Understanding support surface technologies

Advances in Skin & Wound Care,  Sep/Oct 2000  by Brienza, David M,  Geyer, Mary Jo

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rolonged external pressure over bony prominences has long been identifled as the primary etiology in pressure ulcer development. Other related causes include the magnitude of shear and friction forces and the additive effects of temperature and moisture. Each of these factors can be affected by (and is related to) the characteristics of the support surface chosen for a given patient.

In this article, we concentrate on the possible effects of support surface characteristics-pressure distribution, shear, temperature control, and moisture control-on pressure ulcer prevention. We examine these relationships relative to elastic, viscoelastic, fluid-filled, low-airloss, air-fluidized, and alternating pressure support surfaces. Explanations of the support surface characteristics can be found in Defining Support Surface Characteristics.

Support surface technologies can be classified in many different ways. For the purposes of this article, they are classified according to their mechanical characteristics or unique therapeutic function. In practice, most products consist of a combination of materials and incorporate multiple therapeutic strategies. Examples of representative products in these categories are given in Table 1. This list illustrates the types of products included in the categories and is not intended to be all-inclusive.

Elastic Foam

An elastic material deforms in proportion to the applied load: Greater loads result in predictably greater alterations in the shape of the material and vice versa. Support surfaces that are made from resilient foam exhibit this type of elastic response. (If time is also a factor in the load versus deformation characteristic, then the response is considered to be viscoelastic; this will be discussed later.)

Foam support surface products are made from 2 basic types of foam-open cell or closed cell. Foam is said to have "memory" because of its tendency to return to its nominal shape or thickness. The minimum density (weight per cubic foot) of bed support surface material should be 1.3 to 1.6 pounds.1 Convoluted foam should have a minimum depth of 4 inches from the bottom of the foam to the lowest point of the convolution to achieve the optimal pressure-reducing effects of the material.

Foam products typically consist of either foam layers of varying densities or combinations of gel and foam. Other products, including several seat cushion products, have a series of air-filled chambers covered with a foam structure or are available as multidensity closed-cell products 4 to 10 inches deep with deflectable rips. These types of products do not have total memory because only the foam components will return to their unloaded shape. The advantage of support surfaces with a combination of fluid-filled bladders and resilient foam would be to provide a degree of postural stability with a resilient shell and improved immersion and envelopment with a fluid or viscous fluid-filled layer at the skin interface.

An ideal combination of characteristics for an elastic support surface would be resistance that also adjusts to pressure.23 The support surface should have a resistance to pressure that is high enough to fully support the load (prevent bottoming out) without providing a reactive force (memory) that is too high to keep interface pressure low. Over time and with extended use, foam degrades and loses its stiffness. This results in higher interface pressures. Mattresses typically wear out in 3 years and the pressure is then transferred to the underlying supporting structure used to support the foam.2 In other words, the mattress bottoms out.

Foams of varying densities may be combined or cut to relieve or conform to bony landmarks in order to enhance pressure distribution and possibly reduce shear forces. For example, multidensity, closed-cell foam products with deflectable tips provide some shear protection. Many pressure-reducing mattresses (PRMs) have loose-fitting covers to reduce friction.

Foam is limited in its ability to immerse and envelop by its stiffness and thickness. Soft foams will envelop better than stiffer foams, but will necessarily be thicker to avoid bottoming out. Foam seat cushions are frequently contoured to improve their performance. Precontouring the seat cushion to provide a better match between the buttocks and the cushion increases the contact area, thus reducing average pressure. Precontouring also increases immersion and envelopment properties, thus decreasing pressure peaks.4-6

Foam tends to increase skin temperature because foam materials and the air they entrap are generally poor conductors of heat. The heat transfer characteristics of foam mattresses are less than normal physiologic resting heat losses. Heat transfer rates for mattresses with nonstretch and 2-way stretch covers are less (by nearly half) than heat transfer rates for mattresses without covers.7

Moisture does not increase as much on foam products with porous covers as on most foams because the open cell structure of the covers provides a pathway through which moisture can diffuse. Water vapor transmission rates can be reduced by more than half when foam mattresses are covered with nonstretch and 2-way stretch covers.7