VOL: 96, ISSUE: 36, PAGE NO: 9
Shyam Rithalia, PhD, MSc, BSc, is senior lecturer, School of Health Care Professions, University of Salford
Laurence Kenney, PhD, BSc, is research fellow, School of Health Care Professions, University of SalfordUnlike most support surfaces, which are squashed or compressed under weight, tissue is largely incompressible and lies over an articulated structure of rigid bones. When pressure or shear forces are applied, it tends to move away from the affected area.
Unlike most support surfaces, which are squashed or compressed under weight, tissue is largely incompressible and lies over an articulated structure of rigid bones. When pressure or shear forces are applied, it tends to move away from the affected area.
This process is similar to the way in which soft, wet sand moves away from an area under pressure: it is not significantly compressed but moves slowly toward areas that are under less pressure. Sustained distortion of soft tissue can lead to occlusion of the blood supply, damage to the capillaries, disruption of the lymphatic system and, ultimately, tissue breakdown (Reger et al, 1990).
Support surfaces have two main functions: they redistribute pressure, helping to prevent the formation of pressure ulcers, and provide a comfortable surface for the patient to lie on.
The redistribution of pressure reduces the magnitude and/or duration of pressure and shear forces, both of which can cause excessive tissue distortion and damage soft tissues. This is usually done by redistributing the patient's weight in one of several ways, depending on the design of the mattress. This design and the patient's response to it determine the mattress' effectiveness.
The degree of comfort provided by a support surface is an important factor in its acceptability. Researchers and manufacturers are in the process of developing laboratory-based tests that provide quantitative measures of comfort, but it is still usually assessed on the basis of subjective reports (Grindley and Acres, 1996).
The physical variables associated with comfort include skin temperature, weight distribution and vapour exchange between the patient's skin and the mattress. Discomfort is associated with pain, which can be correlated with physical measures such as interface pressure (IP). Comfort is usually measured using either a multilevel descriptive scale or an analogue visual scale.
Types of support surfaces
Pressure-redistributing surfaces fall into two broad categories: those that reduce and those that relieve pressure. According to one definition, the difference lies in the measured ability of the support surface to lower IP to below capillary closing level (Agency for Healthcare Research and Quality, 1992).
This is a useful starting point, but the level of pressure necessary to close the capillaries of patients with certain conditions remains debatable. Then there is the question of whether the device should relieve pressure on all bony parts of the body all the time or if intermittent localised relief is sufficient?
Because of these uncertainties, it may be more useful to categorise devices according to their operating principles, which can be identified using IP measurements.
Fig 1 shows one approach to classification, in which products are categorised by their ability to remove localised pressure (pressure relief) or to redistribute it evenly around the body (pressure reduction).
Pressure-reducing support surfaces tend to be static, which means the IP remains constant when the patient is not moving.
Pressure-relieving support surfaces can be static or cyclical. Cyclical systems create a continuously changing IP, usually to extremely low levels, independent of patient movement. This is important because both the duration and intensity of pressure can influence the development of pressure ulcers.
Pressure-reducing support surfaces maximize the skin-contact area, reducing peak IP. Support surfaces such as air-fluidised beds (high-air-loss systems), low-air-loss systems and static flotation surfaces (foam, gel, air or fibre) all operate according to this principle.
Although tissue damage is not caused by pressure but the resultant tissue distortion, it is often accompanied by high pressure gradients. It is therefore reasonable to assume that high IP readings at vulnerable sites indicate high pressure gradients which could lead to tissue damage. Most pressure-reducing surfaces do not vary IP, but some allow the user to adjust it.
The most common pressure-reducing devices are lightweight polyurethane foam mattresses. Polyurethane is a honeycomb structure made up of polymeric struts and air. When weight is applied, this relatively compliant structure is compressed around the patient's body. The extent of compression depends on the mechanical properties of the foam, namely its density and hardness, and the patient's weight and posture.
Because patients' weight and posture can differ widely, designers have concentrated on creating mattresses that are appropriate for a broad selection of patients. There are several different types, all of which even out the pressure beneath the body. They include:
- Laminated surfaces, which are made up of multiple layers of foam with different mechanical properties;
- Cut-foam mattresses, which have longitudinal and/or transverse slits, creating a number of independent columns. The durability of these mattresses has improved in recent years (Rithalia, 1996);
- Formed and preshaped foam mattresses, which have undulating surfaces that fit the body contours.
Foam is categorised according to its density and hardness (King, 1998). Density is the weight per unit volume of the foam, which is usually expressed in pounds per cubic foot. It is not a measure of firmness but is an indicator of resilience: for a given load-bearing requirement, a high-density foam will usually perform better and lasts longer than one with a low density.
Hardness is determined by the weight required to produce a given compression. Nurses should be aware that there are a number of different standards of hardness. The UK standard is the indentation load deflection test, which measures the force required to produce 40% compression. The load is usually expressed in pounds, and the higher the load, the firmer the foam.
The density of foam can change over time and measurements such as compression set, energy absorption and fatigue durability can help to quantify this. All foam mattresses must have sufficient density and depth to ensure that no part of the patient's body rests on the bed frame.
In assessing the pressure-reducing abilities of foam mattresses, look for quality foam and take into account the evidence of clinical trials and other assessments (Rithalia, 1996).
Early experiments on mattresses reduced the air volume required in high-air-loss systems by containing it in fabric sacks arranged beneath the patient (Scales et al, 1974). The resultant low-air-loss systems also produced currents of air that flowed over the skin, minimising the risk of maceration.
Most low-air-loss surfaces consist of a series of cells inflated by a high-output pump or fan. In some, the air in the cells is zoned to maximize its pressure-reducing effect. Others can be modified to alter the pattern of static pressure gradients on the skin.
Nurses should remember that low air pressure in the mattress cells does not necessarily equate to a low IP: a deflated mattress will register zero air pressure but high IPs. And not all systems eject air from the mattress: 'low air loss' is often used to describe any powered static air product.
Pressure-relieving devices aim to relieve the pressure on a localised area of skin. This can occur statically, as in the case of heel boots or deflated air cells on some low-air-loss systems that lift or suspend vulnerable areas. It can also take place cyclically, providing relief to the entire contact area during each full cycle, as in the case of alternating-pressure air mattresses (APAMs).
APAMs are made up of a number of cells connected in groups to an electrically powered pump. During each cycle, different groups of cells are sequentially inflated and deflated to remove the supporting pressure from beneath the entire body.
These change the IP by periodically deflating air cells under the body, which redistributes the pressure on the soft tissue and encourages the reperfusion of previously supported areas. Because the duration of pressure is as important as its intensity (Bliss, 1990), the way in which the IP varies is crucial.
The degree of pressure relief achieved is related to many variables, including the type of cover material, cycle time and air pressure in the cells. Although there is no consensus on optimal cycle times, systems that change pressure at least once every five minutes mimic normal movement during sleep.
Ensuring that the air pressure in the cells is correct is important as both too much and too little can result in adverse IP.
In some products the air pressure is fixed while others use a sensor to vary it slightly, depending on the patient's shape, size and position. This may reduce the risk of the patient's body coming into contact with the bed frame and has been shown to yield significantly better IP values (Rithalia and Gonsalkorale, 1998).
Nurses are required to assess and select a technologically sophisticated range of support surfaces, so it is important that they understand the way in which mechanical principles govern their effectiveness and suitability.