NO174872B - Support device for reducing bed ulcer formation - Google Patents

Abstract

A support system such as a clinical mattress comprises two sheets of flexible material bonded together to provide a plurality of separate cells (1) that are capable of being alternately and repeatedly inflated and deflated by means of a fluid contained in the cells (1). The flexible material is impermeable to the fluid. The cells (1) are of a size and shape and with an intercellular spacing such that in the width or length, or both, of the system, the distance between centres of adjacent inflated cells (1) is less than the human two-point discrimination threshold and the clinical support system is capable of supporting a human body with bottoming out either of or between the inflated cells (1).

Description

The invention relates to a support device comprising a number of separate cells of selected size and shape in a single layer, the cells being formed of a flexible material which is sufficiently impermeable to a fluid contained in the cells, so that each cell can be alternately and repeatedly inflated and deflated .

The invention is used in connection with support devices for use in reducing the breakdown of human skin, and in particular to reduce the likelihood of bedsores forming in people who have to hold the bed or are bound to a wheelchair or the like. for longer periods of time, or who are otherwise completely or partially immobilized.

As used here, the following terms have the following meaning: "Support system" includes mattresses, pillows and other related support devices, including support devices that can be used for therapeutic or other purposes;

"bottoming out" refers both to the collapse of a cell in a support device, so that the top part of the cell comes into contact with the underlying part or bottom part of the cell under the influence of a weight, e.g. the weight of a person, and that a person makes contact with the underlying part of the support device between cells; and

"human two point discrimination threshold" ("human two point discrimination threshold") is measured on a person's back and is the minimum distance at which two objects can be distinguished by touch when the objects are placed on the skin, as this distance is understood within the anatomy profession and is approximately 30 mm on the back of a person.

People can be tied to a support surface, such as beds, wheelchairs or similar devices, for a number of different reasons, for example as a result of injury or illness, or as a result of the needs or conditions of a work function during employment. Elderly people may be tied to the bed or other devices for longer periods of time.

Bedsores, which are also referred to as pressure ulcers, are a pervasive problem in the healthcare area, with a high price both in terms of individual, human suffering and in terms of the financial costs for society. The incidence of bedsores in hospitalized patients ranges from approx. 3% to approx. 17% and can increase to the range of 20-30% for hospitalized, elderly patients (D. Norton et al., "An Investigation of Geriatric Nursing Problems in Hospital", Churchill Livingstone, Edinburgh (1962)). For neurologically damaged patients, the incidence or frequency may be in the range of 30-60% of patients (Richardson and Mayer, Gerontol. 19 235-247 (1981); Taylor, J. Gerontol. Nurs. 6 389-391

(1980)).

Bedsores are localized cell tissue wasting or cell necrosis that tend to develop when soft tissue is compressed between a bony prominence and a solid surface for long periods of time. External pressure exerts its influence by occluding blood flow, leading to ischemic damage. Upon interruption of blood flow and consequently of oxygen supply, a sequence of intracellular events occurs which continues to an irreversible stage if blood flow is not restored. Ischemic damage results in cell death, i.e. necrosis, and the accumulation of cellular waste in the tissues.

The most decisive factors in the formation of bedsores are the intensity and duration of the pressure exerted, as the relationship between these factors is generally assumed to be a parabolic intensity-duration curve. If the patient remains motionless and in the same position for periods of time that are less than approx. two hours, the ischaemia is reversible and no long-term or irreversible damage is usually done to the soft tissues, i.e. skin, subcutaneous tissues and muscles, over bony prominences. If, however, the period of immobility exceeds approx. two hours, bedsores begin to form, which is sometimes referred to as Stage 1 pressure ulcer formation. It is for this reason in particular that it is the policy of many hospitals and institutions to place patients in a new position approximately every two hours. However, this practice is not entirely effective. There is also a trend towards caring for patients in the home, rather than in a hospital, and under such conditions nursing passes may not be available 24 hours a day.

Both external and internal factors are considered to contribute to reducing tissue tolerance to pressure. External factors that exert an influence on soft tissue include displacement or shear friction, humidity and temperature. Internal factors that determine tissue susceptibility to degradation include loss of sensation or sensation, impaired mobility, advanced age, malnutrition, vascular disease, anemia, incontinence, and infection.

