Low Air Loss Support Surfaces
There are no universal solutions in wound care. Certainly this is true of support surfaces. A variety of support surface technologies are available and necessary to address the diversity of individual needs and circumstances. The most widely used therapeutic support surface technologies are: non-powered and powered air flotation, powered air flotation with low air loss, air fluidized and powered alternating pressure. Each of these technologies has its advantages and disadvantages, its trade-offs and urban legends. This article will focus on one of the most widely used technologies in the United States, powered air flotation with low air loss. It is certainly not my intent to imply that this technology is superior to the others mentioned, however, due to its substantial usage, its benefits and drawbacks should be considered and its fiction must be identified and dispelled.
Management of the following extrinsic risk factors: pressure, shear and friction, need to be addressed with distribution of pressure being paramount. |
What do we want and expect when we select a low air loss product? Clearly management of the following extrinsic risk factors: pressure, shear and friction, need to be addressed with distribution of pressure being paramount. In addition, we expect that the product be designed to allow airflow that directly, or indirectly, interacts with patient moisture, dissipating it, thus preventing and treating skin breakdown secondary to maceration and reduced tensile strength of the epidermis. The first misconception is that all products claiming to be low air loss do this.
Low air loss should not be mistaken with air flotation support surfaces. Although generally, products that provide low air loss also provide air flotation. Some products offer air flotation without low air loss. Confused? Read on and I will clarify the differences.
Humble Beginnings
The mere thought of a low air loss mattress conjures up images of full-thickness chronic wounds, compromised patients and complicated protocols. If an individual needs a low air loss support surface, they are without question, extremely ill requiring complex care. Why does low air loss paint such a vivid picture? Do we really understand low air loss as a therapeutic modality? What are the clinical criteria for recommending a low air loss product?
Dr. John Scales first developed the low air loss support surface for pressure ulcer management in the early 1970s. He described this system and his clinical findings in 1976. Numerous versions of this primary system evolved and became lighter, smaller and more user friendly. Low air loss (LAL) products have gained popularity and are now synonymous with the healing of chronic pressure ulcers. There are various designs to accommodate the needs of the end-user, clinician and care setting ranging from mattress overlays and replacement mattresses, to complicated adjustable fully integrated bed systems. The increase in the number of manufacturers has offered choices in features and benefits, as well as decreasing the prices of these surfaces.
Choosing support surfaces require providers to fully understand categories, technologies and clinical applications. Basically, what are the problems facing the patient and how might a particular support surface alleviate them? The following are factors affecting pressure ulcer formation: pressure, shear, friction, moisture and heat build up. These forces are generally considered to be the primary external causes of tissue deformation, harmful microclimate, lack of blood flow to the capillary beds and cells and resulting ischemia. By managing these forces and factors, we can diminish their detrimental effects on the tissue thus preventing and healing pressure ulcers.
Low Air Loss: A Technical Therapy
Low air loss support surfaces are capable of addressing all four of these forces or factors. Properly designed LAL support surfaces have the ability to evenly distribute a patient's body weight over connected pillows or cushions of air, minimizing pressure and resulting tissue deformation over bony prominences (the areas that are most vulnerable to pressure ulcers). These sections are inflated to specific pressures, according to a person's distributed weight, to provide load resistance. A pump circulates a continuous flow of air through the device, replacing air that is lost through the system's openings. Additionally, a large volume of air supplied to the mattress sections is allowed to escape directly under the patient bed boundary (Bodine, 1995). The coverlets are constructed of low friction, low shear material. These features further foster blood flow.
The heart of the low air loss system however, is a relatively high volume, low-pressure blower. The blower directs air into a mattress composed of a series of interconnected chambers or baffles. On each baffle (preferably the top portion at the patient-bed boundary) there are small holes that allow air from the blower to escape at a controlled rate. By varying the airflow into the baffles, and knowing the rate that air is lost, the pressure in the baffle can be very accurately controlled. The vapor permeable cover, constructed of low friction material, is placed upon the top of the low air loss mattress allowing the diffusion of moisture vapor into the airflow preventing moisture from contacting the mattress. The loose-fitting, air tight, vapor-permeable and moisture-impermeable cover does not allow free airflow to contact the skin, which could ultimately dehydrate the user. The moisture vapor from the patient passes through the top portion of the cover. Beneath the surface, the vapor interacts with the air flow from the system, thus reducing the humidity beneath the patient, which evaporates the excess moisture. This wicking method is capable of removing moisture and heat without drying the skin.
