Patient supports and methods of operating them

In operation of a support for a patient's body, used in medical or veterinary treatment, which applies alternating-pressure to the body in order to reduce or minimize the risk of pressure sores caused by prolonged pressure on the skin, inflatable cells of the support are inflated and deflated cyclically in a predetermined sequence. To provide improved effect in relieving or preventing pressure sores, the cells are deflated in the sequence in such a manner that the interior pressure falls from 10 mmHg (135 Pa) to 0 mmHg in a time period of not more than 15 s. Preferably the interior pressure falls to below 0 mmHg (ambient atmospheric pressure). A vacuum pump or pumps may be employed to achieve this result.

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 
.English Pound.250 million per year..sup.(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.sup.(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.sup.(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.sup.(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.sup.(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.sup.(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.sup.(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.sup.(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.sup.(6) then interface pressures of 1.6-2.6 mmHg are necessary 
to relieve the ischaemic tissue. Kosiak summed this up in 1961.sup.(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 of not more than 15s, preferably not more than 10 s. 
In this application cell internal pressures are expressed relative to 
ambient atmospheric pressure (0 mmHg). 
Furthermore, the invention provides a method in which in the predetermined 
cyclical sequence the cells are deflated in such a manner that the 
interior pressure falls from 20 mmHg (270 Pa) to 0 mmHg in not more than 
30s, more preferably in not more than 20s. 
In order to obtain the desired pressure-reduction curve, preferably the 
cells are deflated in the cyclical sequence to a pressure which is less 
than ambient atmospheric pressure. In this case, the lowest interior 
pressure of the cells in said cyclical sequence is preferably in the range 
0 mmHg to 10 mmHg (135 Pa) (more preferably 0 mmHg to 5 mmHg) below 
ambient atmospheric pressure, in order that the amount of air needed to 
re-inflate each cell is minimized. 
Although any suitable method may be employed to provide the desired 
pressure-reduction curve, preferably the cells are deflated in the 
cyclical sequence by pumping gas from them by means of at least one vacuum 
pump. 
The invention therefore defines in various ways the lower end of the 
pressure-reduction curve of the cell interior pressure during the normal 
alternating-pressure cycle. This concept applies to each cell, and 
depending on the exact nature of the device it is not necessary that a 
plurality of cells are deflated simultaneously. Preferably the 
pressure-reduction rates specified by this invention apply to all 
alternating-pressure cells of the support. 
For convenience of construction and operation, it is preferable that the 
cells are arranged in a plurality of groups, each group containing at 
least one cell and usually a plurality of cells, the cells of each group 
being inflated and deflated together in the cycle out of phase with the 
cycle of the cells of the or each other group. In a mattress for example 
the cells may be transverse tubes, and there are typically two or three 
groups of cells with horizontally adjacent cells belonging to different 
groups. In a chair seat, there may be for example four tubes extending in 
the front-to-back direction and arranged in two groups. 
The invention can further be defined by reference to the interface pressure 
applied to the patient's skin by a support device. In this respect the 
invention provides a method of operating an inflatable body support having 
a patient at least partly supported thereon, which support has a plurality 
of height-displaceable elements which support the patient and are arranged 
in groups each group comprising at least one said element, comprising 
causing the groups of elements to undergo cyclic raising and lowering in a 
predetermined sequence so that the groups sequentially support the 
patient, wherein during the lowering of the elements in the sequence the 
elements are operated in a manner such that interface pressure exerted 
between at least some of the elements and the patient falls from 20 mmHg 
(270 Pa) to 5 mmHg (68 Pa) in not more than 15 s, preferably in not more 
than 10 s. Preferably the interface pressure is reduced to 0 mmHg by the 
lowering of said elements. 
In use of an alternating-pressure support, not all of the support elements 
may be supporting the patient, and some elements may provide only light 
support. The concept of the invention, of rapid interface 
pressure-reduction applies particularly to those support elements applying 
significant interface pressure, e.g. at least 40 mmHg when raised. 
In this aspect the invention is not limited to use of inflatable cells, and 
other arrangements of height-displaceable elements which have been 
proposed in the past. Preferably however, the elements are upper portions 
of inflatable cells of flexible material. 
