Prolonged external pressure over bony prominences has long been identified 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.
This article focuses on the possible effects of support surface characteristics—pressure distribution, shear, temperature control, and moisture control—on pressure ulcer prevention. The relationships relative to elastic, viscoelastic, fluid-filled, low-air-loss, air-fluidized, and alternating pressure support surfaces are examined. Explanations of the support surface characteristics can be found in Table 1.
Support surface technologies can be classified in many different ways. In this article, they are classified according to their mechanical characteristics (ie, materials) or unique therapeutic function (ie, features). In practice, most products consist of a combination of materials and incorporate multiple features.
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.)
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 per cubic foot.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 or fluid-filled bladders and foam. 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 elastic foam 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 the interface pressure low. Over time and with extended use, foam degrades and loses its stiffness.2 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.3 In other words, the mattress bottoms out.
The stiffness and thickness of foam limits its ability to immerse and envelop. 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.7 Heat transfer rates for mattresses with nonstretch and 2-way stretch covers are less (by nearly half) than those for mattresses without covers.7
The increase in moisture in foam products with porous covers is comparatively lower because the open cell structure of the covers provides a pathway for the moisture to 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
Viscoelastic foam products consist of viscoelastic, open-cell foam that is temperature-sensitive. The foam becomes softer at operating temperatures near body temperature, the effect of which is that the layer of foam nearest to the body provides improved pressure distribution through envelopment when compared with high-resilient foam. The viscoelastic foam acts like a self-contouring surface because the elastic response diminishes over time, even after the foam is compressed. The disadvantage of the temperature- and time-sensitive response is that the desirable effects may not be realized when the ambient temperature is too low. The properties of viscoelastic foams vary widely and must be chosen according to the specific needs of the patient for seat and mattress applications. Solid gel products are also viscoelastic in nature and are included in this category.
Mean temperature increases of 2.8°C have been reported for viscoelastic foam.8 Gel products, on the other hand, tend to maintain a constant skin-contact temperature or may even reduce the contact temperature. Gel pads have higher heat flux than foam due to the high specific heat of the gel material. However, in 1 study, the heat transfer rate decreased after 2 hours, indicating that the heat reservoir was filling.8 This suggests that temperature may increase during longer periods of unrelieved sitting (more than 2 hours). In addition, this study found that moisture increased by 22.8% during a 1-hour period. The relative humidity of the skin surface increases considerably because of the nonporous nature of the gel pads.
Fluid-filled products may consist of small or large chambers filled with air, water, or other viscous fluid materials, such as silicon elastomer, silicon, or polyvinyl. The fluid flow from chamber to chamber or within a single chamber is passive in response to movement and requires no supplemental power. The term air-flotation is sometimes used to describe interconnected multichamber surfaces, such as those manufactured by ROHO, Inc, Belleville, IL. For air cushions, care must be taken to maintain the correct level of inflation to achieve optimal pressure reduction. Underinflation causes bottoming out and overinflation increases the interface pressure. For viscous fluid-filled seating surfaces, such as the Jay2 seat cushion (Sunrise Medical, Inc; Carlsbad, CA), the distribution of viscous material must be carefully monitored and the material must be manually moved back to the areas under bony prominences if it has migrated.
