Description

Heart rate is the measure of Beats Per Minute (BPM) which is the number of heartbeats detected in one minute. Normal resting heart rate range from 60 to 100 beats per minute (Laskowski). Resting heart rate is described as the heart pumping the lowest amount of blood needed when one is not exerting a lot of energy. Heart rate data can be collected from various parts of the body some of which include the wrist, fingertip, chest, thigh etc. Heart rate is measured using a monitoring device that allows one to get heart rate data in real time.

The two common medical applications that measure heart rate are optical based heart rate sensors known as photoplethysmography (PPG) sensors and electrical heart rate sensors known as electrocardiography (ECG) sensors. Optical heart rate sensor “uses a light-based technology to sense the rate of blood flow as controlled by the heart’s pumping action”(NeuroSky). While an electrical heart rate sensor “measures the bio-potential generated by electrical signals that control the expansion and contraction of heart chambers”(NeuroSky).

PPG Sensors

PPG is an optical heart rate sensor that is often used for heart rate monitoring purposes (Castaneda et al.). It is often non-invasive and uses a light source and a photodetector at the surface of skin to measure the rate of blood flow as controlled by the heart’s pumping action (Valencell). Optical blood flow sensors were first innovated in the late 1800s this was done by having someone place their hand over a candle in a dark to view blood flow and vascular structure(Valencell). Since then, the evolution of PPG sensors have gotten smaller and more accurate with modern technology.

There are two types of PPG sensors one is the pulse sensor that uses a green LED which emits light and hits surface of the skin, and a photodetector to measure incident light. The pulse sensor is designed to measure pulse waves when the heart pumps blood. The application of pulse sensors is widely used in wearable devices for health and fitness tracking. The second type of PPG sensor is the pulse oximeter that uses a red LED, and an infrared LED, both measured by a single, shared photodetector used to measure blood oxygen levels. In hospitals pulse oximeters are commonly used to measure pulse rate and blood oxygen.

ECG Sensors

ECG is an electrical heart rate sensor that measures and records the electrical activity that passes through the heart. “With each heartbeat, an electrical wave travels through the heart. This wave causes the muscle to squeeze and pump blood from the heart (Heart and Stroke Foundation).” The first practical ECG machine was invented in 1903 by Willem Einthoven who was a Dutch Doctor (Barold). EGC machines work using lead wires connected to electrode sensor pads placed on specific parts of the chest, arms, and legs. The electrical activity of the heart is then measured, interpreted, and printed out either on paper or digitally on a screen(Electrocardiogram).

There are 3 main types of ECG monitoring which include:

Resting ECG – conducted while someone is lying down in a comfortable position.
Stress or exercise ECG – conducted while someone is using an exercise bike or treadmill.
Ambulatory ECG (sometimes called a Holter monitor) – the electrodes are connected to a small portable machine worn at someone’s waist so their heart can be monitored at home for 1 or more days.

The type of ECG monitor depends on symptoms or heart problem suspected (‘Electrocardiogram (ECG)’).

Comparison of PPG vs ECG

The comparison between PPG and ECG sensors include:

PPG	ECG
Uses electrical signal derived from light reflected due to changes in blood flow during heart activity	Uses electrical signal produced by heart activity
Can measure heart rate but only suitable for average measurements	Measures heart rate accurately
Uses electrical signal derived from light reflected due to changes in blood flow during heart activity	Uses electrical signal produced by heart activity
Requires a longer settling time due to the need to measure ambient light	Meaningful readings can be obtained within a brief time

Wearable Devices with Heart Rate Sensors

Consumer wearables such as smartwatches and fitness trackers commonly have PPG sensors as they are cheaper than ECG sensors. In this research I compare two wearable devices that use PPG, ECG and a variation of both sensors.