Among the aging-related skin changes that may predispose an elderly person to the formation of bedsores are flattened skin-epidermal junction (Montagna and Carlisle, Journal of Investigative Dermatology 73 47-53 (1979)), reduced number of Langerhans cells (Kripke, Journal of the American Academy of Dermatology 14. 149-155 (1986)), reduced skin density that becomes relatively acellular and avascular (Montagna and Carlisle, ibid.), changes in collagen and elastic fibers (Shuster et al., British Journal of Dermatology 93 639-643 (1975)), reduced sweat and sebaceous gland function (Foster et al., Age and Aging 5_ 91-101 (1976); Plewig and Kligman, Journal of Investigative Dermatology 70 314-317 (1978)), and impaired immune response (Barrett et al., Clinical Immunology and Immunopathology 17 203-211 (1980). Versluysen (British Medical Journal (Clin. Res.) 292 1311-1313 (1985)) reported that 90% of patients with hip fractures who was over 70, developed bedsores.Inability to heal bedsores ulcers have been linked to an almost six-fold increase in death in the elderly (Reed, MD State Med. J. 30 45-50

(1981)). Complications of bedsores include bone marrow inflammation (osteomyelitis) and wound inflammation (sepsis), and the mortality rate in sepsis approaches 50% (Galpin et al., American Journal of Medicine 61 346-350 (1976); Sugerman et al., Arch . Phys. Med. Rehabil. 66 177-179 (1985); Bryan et al., Arch. Intern. Med. 143 2093-2095 (1983)). Bedsores can thus be a very serious problem in the healthcare area. A number of different systems or devices are available which are intended to reduce the formation of bedsores. Most of these work based on one of two principles, namely static devices, e.g. foam mattresses, air mattresses, waterbeds and sheepskin, which attempt to redistribute support away from bony prominences, and active devices, e.g. alternating air mattresses, which work by alternately changing the support pressure. Although such devices constitute improvements in relation to the use of conventional mattresses, there is a need for further improvement in efficiency and/or performance in use. Many of the static devices often have a limited useful life due to their inability to be cleaned effectively for repeated use by the same or a different patient. A critical problem with the active devices, and some static devices, is that they may be unable to support the weight of a body in areas of bony protrusion. In such circumstances, the support device collapses under the weight of the bony projection, coming to rest on the mattress below, i.e. "bottoming out". This occurs because such devices tend to be composed of one or more air-filled chambers of expandable plastic material, regardless of the design of the chambers. The force exerted by a bony protrusion over a relatively small area of the support device causes the collapse of the associated part of the air chamber, as the rest of the air chamber only has to undergo a small expansion to equalize the pressure in the chamber. Another cause for concern is the design of the air chambers. For the most part, the chambers are drawn into inter-digitating patterns of tubular or diamond-shaped or differently shaped sections or cells, so that when one section is filled with air, the adjacent sections are empty or deflated. However, the five cm or larger cell sizes in typical support devices have not been able to lift the patient sufficiently clear of the mattress under the device to provide effective, alternating pressure, particularly over bony prominences. Although the larger cell sizes in some devices have sufficient deflection or projection, i.e. height, to overcome this bottom contact problem (Bliss et al., British Medical Journal 1 394-397 (1967)), they have been found for others problems, e.g. that large areas of the body are left without support, which causes the patient to experience discomfort and restlessness. Even the five cm cell sizes are unable to prevent small bony projections on a body from falling down between the inflated cells and resting on the mattress below, i.e. touching bottom. Although the application of higher pressures in such tubes could be used to prevent bottom contact, there would be a resulting patient comfort problem and effective alternating pressures would not be achievable. Another limitation of these devices concerns the cycle rate, and more particularly the time required to inflate supporting cells and deflate adjacent cells. A long period of inflation and deflation precludes a decompression phase, i.e. an interface pressure below internal capillary pressure, of sufficient duration to allow normal blood flow and tissue recovery. It is thus an object of the invention to provide a support device which results in an improved combination of reduced tendency to bedsore formation and improved comfort for the patient, by recognizing and overcoming the problems of bottom contact of areas of bony prominences of a body, and to improve the pressure relief phase to allow a more normal blood flow and tissue recovery and reduce physical discomfort for the patient. The above-mentioned purpose is achieved with a support device of the type indicated at the outset which, according to the invention, is characterized by the cells having such a size and shape and such an inter-cell distance that the distance between the centers of adjacent, inflated cells, in at least one of the width of the support device - and longitudinal direction, is less than the human two-point discrimination threshold, that the support device is able to support a human body without touching the bottom either in or between the inflated cells, and that a deflated cell, when the support device supports a body, exerts a pressure at less than the human internal capillary pressure threshold on the body. An advantageous embodiment of the support device also includes a device for inflating and deflating the cells, and the cells are preferably capable of being inflated and deflated independently of each other. The fluid in the cells can suitably be a liquid which is able to evaporate to inflate the cells, and the device for inflating and deflating the cells is then a heating and cooling device. In another advantageous embodiment, the device for inflating the cells can be a compressor. In an advantageous embodiment of the support device, the device for inflating the cells is controlled in such a way that when one cell is inflated, an adjacent cell is deflated. The distance between adjacent inflated cells is preferably less than 30 mm. An advantageous embodiment of