Optical sensing heart-rate monitors may move from the wrist to the clinic
Optical sensing makes personal heart rate monitoring an almost casual activity. Many smartwatches and fitness trackers monitor the wearer continuously, making the data available in real-time and enabling historical analysis with a smartphone app.
So far, optical sensing is less popular in clinical settings. That could change quickly given that cardiovascular disease is a leading cause of death worldwide.
Monitoring heart activity using optical sensing is easier to implement than traditional methods using attached electrodes. Optical sensing is more comfortable for the patient, less expensive and doesn’t require trained practitioners. As populations age and healthcare costs rise, optical sensing enables better, more universal access to heart-monitoring services.
Optical sensing principles
Optical cardio sensing – photoplethysmography (PPG) – is a mature technique. Leveraging differences in the way blood and tissue absorb light, PPG is sensitive to volume changes in blood vessels as the blood circulates round the body. The equipment takes measurements when placed at the skin’s surface. The PPG waveform captures changes in the blood volume with each heartbeat. It also captures changes due to respiration, nervous system activity and thermoregulation.
How optical cardio sensing works
How can you use PPG data?
Pulse wave analysis of the PPG signal can measure the heart rate and capture other valuable information about the cardiovascular system. The analysis can be used to make inferences about many other conditions. Physiological monitoring can include blood oxygen saturation, blood pressure, cardiac output and respiration in addition to heart rate.
Data from PPG sensors is also used to detect arterial disease, monitor arterial ageing, assess the brain’s ability to control the cardiovascular system (vasomotor function), and assess reflexes such as compensating for blood pressure fluctuations as the body changes position (orthostasis). Further clinical applications include monitoring vasospastic disorders that constrict blood flow, monitoring thermoregulation, and monitoring endothelial function, which regulates enzymes in the blood that control properties such as clotting and immune responses.
Identifying the limitations of PPG
The types of heart-rate monitors (HRM) commonly used in sports training use sensors that contain electrodes pressed against the skin, typically worn around the chest.
Medical studies show HRMs perform extremely well, close to how clinical electrocardiogram equipment performs. Tests show correlations above 99%.
Recent investigations compared the accuracy of wrist-worn smartwatches and fitness trackers with professional electrocardiogram equipment and electrode-based HRMs. The best models among these wearables demonstrate better than 95% correlation with known-accurate electrode-based HRMs.
However, several factors affect performance. One is the position of the instrument when worn on the wrist, which can move and become less closely placed against the skin resulting in a loss of accuracy. In addition, the accuracy varies depending on the hardware implementation. Some models feature a single LED light source while others use an array of LEDs, or receivers, to capture several waveforms for comparison. There are also differences between the PPG algorithms that various manufacturers use to convert the waveforms, as captured, into viewable heart-rate readings.
Wavelength determines the depth to which light penetrates human skin. Red and near-infrared wavelengths pass easily. Hence, clinicians use IR LEDs to analyze blood flow in deep tissue. On the other hand, skin absorbs wavelengths in the green-yellow region to a much greater extent. One advantage of this is to show a greater change in reflected light as blood pulses through the vessels, which results in an increased signal-to-noise ratio (SNR). Today’s wearable PPG trackers often use LEDs that emit green wavelengths.
Moreover, PPG sensing can detect light transmitted through the medium or reflected from the tissue. Transmission-mode sensors include fingertip pulse oximeters and tend to use red LED wavelengths that pass more easily through body tissue. However, sensing at the fingertip has some disadvantages. The quantity of blood present may be small and can depend on the ambient temperature. A fingertip sensor is also unsuitable for continuous long-term monitoring. Clinicians can also perform transmission-mode measurement at the nasal septum, cheek and tongue, which are also unsuitable locations for continuous monitoring.
Wrist-worn wearables like watches and trackers are more convenient, although they must operate in the reflected mode. Hence, the receiving photosensor is located adjacent to the emitting (typically green) LED. However, the received signal can contain artifacts due to motion by the wearer as well as pressure disturbances caused by changes in contact force against the skin.
PPG sensing embedded in audio ear buds or inside smart rings that measure blood flow at the base of the finger can alleviate some motion and pressure artifacts.
The popularity of watches and trackers, which can be worn comfortably all day if desired, has raised demands for greater accuracy and other improvements such as lower power consumption. These have driven the emergence of new components that integrate the complete optical array comprising multiple LED emitters and photosensors, analog front-end ICs with built-in artifact correction and ultra-low power operation.
The future of PPG monitoring
The relative inaccuracy of reflectance-mode monitoring limits its application in clinical settings despite its ease of use. But with demand driven by consumer markets for smartwatches and trackers, we can expect PPG performance and affordability to improve significantly and quickly. The technology will become more commonplace, powerful and versatile in clinical settings. Given that cardiovascular disease is the leading cause of death globally, responsible for about 32% of deaths worldwide according to the World Health Organization, leveraging PPG as a convenient source of more and better data could help improve standards of care and patient outcomes.