In the past, it has been thought undesirable to interpose a cover sheet 
between the patient and the alternating-pressure device, because of the 
fear of "bridging" of the sheet between adjacent elements of the device 
which might prevent removal of interface pressure. However with the rapid 
pressure relief of the present invention, this risk is reduced, and 
therefore at least one sheet of flexible material may be present between 
the patient and the height-displaceable elements. 
A device which applies sufficient suction to the inflatable cells of an 
alternating-pressure device can provide the desired rapid 
pressure-reduction. In another aspect therefore, 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 of each cell falls to below 0 mmHg (ambient atmospheric 
pressure). As explained above, it is preferable that the lowest interior 
pressure of the cell in the cyclical sequence is in the range 0 mmHg to 10 
mmHg (135 Pa) below ambient atmospheric pressure, more preferably in the 
range 0 mmHg to 5 mmHg below ambient atmospheric pressure. 
The invention further provides apparatuses for carrying out the methods 
described above. 
In one aspect, the invention provides an inflatable body support having 
a plurality of inflatable cells, 
inflation means for inflating the cells, 
suction means for deflating the cells, 
control means for causing the cells to be connected to the inflation means 
and the suction means cyclically in a predetermined cyclical sequence so 
that the cells are inflated and deflated, 
the suction means being adapted to establish a pressure lower than ambient 
atmospheric pressure in the cells, and the control means connecting the 
suction means to the cells for a sufficient time in the predetermined 
cyclical sequence that a pressure lower than ambient atmospheric pressure 
is established in the cells. 
Preferably there is at least one sensor arranged to sense suction pressure 
applied to the cells by the suction means, the control means operating to 
stop application of suction to the cells when a predetermined minimum 
suction pressure is sensed by the sensor. 
The invention also provides an inflatable body support having 
a plurality of inflatable cells, 
inflation means for inflating the cells, 
suction means for deflating the cells, 
control means for causing the cells to be connected to the inflation means 
and the suction means cyclically in a predetermined cyclical sequence so 
that the cells are inflated and deflated, 
the suction means being adapted to reduce pressure in said cells when 
connected thereto in the predetermined cyclical sequence at a rate such 
that the interior pressure in the cells falls from 10 mmHg (135 Pa) to 0 
mmHg in not more than 15s, preferably not more than 10 s. 
In yet another aspect the invention provides an inflatable body support 
having 
a plurality of inflatable cells, 
inflation means for inflating the cells, 
suction means for deflating the cells, 
control means for causing the cells to be connected to the inflation means 
and the suction means cyclically in a predetermined cyclical sequence so 
that the cells are inflated and deflated, 
the suction means being adapted to reduce pressure in said cells when 
connected thereto in the predetermined cyclical sequence at a rate such 
that the interior pressure in the cells falls from 20 mmHg (270 Pa) to 0 
mmHg in a time period of not more than 30 s, preferably not more than 20 
s. 
Preferably the inflation means comprises at least one air compressor and 
the suction means comprises at least one air pump, the air compressor and 
the air pump being independent of each other, e.g. independently 
controlled and unaffected by each other's operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In arriving at the present invention, the inventor noted some studies, 
which were carried out in connection with pressure sores and some of which 
were performed on healthy individuals, showing that following a period of 
circulatory occlusion there is a period of reactive hyperaemia which has 
the effect of increasing the rate of blood flow to the effected tissues as 
pressure is relieved. The maximum flow occurs immediately after the 
occlusion is released, the larger the occlusion the greater being the 
hyperaemia that follows. Lewis and Grant.sup.(9) showed a close 
relationship between the "debt" accrued during occlusion and the 
"repayment" during the hyperaemia phase. Blair et al.sup.(10) showed that 
if pressure was relieved slowly little or no "debt" was repaid. 
The reason that the rate of fall of interface pressure reduces in prior art 
systems such as the Airwave is that the air is pushed out of the cells by 
patient weight and then the pressure-reduction merely continues by an 
equalisation process to atmospheric pressure dependent on the internal 
flow characteristics of the mattress. The time that patient weight ceases 
to be a factor is determined by the time when the adjacent inflated cells 
provide support for the patient on either side of the deflating cell. 
The level of interface pressure which will occlude the micro-circulation in 
a healthy individual is likely to be at least 30 mmHg, but in a typical 
patient at risk of pressure sores this is often 10-15 mmHg whilst for 
patients with existing sores and at very high risk this is likely to be 
less than 5 mmHg. At these very low interface pressures the rate of 
reduction of pressure is low, and therefore the stimulation of the 
microcirculation is greatly reduced if present at all. 
