Skin temperature as an indicator of stress in soft tissue G Pye, P Bowker Introduction This paper describes the development of a technique for assessing the intensity of the stress state in skin and subcutaneous tissues under compressive loading. The value of such a technique is associated with the development of support surfaces for non-ambulent and long stay hospital patients with the aim of preventing and curing pressure sores (bed sores). These lesions can form at sustained pressures as low as 3.5kPa as a result of irreversible tissue ischaemia following stress induced capillary occlusion.1 Previous measurements of tissue loadings using pressure transducers on the skin surface have been unsatisfactory in that only discrete pressure values have been obtained and the presence of the transducers themselves has affected the experimental results.*,3 Addlitionally, currently available devices measure only direct stress whereas the applied stress generally consists of both direct and shear components whilst geometrical effects are likely to result in the stress state in the subcutaneous tissues being markedly different from that near the skin surface. Concept As it is the flow of blood which mainta,ins peripheral body temperature, the reduced blood perfusion during load applicat,ionwould be expected to lead to a local fall in skin temperature. On subsequent load removal,
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Fig 1. Schematic representation of changes in blood perfusion and consequent changes in heat flow when soft tissue is loaded and unloaded
Fig 2. Infra-red thermographic scanner
there follows a period of enhanced blood flow, termed ‘reactive hyperaemia’, which is commonly regarded as a mechanism for repaying the blood flow ‘debt’. It would therefore be expected that tissue loading would be accompanied by the physiological effects of local cooling dur,ing load application followed by a transient but reasonably long lived temperature rise on load removal, as shown diagrammatically in fig 1. These temperature changes could most usefully be monitored by the use of infra-red thermography, a technique which yields pictorial images of temperature distribution by the detection of the infra-red radiation emitted naturally by the subject. Such a procedure would provide a means of assessing stress induced tissue trauma which would not have the limitations of direct pressure measurement. Theory To estimate the magnitude of the skin temperature changes resulting from these physiological effects a simple heat flow model was set up for the distal segment of a finger. The analysis was performed by analogy to a constant temperature fluid (the blood) flowing through a circular pipe and separated from a constant temperature reservoir (the environment) by an insulating sleeve (the skin and soft tissue sheath). The blood channel was not visualized as any definite physical feature bub simply as part of an idealized model. Load applicatiorl was considered to totally occlude the blood channel and subsequent load removal to readmit blood which, because of the increased flow rate, reached the finger segment at a higher temperature. Computer evaluation of the relationships obtained from a thermal energy
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Engineering in Medicine 0 IMechE. 1976
balance suggested that measurable changes in skin temperature would occur within a few minutes of load application and removal. Procedure and results In the initial investigation temperature measurements were made using an infrd-red scanner, fig 2. Loads were applied via a 35mm diameter perspex indenter to various areas of soft tissue and changes in skin temperature recorded on photographs of the thermographic image. A fall in temperature was indicated by some blackening of the loaded areas but this effect quickly disappeared to leave a thermogram of normal appearance. Additionally, it was unclear whether this cooling was physiological in origin and it was therefore considered necessary to define the: various effects more closely by continuous monitoring of thermistors attached to the skin. Temperature changes were studied in the fingers and forearms of twelve subjects. These sites were chosen for initial investigations as load dependent temperature variations have previously been demonstrated in fingers.’ Thermistor probes were attached to the skin surface and loads in the range 20-33kPa applied using the same perspex indenter. In order to determine the effect of the load alone two transducers were employed; one being placed beneath the loaded indenter and the second being positioned on either an adjacent finger or the corresponding site on the other arm beneath an Temperature - time profiles for arms
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Fig 3. Time variation of the difference in temperature between loaded and unloaded thermistors on the forearms of four subjects, together with theoretical predictions
