Heart Rate Sensors

Heart rate is a crucial measurement for evaluating one’s health. It is usually measure with a stethoscope or armband, however digital solutions are becoming more commonplace with electrocardiography (ECG) and photoplethysmography (PPG) sensors being the two leading technologies. The MetaHealth board uses a PPG sensor and in this post, we will provide an overview of photoplethysmography.

How They Work

Photoplethysmography is a technique that optically measures changes in blood volume through tissue. This is done by illuminating the skin with different wavelengths of light and measuring the amount of light that is absorbed or reflected, a process called pulse oximetry. Transmittance based PPG places the LEDs and photodiodes on opposite sides of tissue whereas reflective PPG has them both on the same surface. Additional high pass filtering is needed to remove the dominant lower frequencies without distorting the pulse shape.


MetaHealth PPG

The MetaHealth PPG circuit and signal processing is designed entirely in-house. Our PPG emits a green light of ~535nm and the photodiode that detects the reflected light is sensitive to light between 400 and 1100nm with a sensitive area of ~1.7mm^2.

Full specifications on the MetaHealth PPG circuit is available on our website.

Use Cases

The purpose of monitoring heart rate is obvious, but the pulse oximetry part of electronic heart rate sensors make them extremely useful over traditional devices in certain use cases. Being extremely small and cheap, pulse oximetry devices a commonplace in hospitals where nurses can use them to constantly monitor a patient’s vitals. For similar reasons, they are used in elderly care where the older patients may require constant monitoring, especially when sleeping.

As pulse oximetry is measuring oxygen concentration, you not limited to only monitoring vitals. This technique can be used to gather information about respiratory system as a whole, measuring the O2 concentration over time.

Further Reading

Several undergraduate students from the Worcester Polytechnic Institute investigated the accuracy of reflective based PPG when applied to the chest and wrist. Their research paper is on the Worcester Polytechnic Institute website, link here.

For more information about pulse oximetry, check out this “How to read SpO2” paper Konica Minlota Sensing, Inc.