pulse interval or PP. That means that the heart rate ‘seen’ from the optical sensor is the inverse of the average of many of those during some time (typically between 5 and 20 seconds). But now we know that the sensor has to ‘wait’ a certain time PAT1 and PAT2 to ‘read’ the two pulses. That means that the heart rate as measured by the optical sensor at your wrist, arm or else, is not exactly the same as the one directly measured at the heat level because it depends on the PAT.

So the answer to this first question is no: the heart rate as measured at the heart level and at the periphery using for example optical sensors are not the same. However, in a quite (no external stimulus) and relaxed situation, both are quite similar and this is the reason why we prefer to measure these heart rhythms at certain time of the day when you are supposed to be mostly relaxed. In all other situations, we need to use all the informations from the pulse wave to have a complete and holistic understanding of the heart rhythm.

The second question is: Are they reliable?

The optical sensors placed on your body are very sensitive to perturbations and mostly movements. Any movements from your wrist, arm or else will impact on the waveform of the pressure wave, sometime to a point when we cannot discriminate easily the heart pulse wave. There are also many other factors that influence the quality of the ‘reading’ by the optical sensor such as the contact to the skin, sweating, colour of skin, texture of the skin, hairs, ambient light, environment and internal body temperature. All these factors will influence on the reliability of the heart rate estimation.

The third question is: Are they accurate?

When measured at rest and in the quietness of your home or a relaxing place, these optical wearable sensors are reasonably accurate and can achieve a precision of few milliseconds for each heart beat interval such as Ts2-Ts1 as discussed above. The accuracy depends on all the factors discussed above in this article. This is why SATHeart prefer to measure these heart pulse wave rhythms and patterns when at rest or during sleep.

The graph illustrates one example of difference between the true Heart rate and the optical heart rate considering an average error on the PAT of 30 milliseconds. This error is quite large in this example to illustrate the effect.

 

(*) Chern-Pin et al. “Combined electrocardiogram and photoplethysmogram measurements as an indicator of objective sleepiness.” Physiological measurement 29.8 (2008): 857.

The fourth question is: Do we need these metrics?

The simple answer is no ! Now can these metrics be useful in some sense in our everyday life? and the answer is: it depends. This statement seems a bit vague but it is actually the honest situation with these Quantify-Me devices.

The first example is a perfectly healthy and happy person. Why this person would need to check her heart rate or other health metrics during day or night? There is absolutely no reason to do that. In fact, if such a person would start to use such smart device, she would probably start to worry about her health by just seeing all these metrics changing all the time, just not being aware that it is Normal!

The second example is the person who is stressed or have difficulties to fall asleep, or is anxious or somehow feels out of balance. In this case, we would say that some devices would indeed help to refocus this person from time to time, if the metrics have real quality for her and brings about true benefits. This can be the case for work stressed people for example.

The third case is a person taking care of herself and wants to improve her capacity in terms of self-awareness by practicing mindful yoga or meditation. In this case, the device should be able to show her progress in that sense and address very specific needs such as a heart-brain-mind balance state during her daily practice and during sleep.

The fourth case is the person who is doing amateur to medium level sport. In this case, heart rate monitoring can be useful along with some various metrics that vendors are offering. Again, check the reliability of these metrics and what do they really mean and if they are useful for you. Otherwise, they might lead you to more troubles than anything else.

The fifth case is a person suffering some mild medical problems, especially linked with the cardiovascular system. Again, we recommend these person to be very careful choosing a device that will suits her need and is accurate (top quality label). FDA approved devices does not by any means prove that they have a required reliability, but is however a good certificate of a certain quality. Only FDA approved with clinical validation have this top quality label. Unfortunately, the true situation is that most FDA approved devices have no such clinical validation and if this is the case, it is hard to find the validation study.

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