Clinical Relevance of Microclimate Tests on Support Surfaces

INTRODUCTION Microclimate is defined as the climate of a local region that differs from the climate in the surrounding region (ambient climate). In the circumstance of pressure injury (PI) prevention, the microclimate of the weight-bearing skin surface is of particular relevance.1 As this confined area of skin becomes moist, the coefficient of friction between the skin and common bedding materials is approximately doubled, increasing the forces transmitted to the skin.2 Skin that is moistened or “wetted” gradually loses much of its mechanical strength and is therefore susceptible to tearing at lower levels of force.3 Because increased local skin temperature is one of the drivers of local perspiration, when the skin warms beyond a “perspiration threshold,” moisture production from the skin itself increases markedly.4–6 The metabolic demand of the skin is increased by approximately 10% per 1 °C of warming, as determined in a rat model.6 When blood flow is limited because of mechanical loading, these effects of ischemia on warm skin may be more detrimental than on cooler skin under the same loading conditions. Warm skin is therefore more susceptible to ischemic injury, as confirmed by a porcine model.7,8 Microclimate management (MCM) is achieved by applying some form of control to the skin surface’s heat and/or moisture levels.8 The direct care clinician can perform MCM through many actions, including turning or repositioning, which may allow for the dissipation of built-up heat and humidity. In addition, full-body support surfaces have many features that can assist the direct care clinician with MCM, such as mattress-cover materials and airflow designs. Knowledge of the result(s) of the microclimate performance characteristic capability of a full-body support surface can enable the clinician to create an individualized plan of care for a given patient. The microclimate testing standard contains three tests: body analog, sweating guarded hot plate, and heated water bladder. The determination of which testing method is appropriate for which full-body support surface is dependent on the construction design of the support surface. Microclimate management therefore is not solely a support surface issue; developing an individualized care plan for a given patient requires: an understanding of how to manage heat and moisture at the skin surface, a review of these effects on the skin, education describing the current standard test methods and results for microclimate, and their relevance to a full-body support surface. COMPOUNDING EFFECTS OF HEAT AND MOISTURE ON SKIN INTEGRITY Microclimate (heat and humidity) is one external risk factor that can decrease tissue tolerance that can then affect two causative factors of PIs: pressure and shear.9 Although the focus of this article is microclimate, the authors would be remiss if they did not provide a review of the causative factors. Pressure in soft tissue results from the weight of the body pressing down through tissue that is supported and constrained by internal and external supporting structures. Injuries that develop from pressure often begin near the muscle-bone interface as a reaction to cellular/tissue deformation and ischemia. Shear stress and strain are measures of internal force and deformation, respectively, that result from external friction and body weight distribution.10 Shear stress and shear strain are collectively referred to as “shear.” The frictional force is proportional to the coefficient of friction, which, for skin, is heavily dependent on its hydration level and thus can be altered if the skin surface tissue has been compromised by dehydration or moisture. Skin is resilient and might be considered a brick-and-mortar structure with corneocytes representing the brick and intercellular lipid layers as the mortar. This wall-like structure provides the water-resistant function of the skin, yet it is only 0.01 to 0.1 mm thick, thinner than plastic cling wrap used to protect sandwiches.4 Thermoregulation by the skin is a complex process involving the multipart system skin structure, thermoreceptors, and sympathetic nervous system, which controls vascular flow via vasodilation or vasoconstriction. Vasodilation results in increased blood flow to the skin, which releases heat by conduction, convection, radiation, and evaporation. As the body senses increased heat, mechanisms are in place to increase the normal rate of body moisture evaporation.4 Overhydration of the skin disrupts the brick-and-mortar structure, which is detrimental to skin integrity; this is a known aspect of skin care, and clinical interventions are well established11 to prevent breakdown and the development of moisture-associated skin damage. In the state of overhydration, the body has perceived a heated area and increases the rate of sweat production to use evaporation for cooling. If there is no evaporation and heat persists, the body makes another attempt to dissipate localized heat and increases its sweat rate again. Microclimate is often thought of as a condition of excessive moisture, yet a lack of moisture could also play a role in skin breakdown.2 To preserve the structure and function of the brick-and-mortar layer, keeping a balance between hydration and drying of the skin is important. Keeping the skin relatively cool and moisture-balanced, through bedside clinical interventions and the use of appropriate full-body support surfaces, should be incorporated into the patient’s plan of care.12 Many full-body support surfaces offer construction features and performance characteristics to assist in reducing excess heat and humidity. Microclimate can be affected by the type of full-body support surface selected for the individual, the number and type of layers between the individual and the full-body support surface, and the interval period of repositioning and other activities performed during the care of the individual. Research