The apparatus shown in FIG. 1 achieves rapid removal of interface pressure 
at the lower end of the pressure curve. This is achieved by connecting the 
deflating cell or cells to pressure which is below atmospheric pressure, 
by the use of one or more air pumps which actively provide such 
sub-atmospheric pressure. However within the invention other suitable 
means of providing suction, such as a vacuum reservoir, may be employed. 
In the diagram of FIG. 1, air lines are shown by bold lines, and the light 
lines indicate control functions. The control system shown is connected by 
four air lines A, B, C, H seen at the left hand side, to an inflatable 
mattress of the standard Pegasus Airwave type, which is substantially as 
shown in GB-A-1 595 417, having a plurality of tubes extending 
transversely across the mattress and arranged in two layers, with each 
tube in the upper layer being supported directly above a tube of the lower 
layer by side formers. In the Airwave mattress, the side formers are also 
inflatable elements, and additionally there are inflatable head cells of 
the mattress. The side formers and head cells are kept permanently 
inflated, during normal operation of the device, by connection to the line 
H. The tubes are 10 cm (4 inches) in diameter. 
The transverse alternating-pressure tubes in the mattress are divided into 
three groups or arrays, which are respectively connected to the lines A, B 
and C. Each of these arrays is cyclically inflated and deflated, in a 
cycle which includes a period in which the tubes of the array are 
maintained fully inflated and a period in which they are deflated. The 
total cycle duration is 8 minutes. The cycles of the three arrays are out 
of phase, so that at any time a patient lying on the mattress is supported 
by two of the arrays which are fully inflated or nearly so, while the 
third array is deflated so as to withdraw pressure from parts of the 
patient's body. Each tube of the upper layer is in the same array or group 
as the tube below it in the lower layer, so that these two tubes are 
inflated and deflated simultaneously. 
The air lines A, B, C, H are connected by a connector device 1 to five air 
lines 2, 3, 4, 5, 6. This connector device 1 is shown and described fully 
in our co-pending UK Patent Application No. 9616769.7, to which reference 
should be made. It is disconnectable into two parts, to allow the mattress 
with the air lines A, B, C, H to be removed from the control system. 
Through relative rotation of two portions of one of these parts, the 
operator can select one of three functional positions of the connector 1. 
In a first position, the connector can be separated into its two parts, 
and in this position, the lines A, B, C, H are all closed at the 
connector, so that the mattress can be removed without deflation. In the 
other two positions, the connector cannot be separated into its two parts. 
In a first one of these positions, normal operation with cycling of the 
cells through their predetermined sequences takes place, the lines A, B, 
C, H being directly connected through the connector 1 to the respectively 
lines 3, 4, 5, 6. In the third position, known as the CPR position 
(cardio-pulmonary resuscitation position), all four of the lines A, B, C, 
H are connected both to a direct vent to atmosphere through the connector 
1, this venting route having a one-way valve, and also to the suction line 
2 which leads by a manifold 7 to two air pumps 8, 9 to be described later. 
In this embodiment, the connector 1, unlike the connector shown in our UK 
Patent Application No. 9616769.7 has two optical sensors 10, 11, which 
detect which of the three positions it is in, and provide output signals 
so that the connector position can be displayed visually on the display 12 
of the device, under control of the electronic control unit (ECU) 13. 
The air lines 3, 4, 5, 6 are connected to ports of a rotary valve 14 which 
contains a stator and a rotor, the rotor being driven by a motor 15 which 
is controlled by the ECU 13. This rotary valve 14 also has ports connected 
to a fill line 16 and an exhaust line 17. The stator and rotor contain 
internal air passages connected to all of these ports, which are connected 
and disconnected to each other by the continuous rotation of the rotor in 
order to provide the desired control of the inflation and deflation of the 
cells of the mattress. The fill line 16 is connected at all times during 
normal operation of the mattress to the air line 6, so that the side 
formers and head cells connected to the line H are maintained permanently 
inflated during normal cycling operation. The fill line 16 is connected 
for predetermined periods in the cycling sequence to the lines 3, 4, 5 so 
that the respective arrays of cells connected to the lines A, B, C are 
inflated and maintained inflated for the desired periods. To cause 
deflation of each of the tube arrays in turn, the rotary valve 14 connects 
the lines 3, 4, 5 to the exhaust line 17. 
