The capabilities of a heart rate sensor extend far beyond counting the number of heart beats per minute. Many people don’t realize that wearable heart rate sensors, using PPG, have made significant advancements in recent years to the point of medical-grade accuracy for some use cases. In fact, some of the more advanced heart rate sensor technology is able to measure the PPG waveform with enough accuracy to detect atrial fibrillation, arrhythmia detection, and blood pressure, for example.

In reality this “new” technology isn’t really that new at all, because much of the science has already been proven with ECG technology. What is “new” is the fact that wearable PPG heart rate sensors have achieved a level of accuracy on par with ECG, particularly in low activity-level situations. As we discuss exactly what today’s PPG heart rate sensors are capable of in this post, we will see the true potential and what will be possible with wearables and hearables in the near future.

What Can a Heart Rate Sensor Already Measure?

As the technology of PPG heart rate sensors continues to improve so too does the level of accuracy. When a device measures a PPG waveform, there are numerous data sets being measured that extends far behind the number of beats per minute. Peak amplitude, the time between individual peaks known as R-R interval (RRi), the changes in frequency and the perfusion variation are just a few examples of what is being recorded.

We are then able to analyze the data to determine outputs like heart rate, cardiovascular fitness, breathing rate, blood pressure, blood oxygenation, cardiac efficiency, and blood perfusion. With Valencell’s current Benchmark heart rate sensors these data outputs can be used in wearable devices to provide the user with assessments such as:

  • VO2 Max: This is a measure of the maximum rate of oxygen consumption during incremental exercise and is a primary measure of changes to cardiovascular fitness.
  • Resting heart rate: An individual’s heart rate during a period of no exertion and be used to determine fitness. Lower resting heart rates correlate with a higher level of cardiovascular fitness.
  • Heart rate recovery: How well an individual recovers following from intense exercise, usually measured in the minute or two after exercise completion, can help to determine the level of cardiovascular health and exercise endurance.
  • Heart rate response: This metric is used to determine how the heart responds to increased exercise.
  • Heart rate variability (HRV): A measure of the variation in the time between heart beats, which can be used for diagnosing overtraining in exercise and for psychosocial stress.

Most wearables on the market today are not taking advantage of the full capabilities of a PPG sensor, but that doesn’t mean these use cases can’t be readily deployed today.

The Lesser Known Capabilities of PPG Heart Rate Sensors

In addition to the data points listed above, current Valencell Benchmark heart rate sensors can provide even more insights than is currently being utilized. Let’s take a look at some of the lesser known use cases that have been scientifically validated in independent studies.

Core Body Temperature

 A recent study led by Dr. Buller at Brown University measured how heart rate changes in time dynamically during exercise when compared to core body temperature. Their findings showed that changes in heart rate can be used to measure core body temperature during exercise with current PPG heart rate sensor technology with an accuracy of +/-0.5 °C over 85% of the time.

Accuracy of core body temperature metrics can be further improved by providing other data to a wearable unit such as activity context (running, cycling, etc.) and cardiac efficiency, or the person’s fitness level—all of which is technology that is already currently available in heart rate sensors today.

Energy Balance

Energy balance is defined as the energy you take in versus your energy expenditure. One of the pioneers in this field is Dr. William Kraus, and he has spent a significant amount of time understanding how an individual’s energy balance correlates to their overall health. What Dr. Kraus and others have determined in their research is that maintaining an energy balance in the body you can help to fight obesity and significantly improve your triglyceride levels, blood pressure, and cholesterol.

In wearables, using a branch equation model that factors in information like type of activity and PPG heart rate data we can accurately determine energy expenditure. The current challenges with energy balance are determining exactly how many calories an individual has consumed. Since there is currently no way to accurately measure this, the results can be skewed one way or the other.

An individual’s energy balance that’s off even by just three percent can indicate obesity in one year’s time. When you consider this data, a wearable that’s off by as little as five percent can’t provide data that’s accurate enough to help someone on a clinical level. While technology isn’t quite there yet, continuing to develop sensor fusion technology may be the answer to providing more accurate data on a clinical scale in the future.

Sleep Stages & Sleep Oriented Diseases

One area of health applications that you will see heart rate sensors being utilized more in the future is determining the quality of our sleep. Current technology in our PPG sensors allows us to determine conditions that negatively impact our sleep.

Sleep apnea studies, for instance, show that you can tell if a person if suffering from the condition by a lack of deep sleep, which can be determined percentages of low heart rate in a sleep session. When a person is able to get into deep sleep, you will commonly see dives in heart rate. If these reductions are absent, it can be an indicator that sleep apnea is occurring.

Likewise, breathing disruptions during sleep can be determined using heart rate data like PPG amplitude pulses and the time between heart beats. This data can help individuals and healthcare professional analyze the time you are actually in a deep sleep state. The ability to differentiate between sleep stages and when the body is in a sleep versus wake stage may also be categorized with a PPG sensor in HRV-based models.

Physical & Mental Stress Assessments

One of the lesser known assessments that can be gathered from PPG data is an indication of human stress levels. HRV measured from RRi has been well documented to be an indicator of stress on the body. However, HRV alone cannot clearly identify the source of the stress. It could be physical stress from marathon training or it could be psychosocial stress from a big project at work, or some combination of physical and emotional stress.

With that said, there are early indications that different elements of the PPG waveform can be used in determining the cause of the stress, using a method of sensor fusion from within the PPG sensor. This is accomplished by looking at the heart rate data in addition to recording found in the PPG wave itself. Intensity and the area under the curve of a PPG waveform can be applied and analyzed in combination with user input data to determine the a more precise cause of the low HRV data.

While heart rate sensors have come a long way in the past 10 years, what we are currently able to measure and analyze is only the tip of the iceberg, and the possibilities for the future are truly endless.

If you are interested in more detail on this topic, I’d recommend this webinar conducted by Dr. Steven LeBoeuf, President & Co-Founder of Valencell: