Understanding support surfaces
01/09/11 | Pressure ulcers | Michael Clark
Support surfaces have been classified in many ways over recent years, often upon the basis of how they modify the pressure exerted upon vulnerable body sites such as the sacrum and heels. This article describes the weaknesses of pressure-based classification schemes and highlights ongoing work in North America, Europe and Japan that seeks to provide explicit test protocols through which support surface performance can be quantified.
The use of specialised beds, mattresses, mattress overlays and seat cushions has been a cornerstone of pressure ulcer prevention for many years. While these devices are in common use in many care settings worldwide, there remains considerable confusion regarding their mode of action and the relative performance of different support surfaces.
This update provides a summary of the current trends in support surface description, classification and evaluation to highlight the global effort by researchers, clinicians and support surface manufacturers to provide clarity to a complex area of tissue viability.
Over the years various research and clinical groups have attempted to provide a classification of support surfaces. Some of the criteria used to classify support surfaces are demonstrated in Table 1.
For many years, common understanding of support surfaces focused upon their ability to keep the pressure on the skin's surface below 32mmHg (the mean pressure within the skin's capillaries). Support surfaces that applied less than 32mmHg of pressure to all the anatomical sites prone to pressure ulcers were deemed to provide pressure relief.
Other support surfaces that applied less than 32mmHg of pressure to particular anatomical sites where pressure ulcers might occur, but not the whole body, were called pressure reduction surfaces.
While pressure relief or pressure reduction was used to define support surfaces there was no agreed pressure measurement protocol through which each support surface could be evaluated. The failure to agree measurement protocols resulted in manufacturers and researchers using a wide variety of pressure measurement sensors, different patient and volunteer groups and several anatomical sites (eg sacrum, ischial tuberosities and heels) to examine how support surfaces influenced the pressure exerted on the body. This reduced the ability of clinicians and researchers to compare the statistics on pressure measurements available in the literature.
A flawed system
Unfortunately there were flaws with this system of support surface classification. Firstly, the selection of 32mmHg as a pressure threshold had limited relevance for the development of pressure ulcers. This is because the mean capillary pressure applied at the skin's surface is a poor indicator of pressure damage given that capillary pressures change over time and upon varying internal and extrinsic stimuli.
Similarly, the transmission of pressure within skin and soft tissue is a complex mechanical process with internal tissue changes not totally dependent upon the pressure measured at the skin surface and with different individuals having an increased or decreased tolerance to applied loads.
Secondly, a support surface classification based upon measured pressure at the skin's surface meant that no support surface appeared to relieve high pressure applied to the heels. Heel pressures are naturally high given the small area of contact between the heel and the support surface that must bear the weight of the foot and lower limb. In one human volunteer study, interface pressure was measured at the buttock or heel while subjects rested upon an air-fluidised bed, often considered to be a pressure-relief surface. However, while mean buttock pressure was low (2.65 kPa while supine, around 21mmHg), heel pressures were much higher (mean heel pressure 7.08 kPa equivalent to 56mmHg and far higher than the 32mmHg 'safe' pressure threshold).
The final flaw with the pressure-based classification lies in the use of terms such as pressure-reducing or relieving. When a person lies or sits upon a support surface, the pressures acting upon their body are the result of their body weight divided by the area of the body in contact with the support surface. Accepting that body weight can be considered as a constant, then altering the pressure depends upon increasing or decreasing the area of the body that is in contact with the support surface. As the pressure at one location, for example the sacrum, decreases, other areas of the body will bear a higher share of body weight burden, in essence redistributing body weight from one anatomical site to another. For this reason the term pressure redistribution should now replace pressure relief or pressure reduction[1,3].
There is, however, one exception to the general rule that links pressure changes to increases or decreases in the area of the body in contact with the support surface. This is the use of alternating pressure air mattresses and cushions, often called dynamic support surfaces, where changes in the pressure at vulnerable body sites are initiated by inflating or deflating air cells under the body in a specified sequence, thereby limiting the duration of the pressure applied to each body part.
The flaws in using terms such as pressure reduction and relief have been discussed for over 10 years but these descriptions of support surfaces still occur in product literature and in publications describing the clinical use of support surfaces. This is undoubtedly the result of there being no alternative classification of support surfaces considered valid by industry, clinicians and researchers.