WO2005067858A1

WO2005067858A1 - Pressure reducing patient support structures

Info

Publication number: WO2005067858A1
Application number: PCT/EP2005/000529
Authority: WO
Inventors: Patrick Noel Daly
Original assignee: Patrick Noel Daly
Priority date: 2004-01-20
Filing date: 2005-01-20
Publication date: 2005-07-28
Also published as: EP1789005A1

Abstract

A support structure for a patient such as a mattress or mattress overlay comprises at least one air-tight chamber defined by an outer fabric. The chamber is provided with a first zone for supporting the major part of a patient's body and a second zone for supporting a part susceptible to pressure induced tissue damage. A first foam material is provided in the first zone and a second foam material is provided in the second zone. The upper surface of the first foam material is attached to the inner surface of the outer fabric. At least part of the upper surface of the second foam material is unattached to the inner surface of the outer fabric and defines and inflatable region. The inflatable region is inflated by transfer of air from the first zone by the action of a patient's weight on the first zone.

Description

Pressure Reducing Patient Support Structures

The present invention relates to patient support structures, in particular to mattresses and similar bed furniture and fittings, more especially to mattress overlays and additionally to cushions, trolley mattresses and the like, all for assisting in the treatment of, or for preventing the occurrence of, pressure induced tissue damage. Elderly and/or immobile patients who may be confined to a bed for long periods of time are at particular risk of such tissue damage and the mattress and mattress overlay according to the invention are especially, but not exclusively, suitable for use with such patients.

Pressure reducing mattress overlays are typically placed on top of an existing base mattress to provide an additional layer of material between the mattress and the bed occupant. The purpose is to reduce interface pressures on the skin tissue thereby helping to reduce the occurrence of pressure related tissue damage (bedsores, ulcers etc.) and also to enhance patient comfort.

Known overlays may comprise a body of foam material in a suitable cover. Alternative overlays include inflatable bladders or cushions which are placed on a conventional mattress.

Existing overlays, even though relatively comfortable, can generate high pressures under certain parts of the patient. The heels are particularly susceptible in this respect, but other extremities such as the elbows, shoulders, back of the head and ischia can also be vulnerable. In fact, the heel (more specifically, the bony protrusion under the back of the heel) is typically the area of the body most prone to pressure induced tissue damage and breakdown. This area has very little tissue, muscle or fat to support the bone protruding inside the skin at the heel. The reduced surface area through which the heel is supported therefore increases pressures on the tissue. The pressure can increase to dangerous levels, resulting in restriction of the circulation in the heel area, which is the main cause of tissue damage/pressure ulcers. The same problem can occur with the other body areas mentioned above. In some known pressure reduction mattresses, inflatable bladders or cells and associated air pumps and control equipment are used to "float" the whole patient or change pressure in different body areas intermittently by mechanically pumping air to certain cells in alternating patterns. These powered systems are very expensive and in a lot of cases unreliable over time. In addition these systems can be intrusive and loud in a person's home bedroom as they are electrically powered pneumatic systems which are inherently noisy.

Other known products such as the "Repose" from Frontier Therapeutics and the "Sofcare" from Gaymar Industries use air filled full bed length overlay pads that are placed on top of a standard mattress and comprise a chamber into which a preset amount of air is pumped, through a valve, to create flotation for the patient. The valve is closed when the overlay is inflated to the designed extent. A significant disadvantage with these products is the lack of uniformity of air inflation. The weight of the user's torso causes the displacement of a large portion of the air to the outer regions of the overlay where the legs and arms are supported. This extra pressure can create excessive pressure to the heel and elbow area which, as noted above, are particularly vulnerable zones in respect of pressure damage to tissues. Also these overlays are sometimes easily punctured and therefore rendered ineffective as pressure reducing devices.

There are known sensor-based systems on the market which claim to automatically sense the amount of air within the mattress. The "Repose" mentioned above, for example, has an air pressure valve which exhausts automatically at a certain threshold pressure when the overlay is being inflated. There are also many other types of self-regulating valves built into mattresses to attempt to reach the lowest pressure possible by reaching a certain pre-regulated threshold. All these designs have inherently complex valves and valving arrangements which promote unreliability and are not necessarily easy to use.

Also known are so-called heel boots or heel and elbow supports. These are foam or air based cuffs or boot-like arrangements that can be strapped around the arm or foot/lower leg to permanently ensure the heel or elbow is not in contact with the underlying pad or mattress. These products are again quite expensive and it is sometimes difficult to find a size and shape suitable for specific patients. These products can also be very uncomfortable for the patient and it can be difficult for a patient to move in bed when these cuffs and boots are in place. This is more especially true if the patient's mobility is already impaired due to illness.