the support device is characterized by the fact that it sequentially comprises the single layer of separate cells, a device for inflating and deflating the cells, a layer of shock absorbing material, and a layer of a material with a high coefficient of friction. The cells preferably have such a shape and size that a weight of 2.5 kg, and with a spherical surface with a diameter of 2.67 cm placed on the support device, will not cause bottom contact in the support device. According to the invention, a cell is also provided which is suitable for alternate inflation and deflation, and which is characterized by the fact that it is formed from a flexible, impermeable, thermoplastic material and contains an inert fluid with a boiling point in the range 0 - 50°C, preferably 20 - 34°C, the cell also having a device for heating and/or cooling the fluid. Although the present invention is specifically described here with reference to clinical support systems and mattress support systems, one must be aware that the systems or devices, particularly in some end applications, need not be in a form commonly referred to as supports or mattresses, but instead in a form of seat supports or other supports, as described below. The invention will be described in more detail below with reference to the drawings, where fig. 1 shows a schematic representation of part of a single row of cells in a support device, where all cells are shown in an inflated state, fig. 2 shows a schematic representation of the cells in fig. 1, where some of the cells are in a deflated state, fig. 3 shows a schematic representation of an embodiment of a support device according to the invention, fig. 4 shows a computer-simulated drawing of a cell, fig. 5 shows a histogram (bar chart) of data obtained in Example I, fig. 6 shows a graphical presentation of data obtained in Example II, fig. 7 shows a graphical representation of data obtained in Example III, fig. 8 shows a graphical representation of the pressure profile as measured in Example IV, fig. 9a and 9b show schematic cross-sectional views of the use of support devices with long, inflated, tubular cells and of support devices according to the invention, fig. 10 shows a graphical presentation of temperature as a function of recovery time as measured in Example V, and fig. 11 shows a graphical representation of heat response for tissue as a function of time, as measured in Example VI. In fig. 1 shows a single row of cells 1, the outer surface of the cells 1 and a substrate 2 being sheets of flexible material, the sheets forming the outer and bottom surfaces of the cells. The cells 1 are separated by spaces 3 which are substantially smaller than the distance d between the centers of the cells, as shown at 4. The cells 1 are shown to be elongated, but one must be aware that the cells can have any suitable shape. Nevertheless, the cells should be of a size and shape which precludes "bottoming", i.e. precludes collapse of the cell such that the top portion of the cell contacts the bottom portion of the cell under the action of a weight, e.g. the weight of a patient. An example of a cell is shown as a computer simulated drawing in fig. 4. When in use, the cells will be connected to devices to inflate and deflate the cells in a controlled manner. Such devices are not shown. The cells can be formed with single or multiple compartments, including foam, with the provision that the cells must be inflatable and deflatable. The cells 1 in fig. 1 is capable of being inflated and deflated, as shown in FIG. 2. In the embodiment shown, inflated cells 11 are separated by a deflated cell 12. The distance between the centers of the inflated cells is less than the human two-point discrimination threshold, and a person lying on the cells is thus unable to distinguish at touch that every second cell is inflated and deflated. Furthermore, the patient is usually unable to feel deflation of cells 11 and inflation of cells 12. In fig. 3 comprises a mattress system or a mattress device shown generally at 20, a layer 21 of closed cells on top of a heating element layer 22, a fiber layer 23 and a high friction layer 24. On top of the closed cell layer 21 is a fabric or fabric layer 25 and an outer, microporous layer 26. The layer 21 of closed cells has a number of cells 27 which may be of the type shown in fig. 1. The cells 27 are shown to be elongated and to be mutually aligned in both the axial direction and the transverse direction of the cells. However, the cells could have alternative shapes and/or be arranged in a more random pattern. The cells are referred to here as "separate cells". However, one must be aware that although the cells physically appear to be separate cells, any cell may be connected to one or more other cells for purposes of inflation and deflation of the cells. The cells 27 are capable of being inflated and deflated. A number of different devices can be used to inflate and deflate the cells. For example, the cells can be connected by means of a pipe system to a system which will alternately supply a compressed gas, e.g. compressed air at a pressure sufficient to inflate the cells 27 when in use, and later applying a vacuum to the cells 27 to the extent necessary to deflate the cells. The degree of vacuum used can be small, i.e. barely sufficient to deflate the cells 27 to the extent that the cells will no longer support a patient on the mattress system 20. Compressors for supplying the compressed air tend to be noisy , and alternatively the supply of compressed gas can take place from a source that is located at a distance from the area of use for the mattress system, e.g. from a compressor or other source of compressed gas in a remote location. Alternatively, the alternating pressure in the cells can be applied by means of hydraulic means to a liquid in the cell. Examples of such liquids include water and silicone oils. A preferred method for inflating and deflating the cells 27 is to incorporate a liquid into the cells. When using such a liquid, the liquid is heated, particularly by means of thermoelectric devices, to cause vaporization to shape and thereby inflate the cells 27. Such heating may increase the temperature of the liquid above its boiling point, but it need not be necessary to do so , provided that sufficient pressure is generated to inflate the cells 27. Upon cooling, the pressure in the cells 27 decreases, and the cells deflate. The liquid must be selected so that sufficient vapor can be generated to cause the cells to inflate while at the same time remaining at a desired or preselected temperature. It may also be necessary to select the fluid for a particular end-use location. For example, the ambient temperature around the patient in some places can be as low as approx. 