A Photoplethysmography (PPG) signal is an optical measurement that detects changes in blood volume within the microvascular bed of tissue, typically in areas like the fingertips or wrists. This non-invasive technique uses light to provide information about the cardiovascular system. PPG technology has become common in modern health monitoring devices due to its simplicity and affordability.
How PPG Signals Are Generated
PPG signals are generated by shining a light, typically from an LED, onto the skin and then measuring the amount of light that is either absorbed or reflected back by a photodetector. Common wavelengths used include green, red, or infrared light; red and infrared penetrate deeper. Blood absorbs light more strongly than surrounding tissues, so changes in blood flow are detected as variations in light intensity.
When the heart beats, blood volume in the skin’s microvascular bed increases, causing more light to be absorbed and less reflected. Between heartbeats, blood volume decreases, leading to less absorption and more reflection. These fluctuations correspond to the pulse wave from each heartbeat. The resulting PPG waveform has a pulsatile (AC) component, reflecting blood volume changes with each heartbeat, and a constant (DC) component, representing light absorption by tissues.
What PPG Signals Measure
PPG signals measure several physiological parameters, primarily heart rate and blood oxygen saturation. Each beat of the heart produces a pulse wave in the PPG signal, allowing devices to calculate heart rate by detecting waveform peaks and measuring intervals between them. This method can also assess heart rhythm by evaluating the regularity of these intervals.
Beyond heart rate, PPG can estimate blood oxygen saturation (SpO2), which is the percentage of hemoglobin in the blood carrying oxygen. Pulse oximeters, for example, use two different wavelengths of light, typically red and infrared, because oxygenated and deoxygenated hemoglobin absorb these wavelengths differently. By comparing the absorption of these two light types, the device estimates the SpO2 level. The PPG signal also provides information about heart rate variability (HRV), which reflects variations in the timing between successive heartbeats, offering insights into the autonomic nervous system.
Everyday Applications of PPG
PPG technology is widely integrated into consumer wearables like smartwatches and fitness trackers, making health monitoring accessible. These devices provide real-time heart rate data, assisting individuals in monitoring their health and adjusting physical activities. Its continuous monitoring capability makes it a popular solution.
In medical settings, PPG sensors are a component of patient monitoring systems and pulse oximeters. These fingertip devices are routinely used in hospitals and clinics to measure blood oxygen saturation and pulse rate. The small, compact, and often wireless PPG sensors are ideal for integration into various devices for continuous, real-time monitoring.
Factors Affecting PPG Signal Quality
The accuracy and reliability of PPG measurements can be influenced by several factors. Motion artifacts, caused by movement of the sensor over the skin or changes in skin deformation, are a common challenge, as they can introduce noise that obscures physiological signals. This noise can lead to inaccurate readings, particularly during physical activity. Advanced signal processing techniques are used to filter out these artifacts.
Other factors impacting signal quality include skin tone, ambient light interference, temperature, and sensor placement or pressure. For instance, darker skin tones may absorb more green light due to melanin, potentially limiting light penetration to deeper blood vessels. External light sources can also interfere with the sensor’s ability to detect subtle changes in reflected light. The exact placement of the sensor and the pressure applied to the skin can also affect the signal, as different anatomical locations have varying vascular structures and skin thicknesses.