EUROPEAN PRESSURE ULCER ADVISORY PANEL

Some remarks about overlays in the prevention and treatment of pressure ulcers
Jeen RE Haalboom, MD, PhD
Associate Professor of Internal Medicine, Dept of Internal Medicine,
University Hospital Utrecht, The Netherlands

In this paper some aspects about overlays used in the prevention (and treatment) of pressure ulcers are discussed. The opinion of the EPUAP is given, followed by some reflections on pressure reduction and the development of tissue damage.

EPUAP made its first guidelines about prevention of PU in 1997¹. Guidelines about treatment followed in 1998. In both, a model was used in which the available literature was judged following strict criteria of scientific proof. The literature was only reviewed starting in 1992, the year the NPUAP guidelines were published, in which an extensive overview of the literature until then was given. For overlays (mattresses, various types of air systems and air-fluidized bed systems) no specific advice was given; in the literature there were no investigations to be found justifying the use of a particular system. The scarce trials in which different overlays or bed systems were compared did not match the quality criteria. In the recent Cochrane survey on beds and overlays it is only stated that ‘foam mattresses of at least 8 cm thick are better than standard mattresses and there is a tendency towards some better effects of air systems; it is considered to be proven that air fluidized systems perform best’.²

It is not easy to state which overlay should be used in prevention or treatment of pressure ulcers. In this paper some aspects about overlays are discussed; it is possible to approach the item from several views.

The easiest and least controversial approach is from the economical and logistical side. It not only concerns such aspects as lifetime, costs, and after-sales service from the manufacturer; but also the time between call and delivery, and the time between the recognition of a failure and reparation. Once inside the institution there are aspects such as cleaning, and the possibility of sterilization; since the MRSA (methicilline resistant staphylococcus aureus) occurs more often in hospitals, this aspect has become more important.

In Europe, foam mattresses are usually bought by an institution, but as a rule the more sophisticated systems, such as the newest low-air-loss systems and especially air-fluidized beds, are leased. Several investigations at institutional level showed that the process of cleaning and refilling these systems with new granules was too expensive and time consuming to undertake inside the institutions. Here, also, the number of units leased is important; in Europe they only account for approximately 6–7% of all patients, the rest using more simple systems or mattresses. When the number of air-fluidized units in use is doubled, purchasing instead of leasing could be more cost effective. Until now the daily contacts with manufacturers and their service (delivery, reparation, costs) play an important role in the selection of materials to use. As a rule, pure anti-pressure ulcer characteristics of the materials seem not to be the main reason to select a specific device.

A second view on anti-pressure ulcer devices could be from the theoretical side. In the prevention of pressure ulcers it is generally accepted that pressure, shear and friction play important roles. So materials and devices that reduce these physical entities should have a positive effect on the incidence of pressure ulcers. Some remarks about this theory are made later on in this paper. Reduction of pressure is usually achieved by spreading the body weight over a larger surface, usually by a kind of semi-submerging of the body in substances like foams (mattresses), or fluids (air-fluidized beds). The larger the supported area of the body surface, the lower the pressures will be. Low-air-loss systems increase the surface by creating air chambers with low pressures and – increasingly popular – by changing the pressure in seperate air chambers following a more or less fixed pattern. During an hour, for example, the sacral area is pressure-relieved for four periods of 10 minutes, and for two periods of 10 minutes there is a normal pressure. This is only an example, different manufacturers have different systems with different pressure relieving schedules. In all these systems one attempts to create a combination of alternating pressures, thereby simulating changes in body position.

A fakir lies rather comfortably on a bed of sharp needles (I am told), he does not like to lie on one needle! The relation between body weight and surface is clear. For overlays there are also some basic principles. Starting with foam mattresses. The characteristics of the foam are important. The lower the specific weight, the more a body can sink into the foam and the larger the supported area of the body will be. In the literature there is only one paper about the changes in the foam itself when pressure is applied: the specific weight changes under pressure³. When the foam is compressed, air is removed and the specific weight increases, thereby creating totally different characteristics as compared with foam without weight upon it. The construction of a test mattress consisting of several layers of foam, each layer with a less specific weight than the one immediately on top of it, illustrated this clearly. Furthermore, the surface of the foam should also be judged: Is it water resistant? Is it resistant to tear (agitated patients sometimes try to tear the mattress to pieces)? Does it inhibit the effects of heat or even flames (fire resistance)? Other aspects are the memory function of the foam (does it return to the shape it had before the patient was placed upon it?) and does it retain this function after some time of intensive use? These characteristics are of especial importance when making decisions to buy larger quantities. In the early ninetees we could demonstrate that an eggbox-shaped camping-site mattress (like hippies used to carry on their trips to India in the flower-power era) placed upon a normal hospital bed created the same low pressures as an expensive anti-pressure ulcer device; the camping-site mattress only costed about $10. The problem was that after two or three weeks it just fell apart. So life expectancy of the material also plays a role.