Xiaomi Mi Band 6

The Mi Band 6 is the latest series of the Xiaomi smart band, it has two main sensors one is a high precision 6-axis sensor (3-axis accelerometer and 3-axis gyroscope) and the other a PPG heart rate sensor. It is used for both health and fitness tracking and the features include:

Heart rate monitoring: Whole-day heart rate manual heart rate, resting heart rate and heart rate curve.
Sleep monitoring: Deep sleep, light sleep, rapid eye movement (REM), naps.
Women’s health tracking: Provides recording and reminders for the menstrual cycle and ovulation phases.
Stress monitoring, breathing exercises, PAI vitality index assessment, idle alerts, step counter, goal setting (Mi Smart Band).

Apple Watch Series 7

The Apple Watch Series 7 has both PPG and ECG sensors which measures blood oxygen level and allows a user to take an ECG anytime, anywhere respectively. Some of the features include heart rate sensing, mindfulness, and sleep tracking for health monitoring. The blood oxygen sensor and app allow someone to take on-demand readings of their blood oxygen as well as background readings, day and night (‘Apple Watch Series 7’).

The ECG sensor works using the ECG app, Apple Watch Series 7 can generate an ECG similar to a single-lead electrocardiogram. Electrodes built into the Digital Crown and the back crystal work together with the ECG app to read someone’s heart electrical signals. It works by placing a fingertip on the Digital Crown to generate an ECG waveform in just 30 seconds. “The ECG app can indicate whether a heart rhythm shows signs of atrial fibrillation — a serious form of irregular heart rhythm — or sinus rhythm, which means your heart is beating in a normal pattern(‘Apple Watch Series 7’).”

Conclusion

Overall, both PPG and ECG have their advantages and disadvantages and have become an integral part of designing health and fitness wearable devices. Each offer their own benefits to the type of device depending on factors such as placement on the body, type of heart rate data being collected, fitness tracking, health monitoring and so on.

Bibliography

‘Apple Watch Series 7’. Apple (CA), https://www.apple.com/ca/apple-watch-series-7/. Accessed 27 Apr. 2022.

Barold, S. Serge. ‘Willem Einthoven and the Birth of Clinical Electrocardiography a Hundred Years Ago’. Cardiac Electrophysiology Review, vol. 7, no. 1, Jan. 2003, pp. 99–104. PubMed, https://doi.org/10.1023/a:1023667812925.

Castaneda, Denisse, et al. ‘A Review on Wearable Photoplethysmography Sensors and Their Potential Future Applications in Health Care’. International Journal of Biosensors & Bioelectronics, vol. 4, no. 4, 2018, pp. 195–202. PubMed Central, https://doi.org/10.15406/ijbsbe.2018.04.00125.

Electrocardiogram. 8 Aug. 2021, https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/electrocardiogram.

‘Electrocardiogram (ECG)’. Nhs.Uk, 18 Oct. 2017, https://www.nhs.uk/conditions/electrocardiogram/.

Heart and Stroke Foundation. Electrocardiogram | Heart and Stroke Foundation. https://www.heartandstroke.ca/heart-disease/tests/electrocardiogram. Accessed 30 Mar. 2022.

Laskowski, Edward. ‘2 Easy, Accurate Ways to Measure Your Heart Rate’. Mayo Clinic, https://www.mayoclinic.org/healthy-lifestyle/fitness/expert-answers/heart-rate/faq-20057979. Accessed 31 Mar. 2022.

Mi Smart Band. Mi Smart Band 6 – No.1 Wearable Band Brand in the World – Xiaomi Global Official. https://www.mi.com/global/product/mi-smart-band-6/. Accessed 27 Apr. 2022.

NeuroSky. ECG vs PPG for Heart Rate Monitoring: Which Is Best? 28 Jan. 2015, http://neurosky.com/2015/01/ecg-vs-ppg-for-heart-rate-monitoring-which-is-best/.

Valencell. ‘Optical Heart Rate Monitoring Technology: What You Need to Know’. Valencell, 15 Oct. 2015, https://valencell.com/blog/optical-heart-rate-monitoring-what-you-need-to-know/.

HEART RATE SENSORS – By Trish