18°C, while the ambient temperature in other places can be as high as approx. 40°C. The liquid placed in the cells 27 is preferably inert, non-toxic and flame-resistant, and of no concern to health authorities with regard to both the patients and persons, such as doctors and nurses, who care for the patients. The cells 27 must also be made up of a material that has sufficient barrier or barrier properties against the liquid, so that a supply of liquid can be kept in place in the cells for at least the expected period of use for the mattress system. Such a material is referred to here as impermeable. As discussed here, the material can be a multilayer structure, including a coated structure, in order to achieve an acceptable level of impermeability. One must be aware that the expected periods for use of a clinical support facility can be six months or as long as two years. Examples of fluids that are incorporated into cells include fluorocarbons, in particular mixtures of chlorofluorocarbons that exhibit changes in vapor pressure above the temperature range used when inflating and deflating the cells 27, and fluids of the type that are being developed to replace chlorofluorocarbons for environmental reasons, e.g. hydrochlorofluorocarbons. Fluorocarbons and Hydrochlorofluorocarbons are available from Du Pont Canada Inc. under the trade name Freon, examples of which are sold under the trade names 114, 113, 22, 11, 123 and 141B. The boiling point of the liquid should be in the range 0-50°C, preferably 10-40°C. Liquids with lower boiling points than this range can possibly be used for cooling purposes, e.g. of limbs or other parts of the body. In certain embodiments, the liquid has a boiling point in a comfortable range for a patient, but below the normal human transpiration threshold, particularly in the range of 20-34°C. The cells 27 shown in fig. 3, is of a type that will contain a liquid. Although the fluid could be heated solely by the body heat of a patient, it is preferred that electrical or especially thermoelectric devices are provided to heat and cool the fluid. In fig. 3 has the heating and cooling layer (the thermoelectric layer) 22, which is located below the layer 21 with closed cells, heating and cooling devices 28 and 29 which can be used to evaporate or condense the liquid. Although reference is made here to a heating and cooling layer, one must be aware that in some embodiments the layer can only be a heating layer or a cooling layer. The heating and cooling devices 28 and 29 are separate electrical circuits and are connected to adjacent cells 27, the heating and cooling device 28 being used to heat and cool one cell, and the heating and cooling device 29 being used to heat and cool it adjacent cell. One of the heating and cooling devices 28 and 29 will normally be connected to each cell, so that inflation and deflation of the cell can be easily controlled. Only two heating and cooling devices 28 and 29 can possibly be used to control the entire mattress system or mattress device, or a number of different heating and cooling devices could be used to control different parts of the mattress device in different ways, for example by using a microprocessor . It is preferred that the heating and cooling devices work on a low, harmless voltage, i.e. a voltage that is significantly lower than that which is normally used for heating and cooling devices. As mentioned above, the flexible material must be sufficiently impermeable or dense to allow the use of the clinical support device during the expected periods of use. The nature of the flexible material to meet such density requirements will depend in particular on the fluid contained in the cells of the clinical support device. For example, flexible materials suitable for use with an inert, gaseous fluid, e.g. a hydrochlorofluorocarbon, may not be suitable for use if water is used as the fluid, and vice versa, as will be understood by those skilled in the field. The flexible material is preferably a polymeric material, and will in particular be a laminated, heat-bonded or coated polymeric material. In some embodiments, the flexible material is a thermoplastic polymer that has been laminated or coated with a polymer material that exhibits barrier properties to the liquid to be contained in the cells of the clinical support device. In one embodiment, the polymeric material is an inert low density polyethylene that has been coated or laminated to polyvinylidene chloride (PVDC). Such a flexible material exhibits both barrier properties and flexibility and toughness properties, which is important with regard to the lifetime of the clinical support device. In other embodiments, the flexible material can be polyethylene, polypropylene, polyvinyl chloride, polyvinylidene chloride, polyester, polyamide, chlorosulfonated polyethylene, vinylidene fluoride/hexafluoropropylene copolymers, polyurethane, ethylene/propylene/diene terpolymers, copolyether ester polymers, silicone rubber, butyl rubber and natural rubber, if necessary coated to achieve the necessary barrier properties. The closed cell layer 21 and the thermoelectric layer 22 are in fig. 3 shown to be located on a fiber layer 23. The layer 23 is intended to provide shock absorption and good pressure distribution on the mattress system, and thus provide greater well-being for the patient. The layer 23 can be formed from a number of different fibers or foam materials, including artificial fibers such as polyamide, polyester and/or polypropylene, natural fibers such as cotton, cellulose or wool fibers including sheepskin and the like. In most cases, the fiber layer will be formed from synthetic fibers that have been given sufficient bulk to provide shock-absorbing effects. An example of a preferred fiber is Quallof ilR polyester fiber which is used in the manufacture of head cushions. In another embodiment, the layer 23 can be an air mattress. In fig. 3, the fiber layer 23 is shown to be located on the friction layer 24. The friction layer is arranged for stability and safety for the patient, in particular to prevent the mattress device from sliding out of the bed or other structure on which it can be used. A number of different friction layer materials are known, including foamed thermoplastic polymers, such as polystyrene, woven textile structures, Velcro<R->materials and

