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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Non-invasive techniques for predicting soft tissue status during pressure induced ischaemia

Author: KNIGHT, Sarah L.

Year: 1997

Supervisor(s): Dan Bader

Soft tissue breakdown occurs in association with biochemical changes that can be attributed to a reduction in blood and lymph flow to a localised tissue area in response to applied pressure. The resulting ischaemia can lead to a reduction in available oxygen and accumulation of waste products. Tissue breakdown leading to the development of pressure sores afflicts patients who are already debilitated, although not all patients appear to be equally susceptible.
Measurement of sweat biochemistry and blood gas tensions may reflect the biochemical process in the underlying tissues and provide a simple and non-invasive method of investigating the status of soft tissues. The potential of specific sweat metabolites to act as markers of soft tissue status during and following loading has been investigated at a clinically relevant site in healthy volunteers, and in two clinically relevant patient groups. A range of validation procedures were undertaken and a series of parameters derived to investigate the temporal profile of sweat biochemistry, and identify various modes of gas tension response.

Investigations at the loaded sacrum of healthy individuals showed a statistically significant increase in sweat lactate, urea, urate and chloride concentrations which were dependent upon the level of externally applied pressure. Mean increases of between 10%-60% were demonstrated for sweat metabolite concentrations at the loaded site compared to the control site for applied pressures in the range 40-120 mmHg. Similar increases were demonstrated in sweat collected from highly loaded tissue areas within the stump socket of lower limb amputees.

A threshold value for pO2 tension was identified, amounting to a 60% reduction from the unloaded value, which was associated with elevated tissue carbon dioxide levels as well as increased sweat metabolite concentrations in the loaded phase.

This finding may provide a useful predictor of soft tissue status during prolonged loading.