Oscillometric finger pressing is a smartphone-based blood pressure (BP) monitoring method. It measures finger photoplethysmography (PPG) oscillations and pressure during a steady increase in finger pressure to compute systolic BP (SP) and diastolic BP (DP). However, the viscoelastic properties of small finger arteries may adversely affect the accuracy of the BP computation. The aim was to evaluate the impact of finger artery viscoelasticity on the BP computation.
Nonlinear viscoelastic models relating transmural pressure (finger BP minus applied pressure) to PPG oscillations during finger pressing were developed. The output of each model to a measured transmural pressure input was fitted to measured PPG oscillations from 15 participants. A parametric sensitivity analysis was performed via model simulations to elucidate the viscoelastic effect on the popular derivative-based BP computation algorithm.
A Wiener viscoelastic model, comprising a first-order transfer function followed by a static sigmoidal function, fitted the measured PPG oscillations with almost half the error compared to an elastic model comprising only the static sigmoidal function. Simulations using the Wiener model revealed that the derivative-based algorithm underestimates SP, particularly in cases of high pulse pressure and low transfer function cutoff frequency (i.e., greater viscoelasticity). Importantly, the mean of the normalized PPG waveform at the maximum oscillation beat correlated inversely and tightly with the cutoff frequency, suggesting that this parameter could be used to conveniently compensate for viscoelastic effects.
These findings indicate that finger artery viscoelasticity negatively impacts oscillometric BP computation but can potentially be mitigated using available measurements. This advancement could enable the development of accurate calibration-free BP monitoring solutions using smartphones for enhancing hypertension awareness and management.