Most fluid-filled products permit a high degree of immersion, allowing the body to sink into the surface as the surface conforms to bony prominences. This effectively increases the surface pressure distribution area and lowers the interface pressure by transferring the pressure to adjacent areas. These products are capable of achieving small to modest deformations without large restoring forces. In a direct comparison of interface pressures with air-fluidized (Clinitron, Hill-Rom, Inc; Batesville, IN) and low-air-loss beds (Therapulse, Kinetic Concepts International; San Antonio, TX), the RIK mattress (a fluid-filled product) was shown to relieve pressure as effectively as the aforementioned technology.9
Skin temperature is affected by the specific heat of the fluid material contained in the support surface. Air has a low specific heat (indicating a limited ability to conduct heat) and water has a high specific heat (greater capacity to increase heat flux). The viscous material used in the RIK mattress also has a high specific heat; skin temperature decreases have been demonstrated with this product.9
Given the large variety of materials used as covers for products in the fluid-filled category, it is difficult to generalize on moisture control characteristics. However, the insulating effects of rubber and plastic used in some fluid-filled products have been shown to increase the relative humidity due to perspiration.8
Air-fluidized beds have been available since the late 1960s and were originally developed for use with burn patients. These products consist of solid particles, usually glass solid particles (75 to 150 mm), encased in a cover sheet. The solid particles take on the characteristics of a fluid when pressurized air is forced up through them. Feces and other body fluids flow freely through the sheet. In order to prevent bacterial contamination, the bed must be pressurized at all times and the sheet must be properly disinfected after use by each patient, and at least once a week with long-term use by a single patient.10
Air-fluidized beds use fluid technology to decrease pressure through the principle of immersion while simultaneously reducing shear. Air-fluidized products permit the highest degree of immersion among those currently available, allowing the surface to conform to bony prominences. Almost two-thirds of the body may be immersed into the surface,11 effectively lowering the interface pressure by increasing the surface pressure distribution area. The high degree of immersion possible with this technology enables the transfer of pressure to adjacent body areas and other bony prominences. Shear force is minimized by the use of a loose (reduced surface tension) cover sheet.
The pressurized air in these products is generally warmed to a temperature of 28°C to 35°C. This warming feature could be beneficial or harmful, depending on specific patient characteristics. For example, the heat may be harmful to patients with multiple sclerosis or beneficial for patients in pain. In any case, the beneficial effects must be balanced against the increasing metabolic demands of tissue.
The high degree of moisture vapor permeability of the system is very effective in managing body fluids. In cases of severe burns, however, air-fluidized beds have been known to cause dehydration.
Low-air-loss describes a support surface feature that lets the air pass through the pores of the cover material. The covers are usually made of a special nylon or polytetrafluoroethylene fabric with high moisture vapor permeability. Many support surfaces employing this feature use a series of connected air-filled cushions or compartments. These cushions are inflated to specific pressures to provide loading resistance based on the patient's height, weight, and distribution of body weight. An air pump circulates a continuous flow of air through the device, replacing any air that is lost through the surface's pores. The inflation pressures of the cushions vary with patient weight distribution; some systems have individually adjustable sections for the head, trunk, pelvic, or foot areas.12 One manufacturer offers the ability to individualize each of the compartments rather than just the sections.12 Support surfaces are available that combine low-air-loss with alternating and pulsating pressure features.
In low-air-loss systems, the patient lies on a loose-fitting, waterproof cover that is placed over the cushions. The waterproof covers are designed to allow air to pass through the pores of the fabric; they are usually made of a special nylon or polytetrafluoroethylene fabric with high moisture vapor permeability. Manufacturers have addressed the problem of dehydration of the skin by altering the number, size, and configuration of the pores in the covers.12 The material is very smooth, with a low coefficient of friction; bacteria impermeable; and easy to clean.11
Low-air-loss beds use fluid (air) technology to distribute pressure using the principle of immersion. The deeper the immersion, the greater the surface area for pressure distribution. Most surfaces employing the low-air-loss feature allow the air inflation level to be adjusted so that the immersion and pressure redistribution can be increased. These devices are capable of achieving moderate to large deformations without large restoring or shear forces.
The volume of air may be adjusted to provide more or less immersion for the entire body, for specific sections, or even for individual chambers or cells. The loose-fitting covers envelop and decrease friction. In fact, care must be taken so that the patient does not slide off the mattress during bed transfers.
As with other fluid-filled surfaces, the temperature of the skin is affected by the specific heat of the fluid material; air does not have a high specific heat. In addition, although the circulating air gets warmed, the constant air circulation and evaporation tend to keep the skin from overheating.