The present invention seeks at least partially to overcome the above disadvantages by providing a patient support structure which, in addition to achieving good pressure care, is of simple construction, requires no user-adjustable parts and so has excellent reliability. The structure of the invention is primarily a mattress overlay, a bed mattress, a trolley, gurney or stretcher mattress or similar product on which a patient is supported in a recumbent position. However, the structure of the invention may also be a product intended to support a particular part of the body only, such as an arm or leg, or a cushion (such as for a wheelchair) which supports a patient or user in a sitting position.

According to the present invention there is provided a patient support structure for use in the treatment or prevention of pressure induced tissue damage comprising: i. an outer impermeable fabric defining at least one fluid tight chamber; ii. first and second patient support zones, said first zone being adapted to support given part of a patient's body, and said second zone being adapted to provide support for a part of the patient's body susceptible to pressure induced tissue damage; iii. a quantity of inflating fluid in said chamber; iv. a first supporting foam material disposed in the first zone; v. a second foam material disposed in the second zone; and vi. at least one fluid communication path between the first and second zones, wherein in the first zone the upper surface of the first foam material is attached to the impermeable fabric, the second zone includes at least one inflation region in which the upper surface of the second foam material is not attached to the impermeable fabric, and the first and second zones are so sized and the first and second foam materials are so selected that the weight of the given part of the patient's body acting on the support structure in use is effective to transfer inflating fluid from said first zone to said second zone to inflate the inflation region whereby the impermeable fabric in said inflation region is displaced from the upper surface of the second foam material.

Most preferably the inflating fluid is air. The impermeable fabric desirably comprises a laminate including an internal layer of a polymeric material, such as a polyurethane, which is impermeable to the inflating fluid (air) and an external layer, typically of a woven material, which allows the fabric to be "breathable". "Breathable" in this sense can mean that the external layer is effective in wicking moisture away from a patient. The external layer may be a separate removable cover, or may be integral with the polymeric material.

Thus, when a patient is first supported on the structure according to the invention, the weight of the patient's given body part acting on the first zone compresses the first zone (in particular, compresses the first foam material in the first zone) and causes air in the first zone to be expelled from the first foam material. Because the first and second zones are in fluid communication, air flows from the first zone to the second zone and the pressure in the second zone increases until the air pressures in the first and second zone become equal again. The fact that in the first zone the upper surface of the first foam material is attached to the impermeable fabric (and that, similarly the lower surface of the first foam material is attached to the adjacent impermeable fabric) constrains possible volume expansion of parts of the first zone which would otherwise accommodate the increased pressure in the first zone. Hence, air is expelled from the first zone to the second zone. The resulting increase in pressure in the second zone causes inflation of that part of the impermeable fabric, i.e. the inflation region, which is not attached to the second foam material. This inflation of the inflation region separates the impermeable fabric in that region from the second foam material. For a support structure such as a mattress or mattress overlay, where the patient is supported in a recumbent position, the patient's given body part supported by the first zone is primarily the torso. Where the support structure is a cushion, the patient's given body part is also primarily the torso, supported on the first zone through the buttocks. Where the support structure supports a particular part of the body only, the patient's given body part may, for example, be the major part of the leg or arm so that the heel or elbow respectively is raised by the inflation region of the second zone.

In effect, the impermeable fabric in the inflation region acts like a localised

"balloon". The "balloon" when inflated provides a formation by means of which a body part can be supported in a raised condition. The structures of the invention may include one or more strategically located balloons (i.e. inflation regions of the second zone) to lift certain areas of the body to reduce pressure on those areas. The balloon may directly support a body part susceptible to pressure induced tissue damage, or may support a part closely adjacent to the susceptible part, such that the contact of susceptible part with the surface of the overlay is reduced to a desired minimum, so reducing the interface pressure on the susceptible part. The susceptible part may be held clear of (that is, held out of contact with) the surface of the overlay or may be held such that it is barely touching the surface. The latter condition is advantageous, for example in the case of the heel as it prevents excessive forward rotation of the foot.

Most preferably, in the first zone, the upper surface of the first foam material may be attached to the impermeable fabric across substantially its entire area.

