Although not enough is known about the clinical benefits of support surfaces, they are being used successfully to prevent and manage pressure ulcers.

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ore than 1.8 million persons in the United States are affected by pressure ulcers.¹ One of the primary therapeutic interventions for pressure ulcer prevention and treatment is the use of support surfaces. Support surfaces may be defined as “special beds, mattresses, mattress overlays, or seat cushions that reduce or relieve pressure while sitting or lying.”² Along with the developing science of pressure ulcers in general, researchers and clinicians are mounting an awareness of the need for additional technical and clinical evidence in order to build an expanded knowledge base related to support surfaces.

Support Surfaces

       Landis³ measured capillary blood flow pressures, and much of support surface technology and design have been based on these findings. Blood pressures in the capillary beds of 50 healthy individuals were measured and reported as an average of 32 mmHg in the arteriole, 20 mmHg at the capillary exchange level, and 12 mmHg in the venule. More recently in 1988, Williams and others⁴ repeated a similar scientific experiment and concluded that the average “functional” operating pressure of the capillary circulatory system is 17 mmHg. These pressures, recorded in healthy individuals without cardiac or vascular disease, have been generally drawn on in order to extrapolate information regarding the obstruction of blood flow in and out of the capillary circulatory system (otherwise known as capillary closing pressure or the amount of pressure necessary to collapse vessels in the capillary area to the point that blood flow is either reduced or totally occluded to local tissue).²
       Based on the findings of the previously mentioned studies, the “gold standard” for external tissue interface pressure, or the pressure measured externally between the skin and the surface using pressure sensors, has had numerous implications for technical measurements and clinical utility of support surface effectiveness.⁵ Knowing that most pressure ulcers occur within populations that are ill, elderly, or otherwise infirmed, these conclusions may be misleading.
       While tissue interface pressure measurements may have an indirect relationship with interface pressures at the bone-muscle interface, this has not been fully explored. There are some data that report that interface pressure internally between the bone and the muscle may be 5 times higher than interface pressures between the skin and the support surface interface. The actual significance of this increase in pressure has recently taken center stage, with an emphasis on the role of shear in deep tissue damage as it relates to pressure ulcer development.⁶
       Animal studies conducted by Linden⁷ and Kosiak⁸ indicated that both the intensity and duration (or length of pressure application) of pressure application obstruct capillary blood flow obstruction, which leads to reduced levels of oxygen and nutritional supply to the tissue and the removal of waste material and metabolites from the tissue, resulting in cell death, tissue necrosis, and pressure ulcer formation.
       Therefore, one can extrapolate that both the amount of pressure and the time of pressure application play a role in pressure ulcer development. Among the list of facts we know about support surfaces:
1. Support surface therapy is widely accepted as an appropriate treatment intervention for patients and residents at risk for pressure ulcer development and for the treatment of pressure ulcers.²
2. There are a number of areas with little or no valid research evidence to substantiate the accurate use of these medical devices.⁵
3. The major body of technical evidence is produced and propagated by marketing teams of support surface manufacturers.⁵
4. There are no standard technical measurements or reporting techniques for support surfaces, making it difficult to compare and contrast support surfaces.⁹
5. Animal studies have shown that not only the amount but also the duration of pressure had an effect on tissue, resulting in tissue damage.^7,8
6. Most technical reports are based on the use of sensor mechanisms to measure the pressure between the skin surface and the support surface otherwise known as tissue interface pressure.⁵
7. Two support surface technologies have evolved over time and been described in the literature.¹⁰

Constant Low-Pressure

       Constant low-pressure support surfaces are designed to decrease the contact pressure by increasing surface area supporting the individual through “immersion” and “envelopment.” These are also referred to as static support surfaces. The construction of the support surface allows the individual to sink into the material and become immersed and enveloped. This results is an increase in contact area, thereby reducing the tissue/surface contact pressure over bony prominences. Constant low-pressure support surfaces directly affect the intensity of the pressure application. While this modality or technology significantly reduces pressure over bony prominences, the tissue interface pressure does not change until the patient moves or is repositioned.⁹

