Cuffless Estimation of Blood Pressure: Importance of Variability in Blood Pressure Dependence of Arterial Stiffness Across Individuals and Measurement Sites.

Objective: Measuring arterial pulse transit time (PTT) to estimate blood pressure (BP) without conventional brachial cuff-based measurement is not new, but is a focus of current wearable technologies research. Much research pertains to efficient, accurate sensing of artery-related waveforms, yet the relationship between PTT and BP receives less attention despite being key for accurate BP estimation. This study investigated BP/PTT calibration by quantifying anatomical site variability (n = 10, 3 female, age 30 9 years) and individual variability ( n = 103, 50 female, age 53 22 years).
Methods: BP and pulse wave velocity (PWV) were measured in both seated and supine. Carotid-femoral PWV (cfPWV), carotid-radial PWV (crPWV), and carotid-finger-volume PWV (cvPWV) were measured with the wrist and hand positioned at the level of the upper thigh to achieve the same hydrostatic pressure effect across all measurements.
Results: The postural change invoked a small (4 7 mmHg) change in brachial diastolic BP with an additional 27 2 mmHg change in hydrostatic pressure. cfPWV decreased in the supine position (1.75 0.17 m s ^-1 , p < 0.001), but crPWV and cvPWV were more variable. The calibration term (BP/PWV) across the sample population varied from 6.6 to 98.3 mmHg sm ^-1 (mean 22 14 mmHg sm ^-1 ) and was correlated with age, heart rate, diastolic and pulse pressure, and weight. These variables did not explain the majority of the variability (R ² = 0.248).
Conclusion: There is anatomical site and between-individual variability in the calibration term for BP estimation from PTT.
Significance: Using and accurately calculating hydrostatic changes in BP within the individual may be one method to increase the accuracy of this calibration term.