Signal processing
Cuffless BP via Pulse Transit Time (PTT)
Cuffless blood pressure estimation using pulse transit time (PTT) exploits the Moens-Korteweg relationship between arterial pressure and pulse wave velocity. As blood pressure rises, arterial walls stiffen, increasing PWV and decreasing PTT, enabling continuous BP estimation from ECG-PPG or dual-site PPG timing measurements.
How it works
Schematic summary of this guide. Waveforms are illustrative, not measured data.
The PTT-BP relationship derives from the Moens-Korteweg equation:
where is the elastic modulus (pressure-dependent), is wall thickness, is blood density, and is vessel radius. Assuming an exponential pressure-modulus relationship , the model becomes , where and are individual-specific calibration constants determined from at least one simultaneous cuff measurement.
The calibration drifts because the E-P relationship varies with smooth muscle tone, hydration, medication effects, and circadian vascular changes. Periodic recalibration against a cuff is therefore required; Samsung, for example, asks Galaxy Watch users to recalibrate blood pressure every 28 days.
Pure PPG-based cuffless BP without PTT uses waveform morphology features — pulse arrival time relative to ECG, systolic upstroke slope, pulse width, augmentation index, and second-derivative features — as BP surrogates.
Frequently Asked Questions
Has any cuffless BP device achieved FDA clearance?
As of 2025, no pure PPG-based cuffless BP device has received FDA clearance for stand-alone BP measurement. Samsung's Galaxy Watch BP feature has CE marking in Europe. Biobeat received 510(k) clearance for a chest-patch BP device using a different measurement modality.
How does posture affect PTT-based BP estimation?
Posture changes alter hydrostatic pressure gradients, modifying the PTT-BP calibration.
What is the minimum number of calibration points?
One calibration point (single cuff measurement) provides a linear fit but assumes fixed slope.