Alternatively, in the first zone, the upper surface of the first foam material may be attached to the impermeable fabric in attachment zones. The attachment zones may, for example define a pattern of attached and non-attached areas. Any non- attached areas of such a pattern are preferably individually small in area so that significant inflation "balloons" are not formed in the first zone. The attachment of the impermeable material to the first foam material must be such that volume expansion of the first zone, or parts thereof, with increasing pressure is substantially prevented. In one preferred construction, the upper surface of the first foam material is bonded to the impermeable fabric. For example, the surface of the first foam material may be adhesively bonded to the impermeable fabric. Alternatively the surface of the first foam material may be heat bonded to the impermeable fabric, such as by heat welding. The lower surfaces of the first and second foam materials are similarly attached to the impermeable material.

In a preferred embodiment, the structure according to the invention comprises a first chamber defining the first zone and a second chamber including the second zone. One or more flow paths, such as pipes or tubes, may be provided to allow fluid communication between the respective chambers. In another embodiment, the impermeable fabric defines a single chamber comprising both the first and second zones. In this case, the flow path for the fluid communication between the first and second zones is simply from cell to cell of the foam material(s).

In the respective first and second zones, the first foam material may be a different foam material from the second foam material. For example the foam materials may have different densities or hardnesses, or one foam material may be more open celled than the other. It is possible also that the foam materials in each zone are the same. In this case the foam material can be one piece of foam extending the full-length of the structure.

In one preferred arrangement, the second foam material is thinner than the first foam material.

In another preferred construction, the inflation region of the second zone is located in the structure so that, in use, it lies below the lower leg of the patient such that said inflation of the second zone is effective in use to raise the lower leg. Most preferably, the inflation region of the second zone is effective in use to raise the lower leg such that the patient's heel is lifted substantially out of contact with the structure. In a preferred construction, the inflation region of the second zone is located in the structure so that, in use, it lies below the arm of the patient such that said inflation of the second zone is effective in use to raise the arm. Most preferably, the inflation of the second zone is effective in use to raise the arm such that the patient's elbow is lifted out of contact with the structure. The inflation region below the arm may be provided in addition to an inflation region for providing support to the heel.

In another particularly preferred construction, at least one inflation region of the second zone is disposed adjacent a second region in which second region the impermeable fabric is attached to the upper surface of the second foam material whereby, in use, a body part susceptible to pressure induced tissue damage may be supported, by means of the inflatable region, above, but not in contact with, said second region. The body part may be the back of the heel and the inflation region is then adapted to support the foot. The body part may alternatively be the heel with the inflation region adapted to support the lower leg. The body part may be the elbow with the inflation region adapted to support the arm. The body part may be the back of the head with the inflation region adapted to support the head elsewhere, e.g. towards the sides of the head, and/or towards the top of the head and the neck. The body part may be the ischia with the inflation region adapted to support the buttocks.

Preferably the first foam material is a visco-elastic foam. Preferably also the second foam material is a visco-elastic foam. Alternatively, the foam materials can be standard Polyether polyurethane foam. The foam materials in the second zone are desirably low density and low hardness to be as soft as possible. It is also most desirable that the foam materials are open celled as this assists and allows air intake during self-inflation.

Visco-elastic foams per se are known and widely available in a variety of grades and types. Visco-elastic foams are also known as "memory foams" (although this terminology is inaccurate), "slow recovery foams" or simply "slow foams". By contrast, conventional high resilience foams (HR foams) are known as immediate recovery foams. This terminology derives from the response of the particular foam to compression. HR foams return immediately to their original state when the compression force ceases; visco-elastic foams have a longer recovery time. Visco- elastic foams are often characterised by their glass transition temperature range which for therapeutic and medical applications is selected to be up to around 40°C, that is, in a range which allows the properties of the foam to be affected body heat, body temperature being around 37°C. This allows the foam to "flow" to a certain extent when heated by the body of a user lying on the foam, so that the foam moulds itself into greater conformity with the body. The surface area of the foam in contact with the body is thereby increased, and consequently the pressure experienced by the body is reduced, especially in key areas such as the heels, shoulders, elbows and ischia.

In a much preferred arrangement the structure further comprises a valve which, when open, allows fluid communication between the first and/or second zone and the atmosphere. The valve is normally closed in use. In this way, the structure can be "self-inflating". That is, for storage of the structure, the valve may be opened and the structure compressed and rolled or folded so that it occupies a smaller volume. In so doing, air is expelled to atmosphere through the valve which is then closed. When the structure is required for use, the valve is opened. The resilient nature of the first and second foams allows them to expand drawing air into the structure from the atmosphere so that the structure regains its expanded configuration, at which point the valve is normally closed.

