Learn Photoplethysmography
Deep dives into core PPG concepts — from signal fundamentals to clinical applications. Each guide is grounded in peer-reviewed literature.
What is SpO2 in Photoplethysmography?
SpO2 (peripheral oxygen saturation) measures the percentage of hemoglobin in arterial blood saturated with oxygen. PPG-based pulse oximeters estimate SpO2 by comparing light absorption at red (~660 nm) and infrared (~940 nm) wavelengths, exploiting the different absorption spectra of oxygenated and deoxygenated hemoglobin.
Read Guide →Interbeat Interval (IBI) in PPG Explained
The interbeat interval (IBI) is the time in milliseconds between consecutive heartbeats, detected in PPG as the period between successive systolic peaks or foot points. IBI is the primary input for heart rate variability (HRV) analysis and serves as a non-invasive surrogate for R-R intervals from ECG.
Read Guide →Perfusion Index (PI) in PPG Sensors
Perfusion Index (PI) is the ratio of the pulsatile (AC) component to the non-pulsatile (DC) component of the PPG signal, expressed as a percentage. It quantifies peripheral blood flow strength and indicates signal quality — higher PI values (>1%) generally indicate better perfusion and more reliable physiological measurements.
Read Guide →Heart Rate Variability (HRV) from PPG Signals
Heart Rate Variability (HRV) quantifies the variation in time between successive heartbeats. PPG-derived HRV uses interbeat intervals (IBI) extracted from pulse peaks as a surrogate for ECG R-R intervals. HRV reflects autonomic nervous system balance and is used as a biomarker for stress, recovery, and cardiovascular health.
Read Guide →AC/DC Ratio in PPG: Pulsatile vs. Static Components
In a PPG signal, the AC component represents the pulsatile variation in optical absorption caused by the cardiac-driven blood volume pulse, while the DC component represents the relatively static absorption from tissues, venous blood, and non-pulsatile arterial blood. Their ratio is the basis for perfusion index and SpO2 calculation.
Read Guide →Signal Quality Index (SQI) in PPG Analysis
A Signal Quality Index (SQI) is a numerical metric that automatically quantifies the reliability of a PPG segment for physiological analysis. SQIs typically range from 0 (unacceptable) to 1 (clean signal) and are used to gate downstream algorithms — only high-quality segments are processed for SpO2, HRV, or other metrics.
Read Guide →Pulse Transit Time (PTT) from PPG
Pulse Transit Time (PTT) is the time for the arterial pressure pulse to travel between two points in the vascular tree, typically measured between the ECG R-wave and the PPG pulse arrival at a peripheral site. PTT is inversely correlated with blood pressure and forms the basis for cuffless continuous blood pressure estimation.
Read Guide →Pulse Wave Velocity (PWV) Measured by PPG
Pulse Wave Velocity (PWV) is the speed at which the arterial pressure wave travels along a vessel, measured in m/s. It is the gold-standard non-invasive marker of arterial stiffness, a major independent predictor of cardiovascular events. PWV can be estimated from the ratio of vascular path length to PTT.
Read Guide →Augmentation Index (AIx) from PPG Waveform Analysis
The Augmentation Index (AIx) quantifies the contribution of the reflected arterial pressure wave to the central aortic pulse waveform, expressed as a percentage of pulse pressure. In PPG, AIx is estimated from the ratio of the late systolic augmentation pressure to the total pulse pressure, serving as an index of arterial stiffness and wave reflection.
Read Guide →PPG Waveform Morphology: Features and Analysis
PPG morphology analysis examines the shape characteristics of individual pulse waveforms to extract cardiovascular information beyond heart rate. Key morphological features include systolic peak amplitude and timing, dicrotic notch position, diastolic peak, pulse width at half-maximum, and features from the first and second time derivatives.
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