similar.

The one in fig. 3 mattress device shown has two layers placed on top of the closed cell layer. The layer shown in the immediate vicinity of the closed cell layer is a tissue layer 25 which is primarily intended as a cover sheet or a sheet that encloses the mattress device according to the invention, in order to preserve the integrity of the mattress system or the mattress device and for aesthetic reasons, as well as for cleanliness and sterility reasons to prevent infections. The outer layer shown is a microporous layer 26 intended primarily for the comfort of the patient. In particular, the microporous layer 26 allows sweat or other moisture associated with the patient to be removed from the patient's location, improving patient comfort. The microporous layer is intended to be a disposable layer. The tissue contact layer 25 and the microporous layer 26 must have such a thickness and be formed of such materials that the beneficial effects of the function of the closed cell layer 21 are not cancelled. In an alternative embodiment, the outer layer can be a non-adhesive layer, in particular such a layer that would be used for burned patients or for some therapeutic end-uses.

When operating the mattress system or the mattress device in fig. 3, a patient is placed on the mattress device, in contact with the microporous layer, or a sheet or similar layer over the microporous layer. It is preferred that the mattress device is constructed so that the cells are arranged at an angle in relation to the axis of the patient's body, and in some embodiments are arranged across the body. The cells in the closed cell layer are then alternately inflated and deflated, e.g. by applying heat using the heating element layer, and then allowing the liquid to cool or actively cooling the liquid.

The inflation and deflation cycle can be varied, from one minute to more than one hour. However, the cycle should be more frequent than once every two hours. Different cycles can be used for different areas of the body, for example the areas where the body exerts greater pressure can be on a shorter cycle than areas where less pressure is exerted, or different cycles can be used for therapeutic or other reasons. It must be expected that there will be different optimal cycle times depending on the intended use of a mattress device or a

clinical support or support device.

Reference is made here to the cycle time for inflation and deflation of the cells. This cycle time essentially includes both the time period necessary for the transfer of fluid out of or into a cell to actually cause the deflation and inflation of the cell, or for the condensation or evaporation of fluid completely contained within a cell, and the time period during which cell is inflated or deflated. Such a period for transferring fluid is limited and may be a few minutes in length. One must be aware that the beneficial effects of deflating a cell, particularly the restoration of normal microcirculation in the layers of the skin near the deflated cell, are primarily limited to the period of time when the cell does not support a patient, which may be significantly shorter than the cycle time. The time period for transferring fluid in relation to the cycle time becomes more important with short cycle times, and may need to be taken into account when operating systems or devices according to the invention.

The inflation and deflation of cells is generally described here in the sense that when one cell is inflated, an adjacent cell is deflated. One must be aware that such inflation and deflation can occur simultaneously or in sequence, the latter involving inflation of a cell followed by deflation of an adjacent cell. The inflation and deflation can also be performed as a wave that passes over the clinical support system, including in accordance with a peristaltic cycle. In some cases, a patient may have a sensation of such a wave or peristaltic effect, but the effect may have e.g. beneficial therapeutic effects and can possibly be used for this reason or for other reasons. In embodiments of the invention, a cell that is inflated will be surrounded by cells that are deflated, and vice versa, or a row of cells may be inflated and the immediately adjacent row of cells deflated, or other configurations of inflated and deflated cells may be used, provided that the arrangement of inflated and deflated cells are able to support a patient as described here.