In a further preferred embodiment the structure of the invention further comprises third zones disposed along respective longitudinal marginal edges of the underside of the structure, in which zones the impermeable material is not attached to the first and/or second foam material, whereby the weight of the given part of the patient's body acting on the support structure in use is effective to transfer inflating fluid from said first zone to said third zones to inflate the third zones whereby the impermeable fabric in said third zones is displaced from the lower surface of the first and/or second foam material. Typically the structure of the invention will be in the form of a mattress or a mattress overlay for placing on a conventional mattress. The term "mattress" hereinafter includes both such constructions, unless the context requires otherwise. Although the invention is primarily described in relation to mattresses and mattress overlays, the principles of construction described are applicable also to other types of structure such as cushions or structures for supporting particular body parts, as noted above.

The outer impermeable fabric defining the first and second zones and foam materials must have fire retardant properties to meet fire regulations. A minimum standard is BS7177 Ignition Source 5 or equivalent. Fire retardant agents are usefully incorporated into these materials to this end.

Thus an important feature of the invention is the utilisation of one or more unbonded air balloons (i.e. inflatable regions) that can be strategically placed to prevent or reduce interface pressure on the limbs of the person, for example, pressure under the elbows and the heels. Although the heels are recognised as being particularly susceptible to pressure damage the structure of the invention can, in principle, be used to lift/raise any part of the body to prevent or reduce pressure in adjacent zones, as indicated above.

Another important feature of the invention is the particular relative constructions of the first and second zones to optimise the reduction of interface pressure under the main body of the patient. Reducing the internal (air) pressure, in a suitably constructed mattress, has the effect of allowing the surface of mattress to conform to the recumbent user over a greater surface area of the user's body, so reducing the effects pressure experienced by the user due to his own weight. Pressure sensing valves are known for such pressure reduction but they are expensive and sometimes unreliable in terms of both accuracy and longevity. In more basic apparatus, such as standard camping mattresses, one can open the valves to reduce pressure and enhance immersion (i.e. extent of conformity) and comfort. This may be difficult to do however and even more so if the user is relatively immobile due to illness or is disabled. Coupled with the inconvenient necessity of having to adjust the prior art mattress by opening the valves, the adjustment itself is commonly subjective or even arbitrary. It can be difficult for a mattress occupant to find the "ideal" immersion level (i.e. the ideal internal pressure which maximises the area of contact with the body and which does not, for example, allow compression to the extent that any part of a patient's body experiences directly the structure (e.g. bed base) underlying the mattress) by manually opening the valves.

In the structures according to the present invention preferably the relative constructions of the first and second zones are selected, for a given patient size and weight, to promote the optimal degree of immersion, in addition to providing the inflation region(s) which promote pressure relief for one or more areas susceptible to pressure induced tissue damage. Thus a range of structures is provided having differing parameters including: foam type and thickness (first and/or second foam material); balloon (inflatable region) size in the second zone; and the relative air volumes in the first and second zones so that the most appropriate structure can be provided for a given patient. The correct degree of compression of the first zone for maximum surface area of contact with the patient and the correct degree of inflation of the second zone are therefore automatically ensured and the need for pressure sensors and automatic sensing equipment is eliminated. Likewise, motorised pumps for movement of air between respective zones are not required since the patient's own weight is exploited to achieve this. Because the structures according to the invention are completely non-powered, reliability is improved and inconvenient noise during use is eliminated.

In other words, a range of mattresses with various foam types and balloon size/locations and relative sizes of the first and second zones can be provided to cater for each segment of the market. For example, a paediatric version providing heel support may have very soft foam in the first zone (and similarly in the second zone) such as a 25kgm^"3 density foam with less than 50 Newton Hardness and a balloon (inflatable region), positioned to be under the lower legs of the patient, measuring about 6 inches (approx 150mm) by about 22 inches (approx 560mm). For a typical adult patient, the foam may have a higher density of about 55kgm^"3 with 120 Newton Hardness and a balloon (inflatable region) measuring about 12 inches (approx 300mm) by about 24 inches (approx 600mm). The balloon sizes and foam type are selected according to the particular requirements of the patient.

