Patient supports and methods of operating them

Details Patient supports and methods of operating them Patient supports and methods of operating them

1999-11-10

2001-04-17

Trettel, Michael F. (Department: 3628)

Beds

Mattress

Having confined gas

C005S715000, C005S933000

Reexamination Certificate

active

06216300

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to supports for a patient's body, used in medical or veterinary treatment, and particularly to supports which apply alternating-pressure to the body in order to reduce or minimize the risk of pressure sores caused by prolonged pressure on the skin. Such supports may be for the whole body, in the form of beds or mattresses, or for a part of the body, for example chair seats such as wheelchair seats and calf supports. The invention also relates to methods of operating such body supports, and is particularly but not exclusively concerned with body supports having a plurality of inflatable cells which are inflated and deflated cyclically in groups, to apply the alternating-pressure to the body.
DESCRIPTION OF THE PRIOR ART
Many such body supports have been proposed and used in recent years. The assignors of the present inventor (Pegasus Airwave Ltd) make and sell two mattresses having arrays of inflatable tubes under the trade marks “Airwave” and “Bi-wave”. The “Airwave” mattress is based on that disclosed in UK Patent 1 595 417 (now assigned to Pegasus Airwave Ltd). They have also disclosed an active wheelchair seat having an array of tubes (WO 94/07396) and an active calf support (WO 96/19175).
In arriving at the present invention, the inventor has paid attention, it is believed for the first time, to the stage of removal of pressure from the patient in such alternating-pressure devices. To gain understanding of the invention it is therefore necessary to collect together and review the clinical data and other reports in the background of pressure sore control.
Clinical data and research reports on this subject are rather sparse, despite the facts that pressure sores can have tragic consequences for patients, including widespread pain, infection, interrupted sleep patterns and impaired rehabilitation. In the most vulnerable patients serious pressure sores can be a cause of death. Recent studies commissioned by the UK Department of Health put the cost of pressures sore treatment at over £250 million per year.
(1)
(A list of references appears below). Pressure sores affect about 10% of hospital patients. A large number of specialised support surfaces are now available to complement skilled nursing practice but there is little evidence by which the efficacy of these products can be analysed. Of a range of 48 products surveyed in 1992
(2)
, 50% had no evidence of effectiveness, a further 21% had only anecdotal information to support claims, and a further 21% only had laboratory interface pressure study evaluation. Only four mattress systems had been studied in a clinical trial and only two of these trials had been adequately designed with randomisation of patients. A clinical trial carried out by Exton Smith et al
(3)
looked at the effectiveness of the Airwave system of preventing pressure sores (mentioned above) compared with a conventional large cell ripple mattress. Superficial or deep sores developed in 42% of the patients nursed on the conventional mattress whilst only 6.5% of the Airwave-nursed patients broke down.
A survey was published in 1992
(4)
of the users of the Pegasus Airwave system. This survey conducted in 1991 represents the largest database in the current pattern of use of any pressure-relieving mattress, surveying 788 patients in 119 sites. In this survey only 4.9% of patients developed new sores, thus supporting the data gathered in the original trial carried out by Exton Smith et al
(3)
. Nevertheless it was apparent that the product was not always successful at preventing pressure sores.
The original premise on which alternating-pressure systems were designed was that arterial occlusion occurred at 32 mmHg interface pressure (1 mmHg equals 13.5 Pa). This is based on work carried out by Eugene Landis in 1929
(5)
, which was not directed at a pressure sore study and in which all the results were obtained from healthy individuals. 32 mmHg was the average pressure of the arteriolar limb of the subjects but venous pressure ranged from 6 to 18 mmHg with an average of 12 mmHg. These figures obtained from healthy subjects are much better than the figures to be expected from a high risk patient. Equipment which merely lowers the interface pressure below this level of arterial closure will not allow blood to flow and hence relieve ischaemic tissue in all patients. This assumes that by reducing interface pressure below the level of internal closure pressure, blood will flow. Le et al in 1984
(6)
showed that pressures are higher within tissue than they are at the skin and that pressure sores would originate within tissue near bony prominences, also that internal pressures may be 3-5 times greater than surface or interface pressures.
Sangeorzan et al
(7)
were able to conclude that tissue pressure should not exceed 8 mmHg when measuring subcutaneous pressures that caused total arrest of oxygen in human tissue. When these facts are looked at in light of Le et al
(6)
then interface pressures of 1.6-2.6 mmHg are necessary to relieve the ischaemic tissue. Kosiak summed this up in 1961
(8)
stating that “Since it is impossible to completely eliminate all pressure for a long period of time, it becomes imperative that the pressure be completely eliminated at frequent intervals in order to allow circulation to the ischaemic tissues”.
The current Airwave system reliably achieves this complete elimination but still nearly 5% of patients using the system broke down. Products of other manufacturers claiming phases of zero pressures have appeared but these also have patients still breaking down. Having noted this apparent contradiction, the present inventors sought to achieve improved reduction of pressure sores.
In the light of the present invention as disclosed below it should be mentioned that in GB-A-1595417 it is disclosed that the tubes of the mattress are deflated by connection to a vacuum source, in the form of a compressor which is said to provide pressure and vacuum for the pressure cycling of the arrays of tubes. The exact arrangement is not disclosed, and it is indicated that the inlet to the compressor from the tubes is also an inlet from the atmosphere. The Airwave mattress as manufactured does not use such an arrangement, but vents the tubes to atmosphere. To the present inventors' best knowledge, no inflatable body support system actually used has employed a source of below atmosphere pressure to deflate its cells during the normal cycling of the cells.
WO 92/07541 on the other hand discloses a mattress of the low air loss type, in which air escapes continuously from the cells via holes or pores in order to dry and cool the patient's skin, so that deflation in normal cycling occurs by this slow air loss rather than by opening of a conduit to atmosphere. To provide for rapid deflation in an emergency requiring cardio-pulmonary resuscitation (CPR) in which the patient must be on a firm surface, there is a CPR mode in which the air cells are connected to the input side of the blower for venting to atmosphere. The aim is rapid total deflation, rather than any control of pressure as in the cycling mode.
SUMMARY OF THE INVENTION
The object of the invention is to provide methods and arrangements for the improved relief and prevention of pressure sores, in systems employing alternating-pressure.
The invention is based on the realization that rapid reduction of the interface pressure applied by the support to the patient, during the pressure removal phase in the cycling of the support, and particularly rapid reduction in the region of low interface pressure, provides improvement in control and avoidance of pressure sores.
In a first aspect the invention provides a method of operating an inflatable body support having a plurality of inflatable cells, comprising inflating and deflating the cells cyclically in a predetermined sequence, wherein the cells are deflated in the predetermined cyclical sequence in such a manner that the interior pressure falls from 10 mmHg (135 Pa) to 0 mmHg in a time period o

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