Another form of low air loss does not employ the use of a coverlet or uses a cover that allows air to flow freely through it. Rather, air is allowed to interface directly with the patient's body, potentially causing drying of the skin, desiccation of wounds and dressings, dehydration and vasoconstriction (decreased blood flow as a result of constriction of the vascular system). Another issue with this type of system is the potential of transmitting particles such as airborne pathogens, into the air. A careful investigation of the LAL mechanisms will allow the decision-maker to choose those systems that provide moisture and heat dissipation without drying and dehydration. Watch out for systems with pumps (rather than a blower) that are smaller than a shoe box. These systems do not have the capacity to keep the system afloat and address moisture and heat. Look for the telltale aquarium pump tubing supplying the airflow to the LAL support surface. This kind of tubing does not have a large enough lumen to provide enough air flow to address excess moisture. These types of support surfaces often become smelly and offensive after several months use, when taken apart to clean.
Physiology of Moisture and Heat Dissipation
Skin water loss ranges from insensible perspiration from respiration and normal metabolism to profuse diaphoresis or sweating to counteract heat build up in order to control body temperature. Cutaneous moisture loss is determined by the ambient temperature (room temperature), humidity and local and neural regulatory mechanisms of the body (Reger, et al, 2001). Elevating the body temperature will increase the metabolic activity. For every 1° C increase in body temperature, there is a concurrent 10 percent increase in metabolic rate, which increases energy and oxygen needs at the cellular level. Thus patients suffering from fever and impaired local circulation from pressure and shear can have resulting ischemia (lack of oxygen to the cells) much faster.
Moisture loss cools the skin via evaporation. Evaporation is driven by the difference in vapor pressure of water at the skin level. If humidity is high or air movement is low or nonexistent, evaporation is not effective. Use of a low air loss surface moves low humidity air to the patient bed boundary where it can be wicked away increasing evaporation and cooling the skin. In spite of the clinical significance of airflow and wicking of moisture from the skin, there are currently no guidelines for how much moisture a LAL system should remove and how much reduction in temperature is considered effective.
It has been hypothesized that there should be sufficient airflow to remove insensible perspiration and the amount of sweat of an average person at rest in a moderate climate (Flam, 1990). Bodine (1994) describes low air loss as an air flotation support surface capable of dissipating a minimum of 200g or moisture per 24 hours from the patient bed boundary via a continuous flow of fresh air in sufficient proximity and volume so that the body can regulate skin temperature through the heat of evaporation. Although a popular misconception, low air loss therapy is not an effective treatment for urinary incontinence. It can however, offer an adjunctive therapy for chronic moisture in addition to an overall incontinence program.
True low air loss support surfaces demonstrate the following characteristics: air can be felt circulating beneath the coverlet across the patient bed boundary, or other evidence of air flow escaping over the bed surface, the moisture removal rates (vs. a standard hospital mattress) should be a minimum of 200g over a 24 hour period, the air blower hose(s) capacity is adequate to supply generous air flow (between 90 liters and 120 liters/minute) and the minimum distributed air flow escaping from the top of the support surface is greater than 23 liters per minute. Be wary of systems utilizing aquarium pump like tubing. The capacity of this tubing is often not sufficient to produce adequate moisture removal.
More Criteria
Another criterion to evaluate is number of pressure zones. A single pressure zone type of low air loss mattress consists of a single bladder mattress or a mattress divided by baffles. The baffles are all interconnected in such a way that the pressure is the same throughout the mattress. This creates a challenge since pressures are different throughout the body.
Although simple to construct and inexpensive, these mattress systems have the following disadvantages over other available LAL systems:
no gradient pressure (hammock effect)
reduced comfort, diminished pressure distribution or pressure reduction in the head (occiput) and foot (heel) regions
inability to adjust pressure in individual sections, reduced deformation of baffles (resembles a camping style mattress)
With multiple pressure zones, the mattress is divided into three or more separate sections. An adjustable restriction valve located in the control module housing controls the airflow, which affects the final pressure in the baffles. Each section receives air from the power module via its own air hose. By controlling the airflow and pressure into each section, the system is better able to accommodate the different pressures required to properly support a body. This allows for the lowest possible interface pressures.