At rest (in the absence of a patient on the structure of the invention), the foam materials in the first and second zones act as air reservoirs. When a patient lies on the structure, air is expelled from the first foam material in the first zone and is accommodated by expansion of the volume (i.e. inflation) of the inflatable region of the second zone. The quantity of air transferred from the first zone to the second zone which is required to lift a given part of the body - i.e. to inflate the or each inflatable region - and to provide an optimum degree of immersion, is determined by a combination of the total patient weight, the type and thickness of the foam materials in the first and second zones and the respective volumes of the first and second zones themselves. For example, if the foam materials are very open celled, they may have a foam/air ratio of, say, 50:50 (50% air and 50% foam). For such foams, as compared with more closed celled foams, at rest, a relatively greater amount of air is retained within a given volume of the first foam material and, consequently, a relatively greater amount of air is expelled from the first foam material when a patient of a given weight lies on the structure. Expressed differently, a relatively lesser degree of compression of such a foam is required to achieve the transfer of a given quantity of air to the second zone. However if the first foam material less open celled, for example with an air/foam ratio is 70:30 (70% foam and 30%) air, a relatively greater degree of compression is required to ensure that the given quantity of air is transferred to the second zone. For such relatively less open celled foams, as compared with more open celled foams, at rest, a relatively lesser amount of air is retained within a given volume of the first foam material and, consequently, a relatively lesser amount of air is expelled from the first foam material when a patient of a given weight lies on the structure.

Thus, in producing a support structure in accordance with the invention, there is a number of design options to provide an optimal design for a given patient. For a relatively light patient it is possible to select a relatively open-celled foam where a lower degree of compression of the foam is sufficient to achieve the transfer of the desired quantity of air to the second zone. For a heavier patient, the degree of compression of such an open-celled foam may be excessive, such that the foam reaches its maximum compression and the patient is not adequately supported. A greater thickness of the foam may then be used, and/or the chosen foam may be less open-celled so that a higher degree of compression is required to achieve the transfer of the desired quantity of air to the second zone.

In summary, the foam materials in the first and second zones of the structure can be selected to optimise the foam/air ratio for different patient weight ranges, e.g. Paediatric or Bariatric. Very light patients (Paediatric) will required greater air volume in the first zone, relative to the total zone volume and proportionately less foam (that is, the more open celled foam) while very heavy patients (Bariatric) will require a proportionately greater foam volume and less air (that is, the less open celled foam).

Thus for a given patient weight (or weight range) the structure of the invention can be constructed so that the properties of the first and second zones are matched in order to achieve a desired inflation of the inflation region and a desired degree of immersion in the first zone. For a given inflation volume required for the inflation region, the degree to which the foam is open celled, the thickness of the foam and the volume of the first zone can all be selected to achieve that inflation volume with a given patient weight. Conversely for a given construction of the first zone, the overall volume of the second zone, the volume of the second zone relative to the volume of the second foam material, the degree to which the second foam material is open-celled and the extent of the non-attached area of the impermeable fabric defining the inflation zone can all be selected to achieve, for a given patient weight, the desired degree of immersion in the first zone and the desired inflation of the inflation region of the second zone. The volume of the second zone relative to the volume of the second foam material can be varied, for example by making the foam material relatively thicker or thinner. If the second foam material is relatively thicker, it will occupy a proportionately greater volume of the second zone. Consequently, a proportionately lower volume of air transfer from the first zone is required to achieve the desired inflation of the inflation region. Conversely, if the second foam material is thinner, the volume of air transferred from the first zone must be greater to achieve the desired inflation of the inflation region.

Most preferably, the selection of the first and second foam materials and the volume of the first and second zones and of the inflation region are each selected so that for the intended patient weight range, the patient is adequately supported by the first foam material without the first foam material reaching its maximum compression ("bottoming out") and with the correct inflation of the inflation region. In a most preferred construction, the overall increase in the internal pressure of the structure after the patient lies on the structure will be close to, but slightly above, zero so that flotation of the patient on the structure occurs.

One could, in theory, simply place an air chamber towards the end of the mattress to support the heels. However, it has been found that the volume of air required to inflate this chamber is very likely to exceed the amount of air within the foam material in the first zone of the mattress. Therefore, the application of a patient's weight to the first zone would cause the first zone to "bottom out", i.e. reach maximum compression before such an air chamber was fully inflated and this would undesirable over the longer term. Controlled flotation is desirable in both the first and second zones, albeit possibly to a lesser extent in the first zone. The second zone clearly needs higher levels of flotation to lift the relevant body part above the foam material.

