Modelling the relationship between peripheral blood pressure and blood volume pulses using linear and neural network system identification techniques

The relationships between peripheral blood pressure and blood volume pulse waveforms can provide valuable physiological data about the peripheral vasc ular system, and are the subject of this study. Blood pressure and volume p ulse waveforms were collected from 12 normal male subjects using non-invasi ve optical techniques, finger arterial blood pressure (BP, Finapres: Datex- Ohmeda) and photoelectric plethysmography (PPG) respectively, and captured to computer for three equal (1 min) measurement phases: baseline, hand rais ing and hand elevated. This simple physiological challenge was designed to induce a significant drop in peripheral blood pressure. A simple first orde r lag transfer function was chosen to study the relationship between blood pressure (system input) and blood volume pulse waveforms (system output), w ith parameters describing the dynamics (time constant, tau) and input-outpu t gain (K). tau and K were estimated for each subject using two different s ystem identification techniques: a recursive parameter estimation algorithm which calculated tau and K from a linear auto-regressive with exogenous va riable (ARX) model, and an artificial neural network which was trained to l earn the non-linear process input-output relationships and then derive a li nearized ARX model of the system. The identification techniques allowed the relationship between the blood pressure and blood volume pulses to be desc ribed simply, with the neural network technique providing a better model fi t overall (p < 0.05, Wileoxon). The median falls in tau following the hand raise challenge were 26% and 31% for the linear and neural network based te chniques respectively (both p < 0.05, Wilcoxon). This preliminary study has shown that the time constant and gain parameters obtained using these tech niques can provide physiological data for the clinical assessment of the pe ripheral circulation.