Usually a foam mattress is protected by a cover of some kind of polyethylene. The characteristics of the mattress, however, are changed dramatically when a cover is used. When the cover is not elastic enough in all directions it stimulates shearing forces and is therefore responsible for the faster development of pressure ulcers⁴. Even tightly fixed blankets can exert such a hammock effect. Pressure recordings as provided by manufacturers are, as a rule, performed with matresses without a cover, so the actual reductions are not correctly presented.

Lastly, it is clear that the overlay should be thick enough to prevent the bottoming of the body on the usually hard under-layer. This is obvious for mattresses, but this effect can also occur in air-fluidized beds (usually with patients weighing more than 100 kg). So for heavyweight patients – and in this context the term may be used for patients weighing more than about 75 kg – other materials should be used than for lightweight patients. Again, in the figures provided by the manufacturers, as a rule the weight of the ‘guinea-pigs’ they used is not given.

The removal of parts of the mattresses under locations at risk should be avoided. Body weight is unchanged, but the supported area becomes smaller, so pressure in other parts simply increases, especially at the edges of the gap that has been created – this is called the ‘doughnut-effect’. Several recordings illustrate this effect dramatically. There are investigations in small groups of patients with specific problems (hip fractures) showing a possible positive effect of such a device, but there are serious methodological problems with the paper⁵. Whatever the results, the theoretical background is incorrect and the possible beneficial effect seems to be coincidental. Patients tend to sit in bed, thereby creating high pressures delivered to the sacral area. Their sitting position also creates shearing forces: patients tend to slip to the foot of their beds when they are in a half-sitting position. In patients suffering with incontinence for urine the combination of pressure and shear is serious, since shearing increases when the skin is humid and less slippery. These effects are in part prevented by bending the overlay in the hips and in the knees; the patient is wedged in the overlay.⁶ For elderly patients especially, this creates another problem, since they tend to slide sideways, thereby creating shearing forces to shoulders and elbows.

In summary, there are many important aspects to consider when making a decision as to which device is advisable or even best. Until now, only pure physical aspects and logistics seem to prevail.

Thirdly, one could look at devices from a medical-theoretical side. In the development of pressure ulcers there are some important assumptions:

1. without pressure – no pressure ulcers,
2. to diminish pressure, body weight should be spread over an area as large as possible,
3. the removal of parts of overlays creates doughnut effects and should be avoided,
4. body positions should be changed on a regular basis, with an optimum frequency of once every three hours, and
5. sitting is worse than lying.

The turning once every two hours, in nursing an axioma, is not based on scientific proof, but is derived from history: in a ward in a British military hospital during World War II the order was given to two nurses to turn every patient as frequently as possible, and eventually it was possible to turn every patient once every two hours. There is no scientific basis for positive effects of this two-hour scheme. Defloor studied the effects on the incidence of pressure ulcers in a large number of patients in Belgian nursing homes, using different turning schemes together with different overlays.⁷ In a beautifully designed and executed study he was able to demonstrate that the use of a good performing foam mattress, together with turning every three hours, implicated the smallest number of patients with pressure ulcers. The incidence of pressure ulcers with three hours turns was less than with two hours and he suggested that it was even possible that the friction caused by the turning itself was harmfull.

Still, it is not so clear as it seems to be. Pressure does not always create pressure ulcers, there must be more. Everyone knows of patients with, for instance, multiple sclerosis, lying for years in the same position, with turning nearly impossible because of contractures etc, but without pressure ulcers. These only appeared when the patient contracted pneumonia. The model of pressure ulcers is now evolving towards a combination of pressure and tissue tolerance. Pressure alone doesn’t do it, there must be combined factors.

In the evaluation of the effects of overlays there is much attention for pressure recordings and attempts are made to measure shearing forces. Contact pressure could be measured using several types of instruments. The first recordings were made using a bed with 1000 nails, each individually connected with calibrated springs, and the possibility to construct isobares lines between points with the same pressures.^8,9 It seemed to be somewhat easier when sensors to measure tissue-interface pressure were used. There are three main types: fluid filled balloons, connected with a pressure transducer; electric sensors, measuring pressure by means of capacity, induction or resistance; and electropneumatic sensors, using the pressure needed to split two contacts on overlying sides of a balloon.