It is possible to consider a mattress or overlay having a single foam containing air chamber with the impermeable fabric fully bonded to the foam on both top and bottom faces. This sort of construction may have advantages in the camping field and to some extent in the medical field as it controls the pressure/volume relationship within the mattress, by avoiding major displacement of air by a person's weight, towards the edges of the mattress. This therefore controls flotation of the person lying on the mattress. In the camping field this is desirable as the air pocket between the ground and person is maximised and therefore insulation is enhanced by maintaining an air insulation barrier. However in the medical field insulation is often not required and can actually be contra-indicated as sweating can cause skin breakdown (maceration).

The structure according to the invention is significantly different in that it provides for the automatic transfer of a volume of air from the first zone (which supports the major part of the patient's body) to the second zone. This allows greater immersion or conformity between the structure (mattress) of the invention and the patient, that is a greater surface area of the structure in contact with the patient, achieved (as compared with the possible fully bonded structure indicated above) through a relatively greater compression by the patient's weight of the foam material in the first zone. These factors have, amongst others the following positive effects: 1. reduced air between the patient and an underlying support means (such as a bed base or conventional mattress) can reduce insulation levels and temperature over time; 2. greater immersion will increase surface area supporting the patient and so reduce the pressure experienced by the patient (recalling that Pressure=Force x Area). For a given patient, Force is the patient's weight exerted downward and is therefore effectively constant. Pressure and Area are inversely proportional. Increasing surface area by allowing air to transfer to the second zone has a beneficial effect in increasing the support for the upper leg and torso/body area; 3. manual operation of valves is not required to achieve immersion of the patient. Opening the valves can be difficult especially, for example, for patients in medical/care institutions who may have restricted mobility and therefore may not be capable of reaching such valves.

In further preferred embodiments of the invention, the first patient support zone may desirably comprise a number of through bores extending from an upper surface thereof to a lower surface (in use). Preferably, the bores are oriented substantially vertically in use. The outer impermeable fabric defines the surface of the bores, so that the fluid tight chamber is not compromised. Of course, the first supporting foam material disposed in the first zone then also has corresponding bores which are typically of a slightly larger diameter, to allow for the presence of the outer impermeable fabric. Typically, that the bores through the first patient support zone may have a diameter of approximately 20 mm and the corresponding bores through the supporting foam material may typically have a diameter of about 40 mm.

Although, as indicated above, the provision of valves which allow air to pass into or out of the fluid tight chamber can have significant drawbacks, such valves are not precluded from the structure of the present invention. As noted above, a valve may be provide to allow compression of the support structure for storage. Where such a valve is provided, a suitably able or qualified user, or a suitably qualified medical practitioner, may be able to use such a valve to achieve a greater level of comfort. Releasing air from the structure of the invention when it supports a user reduces the air pressure within the fluid tight chamber and allows the supporting foam material to have a greater conformity with the patient. Conformity in this sense provides a greater area of contact between the patient and the structure of the invention and greater area of contact in turn leads to a lessening of the pressure experienced by the patient due to his own weight. Clearly, reducing the air pressure in the fluid tight chamber is likely to reduce the degree of inflation in the second patient support zone, to the extent that inflation may not occur and use of the valve in this way is likely to be inappropriate in many cases. This may, however, not be a problem for certain uses. It is possible that where one or more such valves is/are provided the support structure of the present invention may have dual uses. In its primary use for treating or avoiding pressure induced tissue damage, the support structure is normally fully inflated and the valve remains closed so that the "balloon" in the second patient support zone is inflated to the desired extent. In a secondary use, for example where concerns over pressure induced tissue damage are less (or where such damage is not a concern at all), the or each valve may be used to adjust the air pressure within the fluid tight chamber to achieve a desired degree of conformity for the user. In the extreme case, the or each valve may be left open so that the user is entirely supported by the first supporting foam material and the second supporting foam material, with no contribution from the internal air pressure in the fluid tight chamber. In the latter case, in order to achieve the desired level of comfort, the supporting foam materials must be sufficiently thick so that they do not reach their maximum compression under the user's weight.

In a further aspect of the invention, the support structure of the first aspect of the invention may comprise a separate or removable component of an otherwise standard or conventional mattress, or of a modified conventional mattress. Thus, the mattress may be provided with a closure which can be opened to gain access to the support structure of the invention so that, for example, the support structure can be removed for cleaning. The closure can be, for example, an area or region of the mattress cover which is bounded by a zip fastener or hook and loop fasteners or the like. The support structure of the invention may be used in the mattress in place of a layer of foam material, such as a visco-elastic foam material, typically of about 6cm depth, which would conventionally be present. Other conventional mattress layers would remain present. For example, the mattress could include a base layer comprising approximately 8cm of high density, high hardness combustion modified high resilience (CMHR) foam with the support structure disposed on top. Typically the support structure can have a depth of about 6cm. The support structure may have nominally the same width as the mattress or may be narrower than the mattress, so that the support structure is contained in a cavity in the mattress. Where the support structure of the invention is used in a mattress in a medical environment, the impermeable covering of the support structure can assist in infection control, should any fluids penetrate the outer cover of the mattress.

