Biosignals and ECG
A biosignal is a measurable and monitorable signal from a living organism used to learn about biological structures within the being. A type of biosignal is heart rate measured through an electrocardiogram(ECG or EKG). The heart generates electrical pulses that can be measured with electrodes commonly using Einthoven’s equilateral triangle, a method where electrodes are placed on the right wrist, left wrist, and left leg. This forms a visual triangle with the heart in the center. A standard ECG has three lead systems(Lead III) used to detect the electrical activity of the heart over time.
On a monitor, the ECG appears as a waveform known as the QRS complex. The R is the largest spike in the wave while Q and S come before and after respectively. The intervals between each of the letters of the waves provide information about the heart such as beats per minute.
Arrhythmia is a category for irregular heartbeats that can have quick or slow patterns. Specifically, atrial fibrillation is caused when the atria(two upper chambers of the heart) is out of sync with the ventricles(the lower chambers of the heart).
The sinus node, located in the upper right atrium or chamber, acts as a pacemaker to regulate heartbeat. This node stimulates the heart muscles with electricity to pump blood after contraction. Adjusting to the level of activity, the node sends out quicker or slower impulses. As the electrical current travels to the ventricles, the atrioventricular node protects the bottom chambers by delaying signals for contraction. The contraction pumps blood to the rest of the body after. During atrial fibrillation, the sinus node does not control the rhythm because other parts of the atria may have started the impulse, contractions become chaotic, or the atrioventricular node fails to regulate the electrical current.
Causes include high blood pressure, coronary artery disease, congenital heart disease, stress, and stimulants. Symptoms are fatigue, shortness of breath, heart palpitations, chest pain, lightheadedness, and weakness. Over time, atrial fibrillation increases the risk of heart failure, stroke, blood clots, vascular dementia, and death.
Current Devices and PPG
Most consumer devices use photoplethysmography(PPG) to measure beats per minute, stress, and SpO2. PPG works by using light to measure skin absorption change.
Although convenient, PPG devices are sensitive to movement, require calibration based on the light in the environment, and higher power consumption than ECG for portability. PPG detects pulses and is not as reliable as an ECG for a clear QRS waveform.
Electrode based ECG portable devices exist but can be cost more than a PPG device. By simplifying the ECG circuit design, an affordable and small ECG can be designed to monitor heart activity.
Searson, P. C., Bychkov, D., & Jeong, I. C. (2018, September 24). Wearable Devices for Precision Medicine and Health State Monitoring. Retrieved July 1, 2019, from https://ieeexplore.ieee.org/document/8470151
Lazazzera, R., Belhaj, Y., & Carrault, G. (2019, June 04). A New Wearable Device for Blood Pressure Estimation Using Photoplethysmogram. Retrieved July 1, 2019, from https://www.mdpi.com/1424-8220/19/11/2557/htm
Kooij, K. M., & Naber, M. (2019, May 31). An open-source remote heart rate imaging method with practical apparatus and algorithms. Retrieved July 1, 2019, from https://link.springer.com/article/10.3758/s13428-019-01256-8
Ko, J., & Kim, N. H. (2019, March 21). Evolution of Smart Health Wearables: Novel Application for Detection of Arrhythmia. Retrieved July 1, 2019, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6511535/
Kim, J., Chou, E., Le, J., Wong, S., Chu, M., & Khine, M. (2019, April 29). Soft Wearable Pressure Sensors for Beat‐to‐Beat Blood Pressure Monitoring – Kim – – Advanced Healthcare Materials – Wiley Online Library. Retrieved July 1, 2019, from https://onlinelibrary.wiley.com/doi/abs/10.1002/adhm.201900109
NIH. (2016, July 31). Atrial Fibrillation. Retrieved July 13, 2019, from https://www.nhlbi.nih.gov/health-topics/atrial-fibrillation