Cuffless Estimation of Blood Pressure

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 $\pm$ 9 years) and individual variability ( n = 103, 50 female, age 53 $\pm$ 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 $\pm$ 7 mmHg) change in brachial diastolic BP with an additional 27 $\pm$ 2 mmHg change in hydrostatic pressure. cfPWV decreased in the supine position ( $-$ 1.75 $\pm$ 0.17 m $\cdot$ s−1, p $\Delta$ BP/ $\Delta$ PWV) across the sample population varied from 6.6 to 98.3 mmHg $\cdot$ s $\cdot$ m−1 (mean 22 $\pm$ 14 mmHg $\cdot$ s $\cdot$ m−1) and was correlated with age, heart rate, diastolic and pulse pressure, and weight. These variables did not explain the majority of the variability ( R2 = 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.