The sensors are usually placed under bony prominences of the body. The correct placement of the sensors, however, is very difficult and almost impossible to standardize, even in repeated measurements in one patient. It is clear that this aspect is essential in the explanation of differences in recordings.¹⁰ Somewhat later devices were constructed in which large numbers of sensors were integrated. The useability of these mattresses is dependent of the number of sensors, the lack of dynamic changes in time, the influence of the sensor itself on the recordings (size, thickness) and the characteristics of the mattress itself (flexibility for instance).^11-13 Again it is obvious that the comparison of results of measurements is difficult. All systems have advantages and disadvantages.¹⁴

However, despite all these technical achievements the major problem is still the fact that it is not known which pressure relates to the development of pressure ulcers. It seems a very simple question but it is not established that tissue-interface pressure (the pressure between the skin of the patient and the surface of the pressure reducing device) represents the pressure inside the body. Recordings of pressure between deep tissue layers and bone or fascia seem to be better in this respect, but they are only applicable in in vitro experiments for obvious reasons. In 1990 Staarink performed studies with cushions using the pelvis and upper extremities of a skeleton, with sensors placed on the bones.⁴ The body was reconstructed using materials exactly resembling muscles, fat and skin. The results of these studies showed that the measurements were not comparable with those with extracorporeal sensors, as could be expected, but that they had a much larger correlation between pressure ulcers than the more conventional recordings. In the introduction of a quality mark for overlays in the Netherlands (2000–2002) a model for a complete body will be used.

The use of dynamic systems makes things even more difficult, since pressures are changing constantly. Some researchers use minimum and maximum pressures,¹⁵ others use mean pressures in time¹⁶ and even the cumulative time in which a pressure of, for instance, lower than 10, 20 or 30 mmHg was recorded.¹⁷ Shearing forces are difficult to measure, allthough some very sophisticated devices are developed.^18,19

During the last decade the possibility has arisen that not pressure, but merely tissue deformation causes pressure ulcers. Deformation not only causes different pressures to tissue but also the stretching of blood vessels, impairing blood flow even more than rectangular pressure. Some CT-scannings illustrated this phenomenon,²⁰ but magnetic resonance imaging makes it even more obvious. In figures from Utrecht University Hospital is was shown what happened when an active healthy sportsman lies face downwards, with no pressure on his buttocks, and when he lies on a hard table (the figures were shown during the conference).

It was obvious that the muscles are stretched, even in the presence of a normal layer of adipose tissue. In an MRI one actually measures the number of energy-rich phosphate molecules, and it is possible to get an idea about the energy pool in muscle, or the blood supply. It is obvious that the muscle under pressure is more grey and with a patchy aspect. And this is a healthy sportsman. In this view it is important to realise that the perfusion of the skin is derived from vessels out of the underlying muscles. Animal experiences with pressure application showed that the tissue damage not only occurred immediately below the pressure heads, but also beside them, following a vascular pattern. In this experiment it became obvious that not only pressure, but also vascular damage (because of occlusion-reperfusion) caused lesions.²⁰

And there it is: a connection between pressure and what was called tissue tolerance. There are many reasons for vascular or even more detailed endothelial damage, causing this specific reaction. Activation of the immune modulation system is a reason as is the circulation of clusters of leucocytes and complement as is found in the adult respiratory distress syndrome (ARDS) and in septicaemia, serious situations in which patients often develop pressure ulcers. So the concept of only pressure inducing pressure ulcers becomes less obvious, it is changing its position with that of the combination of pressure with tissue tolerance.

As a conclusion: there are still many aspects about overlays that are not sufficiently dealt with by researchers. The necessity to clarify is obvious. Western population grows older but this in some aspects positive evolution is accompanied by increasing numbers of patients with pressure ulcers. Although not so extensively as in the USA, a growing number of patients are suing the hospital for obviously not taking enough precautions. So the choice of materials for prevention and treatment of pressure ulcers is important also from this aspect.

