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.
The PTT-BP relationship derives from the Moens-Korteweg equation: PWV = √(E·h / 2ρr), where E is the elastic modulus (pressure-dependent), h is wall thickness, ρ is blood density, and r is vessel radius. Assuming an exponential pressure-modulus relationship E = E₀·exp(γP), the model becomes BP = a·ln(PTT) + b, where a and b are individual-specific calibration constants determined from at least one simultaneous cuff measurement.
Single-point calibration achieves 5–8 mmHg systolic BP accuracy over 4–8 hours in controlled settings. The calibration drifts because the E-P relationship varies with smooth muscle tone, hydration, medication effects, and circadian vascular changes. Multi-point calibration (3–5 reference readings across the BP range) and periodic recalibration (daily or weekly) reduce long-term drift to 3–6 mmHg. Samsung Galaxy Watch BP (CE-marked) requires monthly recalibration.
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. Machine learning models (random forest, gradient boosting, neural networks) trained on large calibrated datasets achieve 7–12 mmHg accuracy without individual calibration, falling short of ISO 81060-2 requirements but useful for relative BP trending.
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. Standing increases peripheral BP by 5–10 mmHg. Posture-aware algorithms using accelerometer data can apply corrections, reducing posture-related error by 30–50%.
What is the minimum number of calibration points?
One calibration point (single cuff measurement) provides a linear fit but assumes fixed slope. Three or more points spanning 20+ mmHg range enable personalized slope estimation, improving accuracy by 20–40% compared to single-point calibration.