The mattress system or mattress device according to the invention provides alternating support for a patient in such a way that the patient has little or no feeling that the alternating support is provided by the mattress device, i.e. parts of the patient's body are alternately supported and unsupported while the patient has little or no sensation of movement in the bed in which he is lying. Any such sensation could be very disturbing for the patient. However, the separation, in at least one direction, of the inflated cells by distances less than the human two-point discrimination threshold essentially eliminates or overcomes any sensation and allows the mattress device to perform its intended function. The pressure exerted on the patient's body side by side with a deflated cell is also less than the human internal capillary pressure threshold, e.g. 20-32 mm Hg. If this were not the case, blood circulation to the particular area of the patient's skin above the deflated cells would not occur, and bedsores would be the result. The internal capillary pressure will vary from patient to patient, and probably from one area of a patient to another. The capillary pressure threshold, e.g. the surface pressure above which capillaries can be expected to break down is approx. 20-32 mm Hg, depending on the patient and the area of the patient that is in contact with the mattress device. In embodiments, it is thus important that the pressure exerted on the patient by a deflated cell is less than approx. 20 mm Hg. The more generic requirement is that the pressure exerted over the deflated cell is less than the capillary pressure threshold.

As indicated above, the clinical support system is able to support a human body without touching the bottom either in or between the inflated cells. In one embodiment, the human body is simulated by a spherical or spherical surface. In particular, the following method can be used to determine whether a clinical support system is capable of supporting a human body without touching the bottom: The method uses a jig that has a head with a spherical surface with a diameter of 2.67 cm, the head having an actual diameter of 7.5 cm. The jig also has a rod that is axially attached to the head on the side opposite the ball surface, the rod being adapted to accommodate weights. In the test method, the jig is placed on a surface of cells so that the jig is centrally located over a deflated cell and supported by two adjacent, inflated cells. Weights with an axial hole are then added to the jig using the rod, until the surface of the jig contacts the lower surface of the deflated cell. At this point, the total weight of the jig must be at least 2.5 kg. Under such circumstances, the cells in the clinical support device will have such a shape and size that a weight of 2.5 kg and with a spherical surface with a diameter of 2.67 cm placed on the clinical support device will not cause bottom contact in the clinical support facility.

In fig. 9 a is a portion of a human torso shown generally at 40, shown on a mattress or cushion device 41 with large inflatable cells 42 of which only one is shown in cross section. The inflatable cell 42 is shown to have bottomed out in an area 43 which is the area of the cell that lies directly below the seat leg 44 of the torso 45, the gas in the inflated cell 42 being shown to have been pushed away from the area 43 in which the cell has gone to the bottom, in the direction of the arrows 45.

In contrast, the torso 40 of FIG. 9b shown on a mattress device according to the present invention. The mattress device comprises a single cell layer 46 of cells which are shown to be alternately inflated cells 47 and deflated cells 48. The layer of cells is attached to a flexible, thermoelectric layer 49. In the flexible, thermoelectric layer 49, a number of heating and cooling circuits 50, each circuit 50 being located under either an inflated cell 47 or a deflated cell 48. In the shown embodiment, the heating and cooling circuits 50 under an inflated cell 47 heat the gas in the cell, while the heating and cooling circuits 50 under a deflated cell 48 cools the steam in the cell. The flexible layer 49 is shown to be located on a fiber layer 51. As shown in fig. 9b, the torso rests on the inflated cells 47 and does not go to the bottom and touch the surface of the deflated cells 48. Thus, no pressure is exerted on the torso located above the deflated cells 48. Activation of the heating circuits under the deflated cells 48 and activation of the cooling circuits under the inflated cells 47 will cause a reversal, so that the part of the torso now shown to be in contact with the inflated cells will come out of contact with the cells, and vice versa.

The mattress device according to the invention works below both the capillary pressure threshold and the two-point squint threshold, and thus gives the patient the benefits of increased blood circulation and reduced tendency to form bedsores, and at the same time gives the patient well-being. The mattress device is easy to use, especially when a liquid capable of undergoing a phase change is used to provide inflation and deflation of the cells, and it can be cleaned easily and operated in a noise-free manner. In practical embodiments, the mattress device can be operated by means of a microprocessor and be transportable, i.e. it can be adapted for transportable use, e.g. on wheelchairs and other transportable systems, including for limbs and other parts of the body, which gives the patient the opportunity to be mobile. The liquid in the cells can also be cooled, to allow cooling of all or part of a person's body, e.g. as a cooling pack for use in surgery, or for therapeutic reasons.

Although the support systems or support devices according to the invention have been generally described here in connection with medical applications, i.e. as mattress devices, one must be aware that the support devices can be used in a number of different forms and for a number of different end uses. In many such end applications, the systems or devices will more commonly be referred to by other names, including support systems, seats, chairs and the like. For example, the devices described here can be used in nursing, and in transport and recreation industries, and examples of such include airplane, car, truck, office and home seating and other seating.

