Non-contact blood monitor

The first ever method of monitoring blood flow in human body tissue without actually touching the skin has been developed by Loughborough University researchers.

This hands-free technique could one day be used for remote heart monitoring, and for the assessment of patients, either during surgery or the healing of wounds or burns.

The team, led by Professor Peter Smith, research associate Vincent Crabtree and PhD student Sharon Cheang adapted an existing optoelectronic monitoring technique by removing the need for skin contact. The conventional technique, called photoplethysmography (PPG), involves illuminating a section of the body, which is in contact with the detector, and working out how much of the light is absorbed. This is done by detecting a cardiovascular pulse wave, which consists of a large static component relating to the static tissue components and a smaller dynamic component relating essentially to the blood in the arteries. The size and shape of the pulse wave depends on the properties of the blood flowing through the tissue, this is worked out from how much light the tissue sample absorbs.

‘Skin contact probes used in previous PPG monitoring systems were often hard to attach to difficult-to-reach parts of the body,’ explains Peter Smith, Professor of Photonics Engineering and Head of Electronic and Electrical Engineering at Loughborough. He continues, ‘When a patient moves, the probes also interfere with the signal.

Building a non-contact PPG system increased these interference problems, as the longer distances between the body and the detector meant that interference due to movement and the presence of natural light was increased.’

Tracking these signals of interest is rather like trying to find a needle in a moving haystack. Detecting very small light sources at large distances can be done but when the signal is present in a background light source that is varying over levels far in excess of the signal size, then some further intelligence is required. The team pinpointed the relationship between movement and the received signal by conducting a series of experiments. The results of which enabled them to establish appropriate illumination sources, adaptive detection systems and a computer programme that not only predicts how the pulse signal is affected by movements, but automatically corrects it.

Vincent Crabtree concludes, ‘By addressing the issues of signal corruption, a remote PPG system has been demonstrated for the first time. Although the remote PPG signal quality has currently only been investigated over a range of a few centimetres, this represents an exciting breakthrough. The future applications for this technology could be extensive.’

This technology will benefit patients and healthcare professionals in providing more accurate and reliable perfusion measurements in circumstances where contact is undesirable or perhaps not possible; examples include the monitoring of wound healing and peripheral arterial disease.

The approach will also lead to more automated technology for monitoring and diagnostics that should stimulate its use in the community leading to a reduction in false referrals and to the early detection of disease and more accurate management of therapy.

The research was performed under the Gatsby Innovation Fellowship Scheme in 2000 which enabled Professor Smith to advance the new technology towards successful exploitation. The research team has since teamed up with commercial partners, Stuert Medical Devices, in order to successfully trial the unique remote PPG system at Stirling Royal Infirmary under the supervision of vascular surgeon Richard Holdsworth.