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.
PTT is related to blood pressure through the Moens-Korteweg equation, which describes pulse wave velocity (PWV) as a function of vessel wall elasticity and blood density. As blood pressure rises, arterial walls stiffen, increasing PWV and reducing PTT. The relationship is approximately linear over moderate BP ranges: each 1 mmHg increase in mean arterial pressure corresponds to roughly a 1 ms decrease in PTT, though this ratio is highly individual and requires personalized calibration.
Measuring PTT from PPG alone (without a simultaneous ECG reference) requires two spatially separated PPG sensors — for example, one at the ear and one at the finger, or one at the forehead and one at the wrist. The inter-sensor timing difference yields PTT independent of pre-ejection period variability. Alternatively, ECG-PPG PTT (often called pulse arrival time, PAT) incorporates both the pre-ejection period and PTT, complicating BP calibration.
Clinical validation of PTT-based cuffless BP monitoring has been mixed. IEEE Standard 1708 and ISO 81060-2 require mean error < 5 mmHg and standard deviation < 8 mmHg for ambulatory BP monitors. Most consumer PTT-based devices fail these criteria due to calibration drift, posture changes, and individual variability in the PTT-BP relationship. Cuffless BP remains an area of intense research, with Samsung, Apple, and Omron all pursuing FDA clearance through improved multi-parameter fusion approaches.
Frequently Asked Questions
What's the difference between PTT and PAT?
Pulse Arrival Time (PAT) is measured from ECG R-wave to PPG foot, incorporating both pre-ejection period (PEP) and true PTT. Pure PTT, measured between two PPG sites, eliminates PEP variability and correlates more directly with blood pressure.
How accurate is cuffless blood pressure from PTT?
After individual calibration, PTT-based BP estimation achieves 5–10 mmHg accuracy in laboratory conditions. In free-living conditions, accuracy degrades to 10–15 mmHg, below clinical standards.
Does heart rate affect PTT?
Yes. Heart rate influences PTT through both direct hemodynamic effects and the Frank-Starling mechanism. Models that incorporate heart rate as a covariate reduce PTT-BP estimation error by 15–30%.