The invention is illustrated by means of the following examples:

Example I

Holes of circular cross-section and varying diameter were cut in a series of metal sheets of different thickness. The diameters of the holes were as follows: 31.5 mm, 39.0 mm, 45.0 mm and 51.3 mm. The plates had thicknesses of 4.2 mm, 5.4 mm, 6.6 mm and 7.8 mm.

The ischial projection of a human was placed over each of the holes in turn. The person was a healthy man with an age of 46 years, a height of 173 cm, a weight of approx. 84 kg and with a medium build. A pressure-sensing device was placed in or on the opposite side of the hole, so that the desired output was achieved. The sensing device was located on a wooden plate, so that any pressure exerted by the human on the device, i.e. in the plane of the opposite side of the hole, could be measured.

The results obtained are shown in fig. 5. In only three cases did the person's ischial projection fail to exert pressure, i.e. to bottom out, namely the 31.5 mm hole with a projection (measured by the distance from the surface of the plate to the pressure-sensing device located in the hole) of 6.6 and 7.8 mm, and the 39.0 mm hole with a projection of 7.8 mm. Bottom contact therefore did not occur for such combinations of hole diameter and projection. Cells of such dimensions and of smaller diameter would not result in bottom contact for the ischial prominence of the human subject used in this example.

In a number of related samples, cell dimensions that will support a human body in a number of different positions were determined, e.g. the sciatic bone in a sitting position, the largest femoral joint lying in a lateral position, and the sacrum and shoulder blade in the supine position.

Such samples provide guidance with regard to the cell dimensions that are necessary to prevent bottom contact in the clinical support systems or support devices according to the invention. The results obtained vary with the position of the human body.

Example II

Using procedures similar to those described in Example I, except that the holes were rectangular holes, a series of tests was performed to determine the effect of cell geometry on the pressure exerted by the ischial tuberosity ). In all tests, the plate's thickness, i.e. the deflection, was 8 mm. The holes were arranged in the anterior/posterior direction, and had widths in the range from 18 to 34 mm and lengths from 20 to 100 mm. The results obtained are shown in fig. 6. The capillary pressure threshold of 32 mm is also shown in this figure.

Example III

Example II was repeated, with holes aligned in the transverse direction being used. The results obtained are shown in fig. 7.

It will be appreciated that the results in Example II show that where the long axis of the holes was aligned in the front/back direction, only short cell lengths of 20-36 mm with widths of 18-34 mm gave pressure of less than the capillary pressure threshold. In contrast, the results in Example III show that much longer cells could be tolerated.

Example IV

The pressure exerted by a man lying supine on a mattress of the type used in hospitals, and on a synthetic fiber layer on the mattress, was measured in a number of positions on both the mattress and the layer, to illustrate the pressure profile of a patient.

The results obtained are shown in fig. 8. The three areas of high pressure exerted by man were, in descending order, the buttocks, shoulders, and head. The use of the artificial fiber layer on the mattress resulted in a significant reduction in the pressure exerted in the above-mentioned three areas, this reduction being as high as approx. 60% in the area of the shoulders, but the pressure was still about an order of magnitude above the capillary-separation level in all three positions.

Example V

The return to the normal (pre-test) skin temperature of a subject's buttocks after various periods of time in a seated position was monitored using a thermographic camera. The person was a healthy man aged 43, a height of 173 cm, a weight of approx. 84 kg and with a medium build. The subject was seated on a soft cushion or mattress device according to the present invention which operated at a cycle time of ten minutes for various periods of time, and then the time for his skin temperature to return to normal was monitored using an infrared thermographic camera of the type Agema, Model 870, with image analysis.

The results obtained are shown in fig. 10. As skin temperature is directly proportional to blood flow in the skin, the decline of skin temperature to normal values is an indicator of the state of blood circulation in the skin.

The results show that the recovery or restitution time increased exponentially with the length of the sitting period. The results also show that recovery from sitting on a mattress device according to the invention for 30 min. is almost as fast as from sitting on the soft cushion for 5 min., and significantly better than from sitting on the cushion for 7 min. It will be noticed that the regression lines through the data for the pillows at 3 and 5 min., and for the mattress device according to the invention tend to converge at approx. 6 min., while, on the other hand, the regression lines for data with pads at longer time periods indicate a significantly longer recovery period.

For optimal operation, the time for return to normal blood circulation in the pressure relief phase over deflated cells in a mattress device according to the invention must be matched with the pressure duration phase over inflated cells. The results show that a suitable cycle frequency for a mattress device according to the invention for use by the above-described person in the sitting position would be approximately 10 min.