Advantages include:
the ability to tailor each zone's pressure and the whole mattress to each individual patient
reduced pressures in the head and seat regions, reduction of the "hammock" effect and the ability to accommodate unique body weight distributions such as amputations
:Disadvantages include:
multiple hose connections from the power module to mattress system that increases complexity of setting up the system and risk of mechanical failure
increased size, weight and cost of the power module
increased chance of user error such as improper setting of each section causing reduced effectiveness and inability to regulate pressure in each air pressure zone due to the continuous closed loop pressure control
An internal pressure regulation, multiple zone adjustable system offers even further advantages. The most important of these is allowing for lower pressures in the foot and head sections of the mattress. The lower pressures allow for overall interface pressures and relief from the "hammock" effect. These systems can be expensive and difficult to manufacture, require heavy, complicated control modules and a significant amount of caregiver skill to properly set the system. Routing and connection of three or more hoses further complicates setting up and maintaining the system. There are superior low air loss support surfaces offering the main advantage of a conventional zoned system (gradient pressure without the "hammock" effect) with the low cost, simple operation and single hose connection of the single zone mattress. Effectiveness is further improved due to the reduced chance of user error when operating the system. When choosing a support surface for moisture and heat dissipation, this system's innovative approach places it far above the competition. Ask your manufacturers for these capabilities.
Buyer Beware
Since there are currently no industry standards dictating performance and a limited amount of statistically significant clinical data, the choice of quality and clinical efficiency lies in the hands of the decision-makers: clinicians, administrators, payers and end-users. Investigating the competence and integrity of the manufacturers and distributors is of utmost importance. Ask for a detailed explanation of how the system works. How does the performance compare to other products? Has the product been tested for pressure distribution and moisture/heat dissipation capabilities? How does the low air loss feature work, by the wicking action of a moisture-vapor permeable coverlet or by direct air contact, which may cause drying, and dehydration? Where does the air escape? From the patient interface? From the end of the mattress? Out of an exhalation port or hose having no effect on moisture and heat accumulation? How many zones are there? Is there a chance of "hammocking" and bottoming out inherent in some low-end products claiming to be low air loss?
How about technical criteria, what is the moisture vapor transmission rate? Is the minimum distributed airflow escaping from the top of the support surface greater than 23 liters per minute? Can you feel air circulating beneath the coverlet across the patient bed boundary? Is the air blower and hose(s) capacity adequate to supply abundant airflow? How is the system cleaned and serviced? How often must the filter be changed? Answering these questions will better prepare you to choose an effective low air loss system worthy of its name.
Standards and Protocols
Low air loss therapy is currently utilized and funded only for the treatment of full-thickness (stage 3 and stage 4) pressure ulcers. A paradigm shift toward prevention may soon prompt the use of low air loss for avoidance of these burdensome sores. If moisture and heat build up ultimately contribute to the breakdown of skin, why not consider a low air loss surface for the prevention of pressure ulcers. It is the author's opinion that prevention modalities are always more cost effective than treatment from both a financial and a pain and suffering standpoint.
Investigating the competence and integrity of the manufacturers and distributors is of utmost importance. |
Low air loss systems are considered "powered devices" according to the Food and Drug Administration (FDA) and subject to pre-market allowance under substantial equivalence. Under the current regulatory process, each manufacturer still has abundant latitude to market their support surfaces with limited scientific claims or proof of clinical efficacy. Great care and a process of due diligence must be taken when considering low air loss support surfaces. The old adage, buyer beware should be heeded. Many systems are simply developed to fit a Medicare B code, meeting the minimum standards.
The National Pressure Ulcer Advisory Panel (NPUAP) and the European Pressure Ulcer Advisory Panel have assembled groups of experts to develop support surface performance criteria and standards. Each group of professionals has met formally on two occasions and continues to work toward refinement of industry standards. These standards, including terms and definitions, tissue viability and product life span, are expected to be finalized by October 2003 and should allow providers to make prudent decisions related to tissue load management devices, including low air loss, based on fact and sound research, not propaganda. This is especially important given the multitude of products claiming to offer true low air loss currently offered on the market.
This article originally appeared in the November 2002 issue of HME Business.