The compressed air for the filling of the mattress is provided by two fill 
compressors 18, 19 which are also controlled by the ECU 13, and which in 
this embodiment are operated in tandem, i.e. both are on together or both 
off together. Their output lines 20, 21 are connected by a silencing and 
buffer chamber 22 and line 23 to a manifold 24 which has an output 
connected to the fill line 16. The manifold 24 also has an overpressure 
release safety valve 25 which opens to release air to the atmosphere at a 
predetermined overpressure, higher than the normal operating pressure of 
the tubes of the mattress. Also connected to the manifold are a low 
pressure sensor 26 and a high pressure sensor 27, which provide outputs to 
the ECU 13. Sensor 26 operates when the pressure drops below a 
predetermined value and the sensor 27 when the pressure reaches a higher 
predetermined value. The ECU 13 controls the operation of the compressors 
18, 19 to maintain the pressure in the manifold 24 between these two 
values. 
Connected to the rotary valve 24 is an overpressure sensor 28, which senses 
the pressure in the cell group or groups which are in the inflated phase. 
In this embodiment this operates at a predetermined pressure higher than 
that of the sensor 27, to provide an output signal when the pressure 
exceeds this level. On detection of this output signal, the ECU 13 gives a 
visual indication on display 12 that the mattress system is adjusting to 
the patient's weight. Overpressure may occur in the tubes of the mattress, 
when a patient is placed on the previously inflated mattress. 
As FIG. 1 indicates, the ECU 13 has a mains power input 29, and is 
connected to the display 12 to indicate the operational state and provide 
other useful visual signals, and may optionally also be connectable to a 
remote control 30, for example by a cable or by infrared signalling. The 
ECU 13 contains a microprocessor, programmed to perform the desired 
control functions. The design and operation of the ECU 13 is conventional 
for one skilled in the art and need not be described here. 
In the conventional Pegasus Airwave system, marketed hitherto, the arrays 
of tubes of the mattress have been vented to atmosphere by the rotary 
valve corresponding to the rotary valve 14 of FIG. 1, in order to deflate 
them in the normal cycling mode. As FIG. 1 shows, in this embodiment of 
the present invention, the exhaust line 17 is connected to the manifold 7, 
which itself is connected by two vacuum lines 31, 32 to the respective air 
pumps 8, 9 which when operating provide a sub-atmospheric pressure in the 
manifold 7. The outputs from the pumps 8, 9 pass through a silencing 
chamber 33 to atmosphere. These two pumps 8, 9 also operate in tandem, 
under control of the ECU 13. In the manifold 7 there is a chamber 
connecting both lines 31, 32 to the two lines 2, 17. 
During the normal cycling operation of the arrays of tubes of the mattress, 
with a patient on the mattress, the lines A, B, C are connected via the 
connector 1 and the rotary valve 14 in turn to the exhaust line 17, for 
the sequential deflation of the respective tube arrays. The passage of air 
from the deflating cells to the atmosphere occurs as a result of the 
initial overpressure in the cells relative to atmosphere by the suction or 
vacuum extraction caused by the operation of the compressors 8, 9. The 
characteristic pressure-reduction curves are shown by FIGS. 2 and 4, and 
are discussed more below. 
In order that the pumps 8, 9 do not extract excessive air from the deflated 
tubes, which air would need to be replaced on re-inflation of the tubes in 
the next stage of the cycle with extra energy consumption, the manifold 7 
is connected to a vacuum sensor 34 which provides an output signal to the 
ECU 13 when it senses that a predetermined pressure below atmospheric 
pressure is reached in the manifold 7. The ECU 13 then switches off the 
pumps 8, 9. Of course, the pressure in the manifold 7 is not identical to 
the pressure in the tube array being deflated, but it has been found 
possible by trial and error to set a suitable switching level of the 
compressors 8, 9 so that extraction of air from the tubes stops at a level 
of pressure within the tubes of the mattress which is significantly below 
atmospheric pressure but not more than 5 mmHg below atmospheric pressure. 
The fill compressors 18, 19 and the air pumps 8, 9 are small linear motor 
reciprocating compressors or pumps, and may all be identical. Preferably 
each pair is mounted on a support base so that their moving pistons 
reciprocate 180.degree. out of phase, minimizing vibration. Suitable 
compressors are those shown in WO 94/28306, WO 94/28308 and WO 96/18037. 