unloaded but otherwise identical indenter. The temperature difference between the two probes and the absolute temperature of the control were both recorded continuously before, during and after loading. Loads were applied for a period of five minutes after allowing sufficient time for the limb temperature to equilibriate with the experimental environment. The results obtained indicated that only in some cases did a fall in temperature occur at loading, the presence or absence of cooling depending mainly on the initial temperature of the finger relative to ambient. On the other hand, an increase in temperature corresponding to the reactive hyperaemia was observed in all cases although the magnitude of the temperature rise could not be similarly explained; clearly there must be other physiological effects involved here. The shapes of the arm curves varied greatly (fig 3) and showed the overall result of at least two separate effects. As the load was applied the thermistor was pressed into the warmer tissues nearer to the central blood core and this, together with improved thermal coupling, will have tended to increase the measured temperature. This warming effect would then be added to the temperature changes associated with the real physiological effects in a proportion depending on the thermal and mechanical properties of the particular arm. The converse effects would be expected to occur on unloading. In figs 3(a) and 3(b) the physiological effects predominate whilst in fig 3(d) the physical effects are the greater. Fig 3(c) shows an intermedsiate case. The results did suggest however that the physiological effects of tissue loading were of sufficient magnit-ude to be detected by infra-red scanning. Further thermographic trials were therefore carried out in which pressures of 19-28kPa were applied to fingers and thighs for periods of up to 20 minutes and series of thermograms taken after unloading for a further 20 minutes. In each case two sets of thermograms were obtained, one using the loaded indenter and a second using an unloaded indenter as a control. From temperature profiles taken through the site of loading, graphs were plotted of skin temperature against time after load removal for both the loaded and unloaded tests (fig 4). The difference between these two curves was taken to represent the real physiological effects of the applied pressure. To date similar data has been obtained on four subjects at two body sites and two pressure values. Summary and discussion From the thermistor investigations it was found that changes in finger skin temperature did occur at loading and unloading but that their magnitudes depended on the initial temperature of the finger and was subject to large individual variations. Fingers initially warmer than the environment cooled at loading and all fingers exhibited a reactive hyperaemia although no simple pattern was found. The thermistor tests on the forearm showed that temperature changes associated with the physiological results of tissue loading could be detected but that interpretation of the results was difficult as systematic errors resulting from burying of the temperature probe distorted them. The thermographic data was consistent with the
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0 IMechE, 1976. Vol. 5, No. 3
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and the magnitude of the applied stress. The data was in fair agreement with the effects predicted by the simple heat flow model. It has been found that skin temperature is dependent upon a large number of variables; in addition to environmental effects these may include degree of alertness, distraction and agitation, limb size, age, sex, nervousness, fatigue, hormonal variations, medication and immediate history. Whilst these effects clearly complicate attempts to relate absolute surface temperature measurements to particular physiological or psychological changes, they do suggest that skin temperature could provide a simple means of monitoring inany aspects of a patient's physical and mental state. In this connection, the development of both existing and new techniques for making non-invasive physiological temperature measurements could prove to be of considerable value.
Conclusion I t has been found that temperature changes in soft tissues do result as a direct consequence of applied compressive stresses, and that these changes can be easily detected using both discrete temperature sensors and infra-red thcrmography.
Fig 4. Typical therrnographic result for a finger following the removal of loaded and unloaded indenters. The difference is shown as the deduced curve
References
thermistor observations and yielded more valuable results. By use of the deduced curves a characteristic pise in skin teiriperature after removal of load was found in all cascs. This rise was well defined and persisted for a minimum time of about five minutes. These preliminary results have also suggested a possible correlation between the height of the temperature peak
1. Rogers, J Efji,c/.s of el-ter-rzcrl forces on tissrrc,. Annual Reports of Progress, Rehabilitation Eneincerine Ccntte :it Rancho Los Amicos Hospital-University Gf S o u s e r n California, Downcy, California, pp 71-76, DCC. 1972-Nov. 1973. 2. Redfern, S J, Jeneid, P A, Gillingham M E a n d Lunn, H F Local pressures with ten types of paticnt-support sys'tem. The LrrrZcet, pp 277-80. August 11, 1973. 3. Mooney, V, Einbund, M J, Rogers, J E and Stauffer, E S Comparison of pressure distribution qualities in seat cushions. Ult