In systems with low air loss, the patient's skin is in contact with the cover. The local tissue environment is a function of the moisture vapor permeability of the cover and cushion materials, the airflow and porosity of the cover and cushion materials, and the thermal insulation of the cover. The ideal combination of these factors would be a material with a high thermal insulation to prevent excessive loss of body heat, a high moisture vapor permeability to prevent accumulation of excess moisture on the skin, and a moderate airflow to keep the skin from overheating. In one study, low-air-loss cover materials were rated based on a normalized comparison of these parameters.13 The combination of a cover and a cushion made from nylon/high-air-loss Gore-Tex laminate material had the highest scores and was most likely to promote a favorable local climate at the skin-cover interface.13 Devices with low air loss have been shown to prevent build-up of moisture and subsequent skin maceration.11
Alternating pressure describes a support surface feature in which the pressure distribution is periodically altered. Most surfaces employing this feature contain air-filled chambers or cylinders arranged lengthwise, interdigitated, or in various other patterns. Air is pumped into the chambers at periodic intervals to inflate and deflate the chambers in opposite phases, thereby changing the location of the contact pressure. Pulsating pressure differs from alternating therapy in that the duration of peak inflation is shorter and the cycling time is more frequent. The latter appears to have a dramatic effect on increasing lymphatic flow.14
The concept of alternating pressure for prevention of tissue ischemia is not new. Kosiak concluded in 1961 that “since it is impossible to completely eliminate all pressure for a long period of time, it becomes imperative that the pressure be completely eliminated at frequent intervals in order to allow circulation to the ischemic tissue.”15 Houle's conclusion that a dynamic device that alternately shifts the pressure from one area to another would be “the choice to provide adequate protection against the development of ischemic ulcers”16 has been supported over the years by many others.17–24 Rather than increasing the surface area for distribution through immersion and envelopment, alternating pressure devices distribute the pressure by shifting the body weight to a different surface contact area. This may increase the interface pressure of that area during the inflation phase.
The lack of sufficient study of the tissue responses to alternating pressure leaves many questions regarding this type of support surface. For example, what are the ideal characteristics of the support surface (geometry of the surface [size/shape of cells and space between cells] and the material, depth, composition, and shape of the supporting structure)? Also, what are the ideal characteristics of the alternating cycle (rise time, hold time, duration of total cycle, pattern of relief)?
Alternating pressure technology has the same potential as any other fluid-filled support surface to influence temperature at the interface, and care must be taken to maintain the correct levels of inflation. The skin moisture control and temperature control characteristics of an alternating pressure surface will also depend on the characteristics of the cover and supporting material.
Although support surface technologies have been designed to reduce mechanical (pressure, shear, friction) and additive (moisture, temperature) factors implicated in pressure ulcer formation, most support surface comparisons have relied solely on interface pressure measurement, a limited and highly variable method that will not be discussed here. In more recent years, research has gone beyond assuming that pressure ulcers are the result of external pressure alone. Current studies are focusing on the physiologic, biochemical, and biomechanical characteristics of tissue and their interactions. The results of these investigations reflect the limitation of using interface pressure as a sole indicator of the threshold for pressure ulcer formation. Although based on an individual's relative responses, interface pressure measurements may effectively aid in selecting the best support surface for that individual. However, interface pressure alone is not sufficient to evaluate the efficacy of a particular device or class of devices. Many factors make the results of support surface studies difficult to compare. For now, the best evidence regarding the effectiveness of support surfaces appears to be the outcome of a decrease in the incidence of pressure ulcers coupled with multiple measures of tissue response.
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24. West J, Hopf H, Suski M, Hunt T. The effects of a unique alternating-pressure mattress on tissue perfusion and temperature. Paper presented at the European Tissue Repair Society, Padova, Italy, in 1995.