Learn PPG
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, though the slope 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. ISO 81060-2 requires a mean error of no more than 5 mmHg with a standard deviation of no more than 8 mmHg. 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 active research.
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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?
It depends on calibration and conditions. Drift, posture and individual variability degrade accuracy after calibration, so results should be judged against validation to ISO 81060-2.
Does heart rate affect PTT?
Yes. Heart rate influences PTT through both direct hemodynamic effects and the Frank-Starling mechanism.