studies suggest that the low back area has a normal, open-air temperature range of 90 to 95 °F.13 When the skin warms beyond a perspiration threshold, moisture production from the skin itself increases markedly.4 Many full-body support surfaces are designed to help regulate the microclimate at the skin surface interface. Some examples are airflow beneath a moisture vapor-permeable cover, air-fluidized therapy, and a powered microclimate coverlet with airflow. The performance of a full-body support surface with a microclimate feature is not just the rate of airflow through the system.14 Different technologies are used to manage temperature and humidity based on the construction design. These different technologies result in the differing approaches used for testing their effects. In current practice, an individual is rarely on a full-body support surface without bed linen, an incontinence pad, and/or a safe patient-handling device under the lower trunk. These items have both individual and combined effects on the microclimate at the skin interface. The principle of diffusion dictates that heat and moisture will flow from an area of high concentration to an area of low concentration to approach equilibrium. When the individual is warmer and more moist than the full-body support surface, the heat and moisture move into the layer closest to the skin, such as the patient gown. Various design methods and materials are utilized to create a gradient to move excess heat and moisture away from the individual. It has become common practice that fewer layers between the individual and the full-body support surface are the most advantageous for maintaining a more optimal microclimate.13 How does the direct care clinician affect the extrinsic factor of microclimate when bed linen and incontinence pads are expectations of proper care? This can be achieved through regular intervals of turning and repositioning and applying only the essential linen/incontinence care products. When an individual is turned and repositioned, the individual’s previous weight-bearing side is exposed to the environment, allowing the skin temperature to normalize and dissipate moisture.15 If the patient is immobile and not turned, their skin will accumulate heat and moisture, which may lead to localized sweating. FULL-BODY SUPPORT SURFACE SELECTION CONSIDERATIONS When choosing a full-body support surface, it can be challenging to determine which type of surface would best serve the needs of a patient population or a specific type of individual. Currently, no one surface can meet the needs of all patients or specific patient populations. For example, an ICU patient could be immobile and have medication or comorbidities that produce increased perspiration levels. These conditions may require a full-body support surface with greater immersion, envelopment, and MCM performance features. In contrast, a medical/surgical unit patient could be very mobile, which would not require the advanced level of full-body support surface assist features. In the remainder of this article, the authors intend to provide a better understanding of the clinical relevance of microclimate full-body support surface test methods. There is a wide variety of full-body support surfaces offering performance features to assist with microclimate, pressure redistribution, and shear management through their construction design. In the US, the development of hospital-acquired PIs is considered an adverse event by the CMS. Members of the National Pressure Injury Advisory Panel realized a need to standardize full-body support surface feature performance testing and explain the clinical relevance of these tests to healthcare workers.16 Acting upon that realization, a task force was formed in 2001, the Support Surface Standards Initiative (S3I).16 The S3I’s purpose is to establish standardized test methods to provide a way of objectively comparing singular performance characteristics of full-body support surfaces.16 This interprofessional group developed three microclimate testing methodologies for comparing a full-body support surface’s capability to manage temperature and humidity at the full-body support surface interface. FULL-BODY SUPPORT SURFACE STANDARDS FOR MICROCLIMATE The process of test method development began with S3I members designing test(s) from scientific principles to drive test reliability and repeatability in different laboratories. The results of these tests were shared and then approved by voting members of the S3I committee. The test methods were then submitted for approval to the American National Standards Institute and the Rehabilitation Engineering and Assistive Technology Society of North America Standards Committee, the US standards body for many other products, and accepted.16 These standardized test methods are performed in controlled environments (eg, temperature and humidity controls) to provide results that will enable comparisons.16 It is important to note that all test methods may not be appropriate for every full-body support surface depending on the surface’s design, construction materials, and mode of operation. This is why there are multiple test methods for the same performance characteristic. Manufacturers are responsible for sharing this information with their potential buyers. The S3I committee used the currently accepted body model of the 50th percentile male, which is equivalent to 81.6 kg (179.9 lb) as a reference to design the test fixtures or indenters.17 Currently, these three tests evaluate microclimate (temperature and humidity) performance: the body analog method, the sweating guarded hot plate method, and the heated water bladder method.17 These tests are performed with test fixtures or indenters in direct contact with full-body support surface covers for standardization. Each test measures temperature and/or humidity under varying individual conditions. It is important to state that microclimate tests are only one characteristic of a full-body