Example VI

The recovery of the skin temperature of a subject's sacral or sacral region after two hours in supine position was monitored by infrared thermography. The person was a healthy man with an age of 46 years, a height of 173 cm, a weight of approx. 84 kg and with a medium build. The person was placed in a supine position on a normal hospital bed or on a mattress device according to the invention which worked in a 10 min. cycle. After a period of two hours on the bed or mattress, the person was placed in a position on his right side, for immediate monitoring of the sacral area using the thermographic camera according to Example V. The average temperature change with time in relation to the control temperature for the person was measured. The results obtained are shown in fig. 11.

The thermal response or heat response after the two-hour period in the hospital bed indicates an erythema paratrimma, as shown by the persistent increase in temperature relative to the control. Erythema paratrimma is characterized by an immediate reddening of the skin and a rise in temperature after a period of stasis above a pressure point. After the two-hour period on the mattress device according to the invention, however, the heat response approached normal temperature after 15 min., without inducing erythema paratrimma.

Claims (18)

1. Support device comprising a number of separate cells (27; 47, 48) of selected size and shape in a single layer (21; 46), the cells (27; 47, 48) being formed of a flexible material which is sufficiently impermeable for a fluid contained in the cells (27; 47, 48), so that each cell can be alternately and repeatedly inflated and deflated, CHARACTERIZED IN THAT the cells (27; 47, 48) have such a size and shape and such an inter-cell distance that the distance between centers of adjacent, inflated cells, in at least one of the width and length directions of the support device (20), is less than the human two-point discrimination threshold, that the support device is able to support a human body without touching the bottom either in or between the inflated cells, and that a deflated cell (48), when the support device supports a body, exerts a pressure of less than the human internal capillary pressure threshold on the body. 2. Support device according to claim 1, CHARACTERIZED IN THAT it also comprises a device (28, 29; 50) for inflating and deflating the cells (27; 47, 48). 3. Support device according to claim 1 or 2, CHARACTERIZED IN THAT the cells (27; 47, 48) are capable of being inflated and deflated independently of each other. 4. Support device according to one of claims 1-3, CHARACTERIZED IN THAT the fluid is a liquid which is able to evaporate to inflate the cells (27; 47, 48). 5. Support device according to claim 4, CHARACTERIZED IN THAT the device for inflating and deflating the cells (27; 47, 48) is a heating and cooling device (28, 29; 50). 6. Support device according to claim 2 or 3, where the fluid is a gas, CHARACTERIZED IN THAT the device for inflating the cells is a compressor. 7. Support device according to claim 2 or 3, where the fluid is a liquid, CHARACTERIZED IN THAT the device for inflating and deflating the cells is a hydraulic device. 8. A support device according to one of claims 1-7, CHARACTERIZED IN THAT each cell (27; 47, 48) has a geometry that precludes complete collapse of the cell when it is deflated. 9. Support device according to claim 8, CHARACTERIZED IN that the device for inflating the cells is controlled so that when one cell (47) is inflated, an adjacent cell (48) is deflated. 10. Support device according to one of claims 1-9, CHARACTERIZED IN THAT the distance between adjacent inflated cells (27) is less than 30 mm. 11. Support device according to one of claims 1-10, CHARACTERIZED IN THAT the cells (27; 47, 48) are inflated and deflated by using a simulated wave movement over the support device. 12. Support device according to claim 1, CHARACTERIZED IN that it sequentially comprises (a) the single layer (21; 46) of separate cells (27; 47, 48), (b) a device (28, 29; 50) for inflating and deflate cells (27; 47, 48), (c) a layer of shock absorbing material (23; 51), and (d) a layer (24) of a material with a high coefficient of friction. 13. Support device according to one of claims 1-12, CHARACTERIZED IN THAT the cells (27; 47, 48) have such a shape and size that a weight of 2.5 kg, and with a spherical surface with a diameter of 2.67 cm placed on the support device, will not cause bottom contact in the support device. 14. Support device according to one of claims 1-13, CHARACTERIZED IN THAT a fabric layer (25) is located above the layer (21) of flexible material, the fabric layer (25) being located between a moisture absorption layer (26) and the layer (21) of flexible material. 15. Support device according to one of claims 1-14, CHARACTERIZED IN THAT the fluid is a fluorocarbon or a mixture of fluorocarbons. 16. Support device according to one of claims 1-14, CHARACTERIZED IN THAT the fluid is an environmentally acceptable substitute for a fluorocarbon. 17. Support device according to one of claims 2-16, CHARACTERIZED IN THAT the device for inflating and deflating the cells (27; 47, 48) has a cycle time which promotes the restoration of normal microcirculation in human skin while the cells are deflated. 18. A cell suitable for alternating inflation and deflation, CHARACTERIZED IN that it is formed of a flexible, impermeable, thermoplastic material and contains an inert fluid with a boiling point in the range of 0-50°C, preferably 20-34°C, wherein the cell (27; 47, 48) also has a device (28, 29; 50) for heating and/or cooling the fluid.