These compressors have valves which seal the air passages when the 
compressors are not operating, so that there is no loss of air through the 
compressors 18, 19 when they are not operating, and no back leakage of air 
from atmosphere through the pumps 8, 9 when they are not operating. In the 
event of-power failure, therefore, the mattress remains as it is, i.e. 
deflation is prevented. 
The mattress has air conduits extending longitudinally along it, and 
connected to the tubes of the respective tube arrays. In the present 
embodiment, the air lines A, B, C are connected to these longitudinal air 
conduits at the middle region of the mattress, so that the tubes at the 
centre of the mattress tend to be inflated and deflated before the tubes 
at the respective ends of the mattress. In an alternative possible 
arrangement, the lines A, B, C are connected to these longitudinal 
conduits at one end of the mattress. A patient lying on the mattress may 
experience slightly different sensations with these two arrangements, as 
each array inflates and deflates. 
In the CPR mode of the connector 1, all three arrays of tubes and the side 
formers and head cells are rapidly deflated, both by venting to atmosphere 
through the direct outlet path through the connector 1, for as long as 
there is sufficient pressure in the lines A, B, C, and also by the pumps 
8, 9 via the line 2 and the manifold 7. When the CPR mode is detected by 
the optical sensors 10, 11, the ECU maintains the compressors 8, 9 in 
operation irrespective of the pressure in the manifold 7. This provides a 
more rapid complete deflation than is obtained by merely venting the tubes 
directly to atmosphere. Saving a few seconds of time is of great 
importance when the emergency CPR mode is required. 
FIGS. 2 and 3 respectively show cell (tube) internal pressure curves 
obtained experimentally for the embodiment of the invention described 
above in which the standard Pegasus Airwave mattress is operated by the 
control system shown in FIG. 1, and for the standard Pegasus Airwave 
mattress in which the arrays of mattress tubes are vented to atmosphere 
only by the rotary valve during the normal cycling operation of the tubes 
of the mattress. The pressures within the mattress tubes were measured by 
attaching a conventional pressure-sensing device to the respective tubes. 
A standardised dummy patient weighing 83 kg was lying on the mattress. 
FIG. 3 shows the cycling of the three tube arrays, identified here as A, B 
and C, respectively connected to the air lines A, B and C, and also the 
continuously maintained high pressure of the head cells and side formers 
attached to the line H. Each tube array is maintained inflated for a time 
period which is about twice as long as its deflation phase. When each 
deflation phase starts, the pressure drops rapidly, due to the weight of 
the patient, but the pressure drop rate decreases significantly below 10 
mmHg, and 0 mmHg is only slowly approached. The sensitivity of measurement 
does not allow detection of whether or not a true pressure of 0 mmHg was 
actually achieved, but it is clearly impossible in such a system for a 
pressure lower than 0 mmHg to be obtained. 
The pressure curves of FIG. 2 show that, on initiation of deflation of each 
cell array, there is initially a rapid pressure fall, similar to that of 
FIG. 3, but that this relatively rapid fall continues with only a slight 
rate reduction until 0 mmHg is obtained, and that a sub-atmospheric 
pressure is maintained within the tubes for a significant period of time. 
More precise measurements have shown that in the curves of FIG. 2, the 
internal pressure of the cells drops from 20 mmHg to 0 mmHg in about 15 
seconds, and drops from 10 mmhg to 0 mmHg in much less than 10 seconds. 
FIGS. 4 and 5 show interface pressures between the mattress and a human 
patient lying on it, plotted against time, for a mattress using the 
control system of FIG. 1 and for the standard Pegasus Airwave system. 
These pressure curves have been measured using a Numotech pressure-mapping 
device, made by Jasco Products Inc. of Sun Valley, Calif., USA. This 
device is a thin sheet containing a very large number of pressure sensors 
which are arranged in a rectangular array and are interrogated by data 
processing techniques to provide a pressure map. FIGS. 4 and 5 show the 
deflation curve only. FIG. 4 shows that with the Airwave mattress 
connected to the control device of FIG. 1, interface pressures of 0 mmhg 
are achieved and that, where the patient has, during the inflated phase of 
a tube array, an interface pressure of above 20 mmHg, for example 50 mmHg, 
in the deflation phase the interface pressure falls from 20 mmHg to 5 mmHg 
in less than 10 seconds. In FIG. 5 by contrast even after 1 minute, zero 
interface pressure is not obtained, and the pressure fall rate below 20 
mmHg is reduced. Below 10 mmHg it is slow. 