For a better understanding of the invention, reference will be made, by way of example only, to the following drawings, in which:

Figure 1 is a schematic plan view of a typical structure according to the invention;

Figure 2 is a schematic side view of a structure according to Figure 1;

Figure 3 is a schematic section through a structure according to the invention and Figure 4 is a schematic section through a further embodiment of a structure according to the invention.

Referring now to the drawings, the structure 1, in the form of a mattress overlay, has a first zone 10 which is adapted to support the major part of a patient's body, specifically the torso (trunk) and head. The position of the patient's head is indicated at A. Arrow D indicates a typical length of the overlay, which may be about 2000mm or more. The overlay also has a second zone 20 which is inflated in use to lift the patient's legs. Arrow B indicates a typical distance between the head and the second zone. In the case of a male patient this may be around 1700mm and in the case of a female patient this may be around 1500mm. These figures are selected to accommodate the 5^th percentile (i.e. 95% of the population) from known anthropometric data. See, for example International Journal of Occupational Safety and Ergonomics 2001, Vol 7, No 1, 15-34. Areas 30 and 40 indicate respectively typical heel positions of female and male patients. In Figure 2, the dotted line 50 indicates a typical outline of a patient, with the heels at the left side and the neck and shoulders at the right side of the Figure. It can be seen that the patient's torso is supported in the first zone 10 and that the foam material in the first zone 10 has compressed to accommodate the shape of the patient, so increasing the area of contact between the overlay and the patient. In the second zone 20, the inflation region is inflated, so that the patient's legs are lifted. The contact (and hence interface pressure) of heels with the overlay is thus reduced and the heels are not then subject to pressure induced tissue damage.

Figure 3 shows one possible construction of a typical overlay in more detail. The first zone 10 occupies the major part of the overlay and includes a first foam material 12. The second zone 20 occupies an area of the overlay proximate the patient's feet in use and includes a second foam 22. The overlay is enclosed by an impermeable material 60 which defines an air-tight chamber. A valve 70 is provided at a convenient location for the ingress and egress of air, to allow the overlay to be self-inflating after storage and compressible for storage. In the first zone 10, as indicated by the arrow X, the first foam material 12 is attached, such as by heat welding, to the impermeable material 60, which is preferably a polymeric material. In the second zone 20, the second foam material is not attached to the impermeable material 60. The region indicated by the arrow Y thus indicates the inflatable region of the second zone.

For the avoidance of doubt it is noted that the first and second foam materials may be one and the same, that is, the first and second foam materials may be identical and may comprise a single unitary foam material. The inflation region is then defined only by the area of the impermeable material which is not attached to the foam material.

Figure 4 shows a section through a structure 1' according to one aspect of the invention mounted on a bed base 70. The structure includes the first zone 10 and a second zone 20 (not specifically illustrated) in the same manner as the above- described embodiments. Further, the structure 1' comprises third zones 80 disposed on the underside of the structure 1' that is, on the side facing the bed base in use. The third zones extend parallel to and close to the longitudinal marginal edges of the structure, preferably for substantially the whole length of the structure. The third zones are similar in principle to the inflation regions of the second zone in that they define areas of the lower face 84 of the structure in which the impermeable material is not attached to the adjacent first (or second, as appropriate) foam material, in contrast to the remainder of the lower face 84 where the impermeable material is attached. Thus, in much the same manner as for the inflation regions of the second zone, when a patient lies on the structure the foam material in the first zone is compressed and air is expelled from the first foam material. Air is thus transferred to the third regions causing the impermeable material in the third regions to become displaces from the first (or second, as appropriate) foam material to form inflated "balloons" extending along the longitudinal marginal edges of the structure. The effect of these inflated third zones 80 is to raise the longitudinal marginal edges of the structure. This effect "cradles" the patient and inhibits the patient form rolling off the mattress and may inhibit a possibly incoherent patient from leaving the mattress. This construction is especially useful for heavy immobile patients.