support surface’s performance; it is therefore not intended as an overall rating of full-body support surface performance.18 Body Analog Method This test method is designed to show the concurrent measurement of the dissipation of heat and water vapor of a full-body support surface. The body analog method uses an indenter to simulate the water vapor and heat production of a 50th percentile male and measure how they accumulate over time at the full-body support surface interface17 (Figure 1).FIGURE 1: BODY ANALOG INDENTERReprinted with permission from the Rehabilitation Engineering and Assistive Technology Society of North America.The results include temperature (°C) and relative humidity at the full-body support surface interface. Surfaces that tend to provide more resistance to heat flow and less breathability will have a higher temperature and relative humidity; the inverse is also true. This test simulates how the full-body support surface allows heat and moisture vapor to accumulate (a high test result value) or not accumulate (a low test result value) under an average afebrile individual. The indenter is warmed to simulate normal body temperature and is placed on the full-body support surface for a total of three trials of 3 hours 15 minutes with the sensors recording at least once per minute, to simulate the body in place before repositioning. The indenter is raised off the surface for 45 seconds and then returned to the full-body support surface, completing one trial. After the three trials are performed, the data are tabulated and compared before a final result figure of microclimate performance capability is assigned. A low test rest value indicates that the heat and moisture vapor did not accumulate. Sweating Guarded Hot Plate Method The skin’s natural flow of heat and humidity to the environment is impeded when in contact with a full-body support surface. “The resistances to the flow of heat and humidity imposed by the full-body support surface are therefore meaningful measures of the degree to which a surface is likely to cause skin warming and/or moistening, both of which have been shown to have significant and harmful clinical effects.”19 This test method consists of a series of tests that provide data on the heat and moisture transfer rate through materials in contact with a source of heat. These tests consisting of dry heat flux (W/m2; flux is also known as the term flow), wet heat flux (W/m2), and evaporative capacity (g/m2 per hour) and designed to simulate and calculate the rate heat and moisture are transmitted from the warmed indenter (simulating skin) through the full-body support surface and into the environment (Figure 2).FIGURE 2: SWEATING GUARDED HOT PLATE DEVICEThermal sensor is placed on the surface to be tested and then under the indenter.Reprinted with permission from the Rehabilitation Engineering and Assistive Technology Society of North America.Three 30-minute trials are performed with the head of the bed at 0° and at 45° for each component of this test The dry heat flux test method the degree to which heat is away from a dry indenter (simulating The wet heat flux test simulates how much heat and moisture vapor are away from an indenter moisture (simulating sweating If the dry or wet flux result is this indicates heat can accumulate and increase the temperature of the individual’s the higher the test result the more heat is away to excessive warming of the capacity provides into the capability of a full-body support surface to moisture away from The higher the the more significant the in heat dissipation and evaporative moisture the lower the the less in heat dissipation and evaporative moisture Method The heated water bladder method measures the full-body support surface’s to dissipate water This method uses a water bladder to simulate a at A wet is placed between the heated bladder and the full-body support surface to simulate The measurement of water is every minutes for or and the rate of moisture is The higher the as in per indicates more moisture was from the support surface interface (Figure with permission from the Rehabilitation Engineering and Assistive Technology Society of North higher moisture vapor rate value can be for individual such as are or or have comorbidities that result in higher perspiration a lower moisture vapor rate more moisture may accumulate at the interface between the skin and the full-body support surface and to Skin It should be that the microclimate testing is solely on the heat and water vapor that is through the skin and the that on the skin and does not moisture that is with or A full-body support surface can use many different principles to manage of A variety of or mechanisms can move heat and/or moisture away from the interface with the individual’s methods a powered source such as methods use specific characteristics of materials and Some of these methods solely to some manage moisture, and to only is the internal design to the principles of microclimate, the design is also to enable moisture vapor and heat to more through it and be away from the surface of the individual at the interface. The body analog test is performed only the of moisture from an individual during a state and how the surface when the individual is The results from the body analog test show how heat and water vapor accumulate over Surfaces that not allow for a in temperature or humidity at the interface and to temperature with repositioning would be for this afebrile individual. In a support surface for a individual, the results of the heated water bladder test would be most The heated water bladder test how much moisture vapor is from the full-body support surface interface. In this a high result would more moisture vapor is which would be to the individual. The results from the sweating guarded hot plate test could also serve as a of how much moisture vapor and heat can be from the full-body support surface interface. 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