Alternating Air Pressure

       Alternating air pressure technology is designed so that air cells systematically inflate and deflate in an alternating sequence, thereby changing body surface

Alternating air pressure technology is designed so that air cells systematically inflate and deflate in an alternating sequence, shifting body surface pressure

pressure from one site to another.⁹ This technology is referred to as dynamic support surfaces because contact pressure on the body surface change using specific, set-timed intervals and alternate by deflation and inflation of air-filled cylinders or cells.¹¹ The alternating support surface changes not only the intensity of the pressure application but also the duration of the pressure application. As the cells alternate, the action removes pressure from one area of the body and then applies pressure to the adjacent tissue area, affecting the duration of the pressure application.⁵
       Although much is known about support surface therapy, many questions remain unanswered, including:
1. What are the actual roles of shear, friction, moisture vapor, and heat in relationship to pressure in pressure ulcer development?
2. What is the exact pressure at which blood flow in capillary circulatory system ceases to flow for every individual?
3. What is the exact amount of external pressure required to occlude internal blood flow?
4. What are the best methods or tests to use to compare and contrast support surface therapy effectiveness?
5. What other methods of measuring support surface therapy are advised?
6. What else do we not know?

As the air cells alternate, the action removes pressure from one area of the body and applies it to the adjacent tissue area, affecting its duration.

       Pressure has been the major focus of support surface technology and is used to describe the cause of pressure ulcers in this well-known definition: “any lesion caused by unrelieved pressure that results in damage to underlying tissue.”² External pressure, defined as “the force per unit area that acts perpendicularly between the body and the support surface,”² has been recognized as a major cause of pressure ulcer development. Pressure (or perpendicular force) must be addressed in nursing interventions for patients and residents across the healthcare continuum at risk for pressure ulcer development as well as for those who require treatment for existing pressure ulcers. Support surfaces as an intervention for pressure ulcer prevention and treatment have been widely accepted as “best practice”¹² and “standards of care” in the prevention and treatment of pressure ulcers.^12,13
       Other areas that are not as well-understood by clinicians and researchers as reasons for pressure ulcer development are shear, friction, moisture, and heat.⁹ New testing methods in the developmental stage include the re-evaluation of the “interface pressure measurement,” transcutaneous oxygen and transcutaneous carbon dioxide recordings, blood flow measurements with laser Dopplers, and the effects of heat and moisture.
       Tissue interface pressure measurements are currently being adapted to integrate not only the degree of pressure but also the length of time that the pressure is applied. It has been noted that humans can withstand high pressures for a short period of time and low pressures for a longer period of time.¹⁴ Since the time/pressure interval is necessary to accurately measure alternating pressure mattresses, standardized measuring and labeling techniques are necessary in order to compare technical measurements among these types of support surfaces.
       Testing on blood perfusion and blood flow to the skin is also under development as a potential measurement of the effectiveness of support surfaces. Researchers have used tcPO2 and tcPCO2 to evaluate blood flow in the capillary bed.¹⁵ Others have used laser Dopplers to compare blood flow with pressure on and with pressure off. The use of heat and moisture are also currently being investigated.⁵
       Clinical outcome trials provide the best clinical evidence to date. These are necessary to determine the appropriate use and clinical effectiveness of specific support surface designs or technology with regard to the reduction of pressure ulcer development or the rate of pressure ulcer healing. Most systematic reviews and guidelines reserve review for randomized controlled trials, but these are difficult to carry out and very costly. Therefore, there are limited references available in the literature, leaving clinical decisions regarding support surface selection to the individual clinician’s judgment.

Conclusion

       While there is little evidence regarding the clinical benefits of support surfaces for the prevention and management of pressure ulcers, these devices are used successfully on a day-to-day basis in the United States to achieve pressure redistribution. Additional support surface research, including the development of common terminology and comparable testing methods, is needed in order for clinicians to choose the right support surface for the right patient. (For additional information on the ongoing development of standardized support surface nomenclature and testing methods, visit the National Pressure Ulcer Advisory Panel, NPUAP, website at www.npuap.org for information about the Support Surface Standards Initiative, S31.)