Claims

1. A patient support structure for use in the treatment or prevention of pressure induced tissue damage comprising: i. an outer impermeable fabric defining at least one fluid tight chamber; ii. first and second patient support zones, said first zone being adapted to support a given part of a patient's body and said second zone being adapted to provide support for a part of the patient's body susceptible to pressure induced tissue damage; iii. a quantity of inflating fluid in said chamber; iv. a first supporting foam material disposed in the first zone; v. a second foam material disposed in the second zone; and vi. at least one fluid communication path between the first and second zones, wherein in the first zone the upper surface of the first foam material is attached to the impermeable fabric, the second zone includes at least one inflation region in which the upper surface of the second foam material is not attached to the impermeable fabric, and, the first and second zones are so sized and the first and second foam materials are so selected that the weight of the given part of the patient's body acting on the support structure in use is effective to transfer inflating fluid from said first zone to said second zone to inflate the inflation region whereby the impermeable fabric in said inflation region is displaced from the upper surface of the second foam material.

2. A structure as claimed in claim 1 wherein, in the first zone, the upper surface of the first foam material is attached to the impermeable fabric across substantially its entire area.

3. A structure as claimed in claim 1 wherein, in the first zone, the upper surface of the first foam material is attached to the impermeable fabric in attachment zones.

4. A structure as claimed in claim 2 or 3 wherein the upper surface of the first foam material is bonded to the impermeable fabric.

5. A structure as claimed in claim 4 wherein the surface of the first foam material is adhesively bonded to the impermeable fabric.

6. A structure as claimed in claim 4 wherein the surface of the first foam material is heat bonded to the impermeable fabric.

7. A structure as claimed in any preceding claim comprising a first chamber defining the first zone and a second chamber including the second zone.

8. A structure as claimed in any preceding claim wherein the first foam material is a different foam material from the second foam material.

9. A structure as claimed in any preceding claim wherein the second foam material is thinner than the first foam material.

10. A structure as claimed in any preceding claim wherein the inflation region of the second zone is located in the structure so that, in use, it lies below the lower leg of the patient such that said inflation of the inflation region is effective to raise the lower leg.

11. A structure as claimed in claim 10 wherein the inflation of the second zone is effective in use to raise the lower leg such that the patient's heel is lifted substantially out of contact with the structure.

12. A structure as claimed in any of claims 1 to 9 wherein the inflation region of the second zone is located in the structure so that, in use, it lies below the arm of the patient such that said inflation of the second zone is effective to raise the arm.

13. A structure as claimed in claim 12 wherein the inflation of the second zone is effective in use to raise the arm such that the patient's elbow is lifted out of contact with the structure.

14. A structure as claimed in any preceding claim wherein at least one inflation region is disposed adjacent a second region in which second region the impermeable fabric is attached to the upper surface of the second foam material whereby, in use, a body part susceptible to pressure induced tissue damage may be supported, by means of the inflation region above, but not in contact with, said second region.

15. A structure as claimed in claim 14 wherein said body part is the back of the heel and the inflation region is adapted to support the foot.

16. A structure as claimed in claim 14 wherein the body part is the heel and the inflation region is adapted to support the lower leg.

17. A structure as claimed in claim 14 wherein the body part is the elbow and the inflation region is adapted to support the arm.

18. A structure as claimed in claim 14 wherein the body part is the back of the head and the inflation region is adapted to support the head.

19. A structure as claimed in claim 14 wherein the body part is the ischia and the inflation region is adapted to support the buttocks.

20. A structure as claimed in any preceding claim wherein the first foam material is a visco-elastic foam.

21. A structure as claimed in any preceding claim wherein the second foam material is a visco-elastic foam.

22. A structure as claimed in any preceding claim further comprising a valve which, when open, allows fluid communication between the first and/or second zone and the atmosphere.

23. A structure as claimed in any preceding claim for supporting a recumbent patient, the structure further comprising third zones disposed along respective longitudinal marginal edges of the underside of the structure, in which zones the impermeable material is not attached to the first and/or second foam material whereby the weight of the given part of the patient's body acting on the support structure in use is effective to transfer inflating fluid from said first zone to said third zones to inflate the third zones whereby the impermeable fabric in said third zones is displaced from the lower surface of the first and/or second foam material.

24. A structure as claimed in claim 1 substantially as hereinbefore described with reference to any of Figures 1 to 4.

25. A mattress including the support structure as claimed in any preceding claim as a separable component.

26. The mattress as claimed in claim 25 including an openable cover portion by means of which access is gained to the support structure in use.