This paper illustrates that there are still aspects not fully understood. Governments ask for more and more advice based on evidence. For the pharmaceutical industry the development of new medication follows a distinct path, starting with theoretical aspects, phase 1 and 2 trials, and eventually release on the market. For most anti-pressure ulcer devices the first steps are skipped: they seem to be simply released. In Europe there is a growing tendency to demand scientific proof before such a device is allowed to be used. The EPUAP has recognized the imminent problem and during the 2000 Amsterdam meeting a working group was installed dealing with devices: which theory should be followed (for instance pressure or tissue deformation), which tests are needed, and in which direction should further development go. In January 2000 the highest advisory board of the Dutch Government, the Health Council, advised the installation of a quality mark for anti-pressure ulcer devices.²¹ In May 2000, a special committee started with the definitions to which materials should perform, eventually leading to a selection of the devices currently available for extensive testing. This will be done using the advice of the EPUAP working group. The project will last for two full years and eventually a quality mark will exist, that fits into the known ISO, DIN and CE markings.

Regrettably this paper cannot give the answer to the question which overlay is best, however, it does give some thoughts about the aspects needed to select one eventually.

Literature

1. EPUAP Guidelines, on the prevention and treatment of pressure ulcers. European Pressure Ulcer Advisory Panel, Oxford 1997, 1998.
2. Cullum N, Nelson EA and Nixon J. Pressure sores. Clinical Evidence, 1999; 1: 211–217
3. Zielhuis MRP, Goossens RHM and Haalboom JRE. Comparison of different foam cushions; peak interface pressure and pressure gradient. EPUAP 4th Conference, Amsterdam 2000.
4. Staarink HAM. Sitting posture, comfort and pressure; assessing the quality of wheelchair cushions. Thesis Delft, Technische Universiteit Delft (the Netherlands), 1995.
5. Hofman A, Geelkerken RH, Wille J et al. Pressure sores and pressure-reducing mattresses: controlled clinical trial. Lancet 1994; 343: 568–571
6. Defloor T. The effect of position and mattress on interface pressure. Applied Nursing Research, 2000, in press.
7. Defloor T. Drukreductie en wisselhouding in de preventie van decubitus. Proefschrift Universiteit Gent, 18 january 2000.
8. Lindan O and Greenway R. Pressure distribution on the surface of the human body. Arch Phys Med Rehab 1965; 46: 378–385
9. Ferguson Pell MW, Bell F and Evans JH. ‘Interface pressure sensors: existing devices, their suitability and limitations.’ In: Kenedi RM, Cowden JM and Scales JT (eds): Bedsore Biomechanics, 189–197; London; MacMillan.
10. Barbenel JC. ‘Measurement of interface pressures.’ In: Barbenel JC, Forbes CD and Lowe GDO (eds). Pressure sores, 67–78. London: MacMillan.
11. Barnett RI, Shelton FE. Measurement of support surface efficacy: pressure. Advances in wound care, 1997; 10: 21–29.
12. Zhou R. ‘Bladder pressure sensors.’ In: Webster JG (ed). Prevention of pressure sores. Engineering and clinical aspects, 109–120. Bristol: Adam Hilger.
13. McLeod AG. Principles of alternating pressure surfaces. Advances in Wound Care 1997; 10: 30–36.
14. Krouskop TA, Garber SL and Noble P. ‘Pressure management and the recumbent person.’ In: Bader DL: Pressure sores. Clinical practice and scientific approach. 235–248. London: MacMillan.
15. Sideranko S, Quinn A, Burns K, et al. Effects of position and mattress overlay on sacral and heel pressures in a clinical population. Research in Nursing and Health, 1992; 15: 245–251.
16. Rithalia S and Gonsalkorale M. Comparison of four alternating pressure air mattresses using a time based pressure trehold technique and continuous measurements of transcutaneous gases. In: First European Pressure Ulcer Advisory Panel Open Meeting, Oxford, 19.
17. Bennett L, Kavner D, Lee BY et al. Skin stress and blood flow in sitting paraplegic patients. Arch Phys Med Rehab 1984; 65: 186–190
18. Goossens RHM. Klinisch fysische aspecten van decubitus. Klinische Fysica 1995; 21–24.
19. Conner LM and Clack JW. In vivo (CT scan) comparison of vertical shear in human tissue caused by various support surfaces. Decubitus 1993; 6: 20–3, 26
20. Houwing R, Overgoor M, Kon M, et al. Pressure-induced skin lesions in pigs: reperfusion injury and the effects of vitamin E. J.Wound Care, 2000; 9: 36–40
21. Health Council of the Netherlands: Pressure ulcers. The Hague: health council of the Netherlands, 1999; publication no. 1999/23.