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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: Can Interface Conditions be Modified by Support Surfaces to Minimise the Risk of Pressure Ulcer Development?

Author: CHAI, Chanyuan

Year: 2011

Supervisor(s): Wen Wang, Dan Bader

The characteristics of a patient support interface can influence the susceptibility of subjects, particularly there who are immobilised and insensate, to pressure ulcer development. Accordingly, externally powered alternating pressure air mattresses (APAM) are utilised to produce intermittent pressure relief and control of the interface microclimate. These conditions will permit adequate blood and lymph flow within the soft tissues and favourable conditions at the loaded skin surface and thus minimise the risk of ulcer formation. Two sets of measurements were performed. Tissue viability was estimated, from a measure of transcutaneous gas tensions and sweat content, from healthy volunteers subjected to various alternating pressure regimens. The latter was achieved by two different strategies a) a custom–made controller which imposes the pressure profile on the subject and b) a prototype APAMs incorporating a novel sensor, which adjusts the profile according to individual subject characteristics. The latter prototype was placed on an articulated hospital bed, with an adjustable Head of Bed (HOB) angle. The second set of measurements involved monitoring the microclimate, namely temperature and humidity, at the interface loaded with a human analogue model supported on an APAM system. The interface was saturated with moisture to simulate sweating. The human studies, involving healthy subjects with BMI values ranging from 18.9 to 42.5 kg/m2, revealed significant differences in soft tissue response under various support surface profile by both strategies. In many cases, the TcPO2 levels either remained fairly stable during the loaded period or fluctuated at a periodicity equivalent to the cycle period of the APAM system, with the corresponding TcPCO2 levels remaining within the normal basal range. These findings were associated with II maximum interface pressures generally not exceeding 50 mmHg (6.67 kPa). By contrast in some cases, there was a significant compromise to the TcPO2 levels during the loaded period, which was often associated with an increase in TcPCO2 levels. These cases generally corresponded with the internal pressures in the mattress prescribed at a maximum amplitude of 100 / 0 mmHg or when the Head of Bed angle was raised to 45º or above. Changes in prototype covering sheet and air flow rates of the APAM system were found to influence both interface temperature and humidity. These results revealed enhanced levels of humidity often reaching 100% RH at the high simulated sweat rates. By contrast, at the lower sweat rate of 1.5 ml/min, the nature of the prototype covering sheets had an effect on the interface humidity profile, with values considerably reduced in the latter stages of the monitoring period. These results were compared with a compartmental model, which predicted the transport of moisture and heat using various mattress support systems. The results offer the potential for the development of intelligent APAM systems, whose characteristics can be adjusted to an individual morphology. These systems will need to be designed to ensure adequate tissue viability and maintain appropriate microclimate at the loaded interface. Such an approach will be directed at those subjects considered to be at high/medium risk of developing pressure ulcers.