The measurement of interface pressure
This source preferred by Ian Swain
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Authors: Swain, I.
There are a number of factors that can predispose an individual to a high risk of developing pressure ulcers. These can be divided into external factors, including pressure, shear, time, temperature, humidity and their interactions, and internal factors, which determine the level of loading tolerated by tissues before damage occurs [ii. The internal factors can be affected by the underlying disease such as diabetes or certain neurological conditions in which the tissues are more liable to be damaged by a given level of pressure and are dealt with elsewhere in this book. This chapter is primarily concerned with the external factors, in particular pressure, which can be affected by a loss of muscle bulk and tone as in flaccid paraplegia, or by the weight loss associated with the latter stages of cancer. Although the majority of this chapter will deal with the interface pressures measured on mattresses and cushions it should be remembered that pressure ulcers can occur in other situations. They may be found on the feet due to poorly fitting footwear, especially if the patient is diabetic, on limb stumps due to prostheses, or under orthoses, especially if the orthosis is exerting significant force to try to prevent spinal or bony deformity. There are, basically, only two ways associated with pressure in which a support surface can operate in order to reduce the probability of a pressure ulcer developing. Firstly, there are static systems which seek to minimise the interface pressure by increasing the contact area, and secondly dynamic systems which produce an alternating action that subjects the tissues to periods of high pressure followed by periods of low pressure during which it is anticipated that the pressure is sufficiently low to enable blood flow to return. The development of accurate pressure-measuring systems is important in assessing such support systems. However their exclusive use in determining risk of breakdown is critically dependent on a reliable indicator of safe pressure, or band of pressures, in association with time [21, that would be appropriate for all patients at risk. This remains a holy grail for medical engineers involved in the prevention of pressure ulcers. Many attempts have been made to determine the minimal degree and duration of compression that will consistently produce tissue damage [35j. Often quoted as a cut-off figure is 32 mmHg, which is the capillary pressure as measured by Landis [6j. However, this value, measured in 1930, was determined in a nail fold capillary at heart level and it is therefore difficult to see how this relates to the external pressure needed to stop blood flow in capillaries when a person is sitting on a cushion or lying on a mattress. In such cases the picture is much more complicated, as there are supporting structures such as collagen and muscle fibres around the capillaries which will serve to distribute the applied load. In addition, the blood pressure in the capillary will vary due to both systemic blood pressure and the hydrostatic head of blood, which will depend upon the persons posture. What is undeniable, however, is that high pressures are sustainable for short times only and, if maintained, will lead to tissue breakdown.