In a conventional use of an alternating-pressure cells mattress such as the 
Pegasus Airwave mattress, it is normal to avoid use of a cover sheet over 
the mattress, because of the fear that "bridging" of the cover sheet 
between two inflated cells, may occur when the cell between them is 
deflated, so that the cover sheet might maintain pressure on the patients 
skin even during the deflation phase of the cell. With the positive 
driving of the cell pressure to below atmospheric pressure in the device 
of the present invention, it has been found that this risk in use of a 
cover sheet is avoided or minimised, so that a cover sheet, of suitable 
flexibility and preferably extensibility, can be employed. Use of a cover 
sheet is advantageous, for reasons of hygiene and also for improvement of 
the appearance of the mattress to the patient. 
Preliminary clinical evidence indicates that the rapid removal of pressure 
provided by the present invention, gives significant benefits in the 
prevention and treatment of pressure sores. As discussed above, there 
appears to be a "pressure-induced debt" in the blood flow of patients 
whose circulation is occluded at low interface pressure levels. To achieve 
"repayment" of this debt, advantage can be taken of the reactive 
hyperaemia effect, by rapid removal of interface pressure at the low 
levels at which occlusion is taking place as a result of the rapid 
reduction of cell pressure particularly in the range from 10 mmHg to 0 
mmHg. By providing positive air extraction, using suction pressure, the 
rate of removal of interface pressure is maintained even when the body 
weight of a patient no longer forces the air out of the deflating cells of 
the mattress. It is believed that improved reactive hyperaemia is 
obtained. It is possible to achieve a reduction of interface pressure at 
the rate of 5 mmHg/s from the maximum pressure (inflated pressure of the 
cells) to the level of 10 mmHg, reducing to 2.5 mmHg/s between 10 and 5 
mmHg and then reducing to 0.5 mmHg/s below 5 mmHg, i.e. a time of about 6 
s from 20 mmHg to 5 mmHg, and a time of about 12 s between 10 mmHg and 0 
mmHg. The overall fall from interface pressure at full inflation of the 
tubes to 0 takes place in less than 20 s. This provides stimulation of the 
micro-circulation of the patient, even at very low interface pressures, 
which it is believed was not possible with a system relying on patient 
weight to force the air out of the deflating cells. It is possible also 
that there is a benefit in improved lymphatic flow. 
The control system of FIG. 1, in which the deflation means (pumps 8, 9) are 
controllable independently of the inflation means (compressors 18, 19) 
allows two further useful modes of operation of the mattress system. 
On initial inflation of the mattress, in preparation for its use, all of 
the mattress cells (tubes) being at first deflated, the control unit (ECU 
13) operates the compressors 18, 19 and the rotary valve 14 but suppresses 
operation of the pumps 8, 9. After all cells have become inflated, by 
their connection via the rotary valve 14 to the compressors 18, 19, the 
control means 13 switches itself to the normal cycling mode in which the 
pumps 8, 9 operate to deflate each group of cells in turn. In this way, 
the mattress can be made ready for use as quickly as possible, since no 
air loss occurs during this initiation mode. 
During normal cycling operation of the mattress, an operator can select a 
"static mode" by pressing a control button on the ECU 13. This is done 
when it is desired that the normal cycling stops but the mattress remains 
inflated, which is convenient for certain aspects of patient care. When 
this "static model" is selected, the ECU 13 continues operation of the 
compressors 18, 19 and the rotary valve 14 but stops operation of the 
extraction pumps 8, 9. Consequently any uninflated cells become inflated 
but no cells are deflated, and the mattress soon becomes fully inflated 
and remains so. For patient safety, the ECU 13 is programmed to permit 
this "static mode" to continue for at most a predetermined period, in this 
embodiment 30 minutes. After 25 minutes an audible warning is given by the 
ECU 13. The operator is permitted to start the "static model" again for 
another period of at most 30 minutes, but the ECU 13 thereafter reverts 
automatically to the normal cycling mode so that the total duration of 
"static mode" is one hour. The ECU 13 prevents re-selection of "static 
mode" for one further hour following its cessation. At any time, the 
operator may exit from "static mode" into the normal cycling mode, by 
pressing the normal operation command button on the ECU 13. 
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