Continuous non-invasive blood-pressure measurements

A problem presented at the UK MMSG Southampton 2007.

Presented by:
Dr Tony Birch (Neurological Physics Group, Southampton University Hospitals NHS Trust)
Participants:
T Birch, CJW Breward, S Campbell, I Chernyavsky, T Fischer, S Glavin, OE Jensen, A Lovrics, S McBurnie, J Moles, C Please, J Siggers, P Stewart, C Voyce, RJ Whittaker, T Witelski

Problem Description

The primary components of healthy living are recognized as a good diet, plenty of exercise, avoiding smoking and reducing stress. These measures are largely aimed at improving the health of the arterial system. The arterial system is much more than a set of pipes, it is an extraordinarily well-regulated blood delivery network capable of responding within a few seconds to challenges such as an altered body position or a change in demand. There is rapidly growing interest in measurement techniques that can assess the rapid dynamics of the arterial system. These may enable deteriorating function to be identified before it becomes a problem. Much of the research in this area requires a continuous but non-invasive measure of arterial pressure.

Arterial blood pressure may be measured continuously by two different non-invasive techniques. Both of which are available as commercially produced medical devices. One technique, the Finapres, uses a cuff around a finger. Light transmission measurements through the finger are used to provide continuous feedback control of the pressure in the cuff. This applied pressure cancels out pulsatile volume changes in the finger and closely reflects the arterial pressure waveform. The other technique is a direct tonometry measurement at the pressure point in the wrist. When the artery is partially flattened against a bone, pressure changes inside the artery should be faithfully transmitted to the external pressure sensor. However when a transient change in arterial blood pressure is provoked these two devices have been shown to consistently record different patterns of change.

The Study Group should therefore explore possible explanations for this disagreement? A model of the behaviour of both devices and vascular response to the provocation may provide the answers.

Study Group Report

A variety of modelling techniques were used to model the behaviour of the Finapres and tonometer. For the tonometer a simple spring and dash-pot model was used to capture the properties of the blood vessel, vessel wall and skin, and the tonometer itself. An improved model using a linear visco-elastic slab to represent the wall and skin tissue was also considered. For the Finapres, we considered simple lumped-parameter models, balancing blood flow rates with resistances calculated using a linear tube law. Both asymptotic and numerical solutions were obtained.

Download the full report

Follow-Up Activities

The following student projects have been inspired by this problem:

Mathematical model of a non-invasive blood pressure measurement technique: the Finapres
Final-year undergraduate project, Imperial College, October 2007 to April 2008.
Supervised by JH Siggers.