Photoplethysmography (PPG) is a simple, non-invasive optical technique used to detect changes in blood volume within the microvasculature beneath the skin’s surface. This technology works by illuminating the tissue and measuring the resulting light absorption or reflection, which fluctuates with the cardiac cycle. PPG has moved from specialized medical settings into everyday life, becoming a ubiquitous feature in modern consumer wearables like smartwatches and fitness trackers. The signal provides information about the cardiovascular system, making it a tool for continuous health monitoring.
The Science Behind PPG
The core mechanism of photoplethysmography relies on the interaction between light and biological tissue, specifically the blood flowing through the arteries and capillaries. A PPG sensor consists of two main components: a light source, typically a light-emitting diode (LED), and a photodetector. The LED emits light into the tissue, and the photodetector measures the amount of light that either passes through (transmission mode) or is reflected back (reflection mode).
Light absorption by the tissue is dominated by several chromophores, with hemoglobin in the blood being the most significant in the context of the cardiac pulse. As the heart beats, it pumps blood into the arteries, causing a pulsatile expansion and contraction of the blood vessels. This temporary increase in blood volume during systole means there are more red blood cells present to absorb the emitted light.
The sensor detects this volumetric change as a corresponding change in light intensity; less light reaches the detector when blood volume is high, and more light reaches it when blood volume is low. Different wavelengths of light are used depending on the desired measurement depth. For instance, green light penetrates only the upper dermis, making it ideal for surface heart rate monitoring in wrist-worn devices. Red and infrared light penetrate deeper into the tissue, necessary for measurements involving deeper arterial structures, such as those used in pulse oximetry.
Decoding the PPG Waveform
The output of the photodetector is a continuous, oscillating waveform that represents the changes in light intensity over time, reflecting the rhythm of the heart. This signal can be mathematically separated into two primary components: the Direct Current (DC) component and the Alternating Current (AC) component. The DC component represents the relatively constant, non-pulsatile part of the signal, which is primarily influenced by the average volume of tissue, skin pigment, bone, and venous blood.
The AC component is the dynamic, oscillating part of the signal that is superimposed on the DC baseline. This AC waveform is directly correlated with the pulsatile changes in arterial blood volume driven by each heartbeat. It is this fluctuation that is analyzed to extract most physiological metrics.
A characteristic PPG waveform features a sharp upstroke followed by a more gradual downstroke. The peak of the waveform corresponds to the maximum blood volume in the tissue during the systolic phase of the heart cycle. Following this peak, the downstroke often exhibits a small inflection point known as the dicrotic notch.
The dicrotic notch marks the closure of the aortic valve, which signals the transition from systole to diastole. Analyzing the position and amplitude of this notch can provide insights into the elasticity of the arteries and the timing of reflected pressure waves. While the notch is more pronounced in younger individuals, it tends to become less visible or disappear entirely as arteries stiffen with age.
Health Metrics Derived from PPG
The analysis of the AC and DC components of the PPG waveform allows for the extraction of several important health parameters.
Heart Rate (HR)
The most straightforward metric is Heart Rate (HR), calculated by measuring the time interval between successive peaks in the AC component. Converting the time between these peaks, often called the pulse-to-pulse interval (PPI), into beats per minute yields a real-time heart rate.
Oxygen Saturation (\(\text{SpO}_2\))
Oxygen Saturation (\(\text{SpO}_2\)) measures the percentage of hemoglobin carrying oxygen in the blood. Determining \(\text{SpO}_2\) requires using two different wavelengths of light, typically red (around 660 nm) and infrared (around 940 nm), because oxygenated and deoxygenated hemoglobin absorb these wavelengths differently. The ratio of the AC component to the DC component is calculated for each wavelength, and the ratio of these two resulting ratios is then used in a calibration equation to estimate the oxygen saturation level.
Heart Rate Variability (HRV)
Heart Rate Variability (HRV) is calculated by analyzing the beat-to-beat variations in the timing of the pulse peaks (PPIs) over a sustained period. Since the PPG peak timing is closely related to the R-R interval measured by an electrocardiogram (ECG), the PPG-derived pulse intervals can be used for HRV analysis. HRV reflects the activity of the autonomic nervous system, providing insights into stress levels and overall physiological resilience.