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evaluation of parkhouse elite 5550 mattress replacement system duncan bain phd 28th july 2005 duncan bain consulting 22 gypsy lane kings langley herts wd4 8pr tel 01923 290033

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commercial in confidence 1 introduction alternating pressure air mattresses apamsare designed to prevent or treat pressure ulcers by a different principle from conventional support surfaces conventional pressure reducing support surfaces seek to achieve lower values of maximal interface pressures on the skin by means of even distribution of pressure over the supported area the aim is thus to bring interface pressures down to a continuously tolerable level in contrast to this approach apams are designed to provide cyclic loading to the skin so that each area of skin experiences pressure only intermittently correct functioning of the apam therefore relies on pressures not being evenly distributed pressure differentials between adjacent regions must be created to provide cyclic loading for this reason evaluation of apams cannot adopt the approach commonly used for conventional support surfaces of simply measuring maximum values of interface pressure at a given moment but must characterise the time-varying behaviour performance measures have been proposed1 2 in the scientific literature for quantifying the pressure relief behaviour of apam systems these measures are based on the proportion of the cycle time during which the skin interface pressure at a given location is maintained below a threshold value the threshold value is arbitrary and has variously been set at 10mmhg 20mmhg and 30mmhg it has also been shown 2 that the pressure profile can be improved by adjusting the air pressure according to the body mass of the bed occupant more recently concerns have arisen over the use of apam systems when the backrest of the bed is elevated or in the gatch position where the backrest is elevated and the thigh section is also raised to resist sliding towards the foot of the bed as mentioned previously apams rely for their effective operation on the maintenance of pressure differentials between adjacent areas of skin if pressure differentials are not maintained alternating behaviour is lost and the skin does not experience an off-loaded part of the cycle where reperfusion may take place it the backrest-elevated or gatched positions air cells are often squeezed together potentially causing equalisation of pressure between cells questions have been raised as to which geometries and arrangements of cells will be most susceptible to this problem 2

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commercial in confidence 2 aims the aims of this study are as follows 1 to examine the performance of the device set up according to the prescribed settings 3 subjects of different body weights will be used evaluation will be based on a pressure and time-based performance index to be defined in the methods section and compared .with the huntleigh nimbus 3 which has been shown by randomised controlled trial to be clinically effective in reducing pressure ulcer incidence 2 to examine changes in the alternating behaviour in the profiled position using 3 subjects of different body weights to determine if correct function is maintained in this position 3

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commercial in confidence 3 methods apam performance index api understanding of the aetiology of pressure ulcers is as yet at a very simplistic stage gross assumptions have therefore been made in deriving a performance index and it must be noted that performance measured in this way can not necessarily be extrapolated to clinical outcome however randomised controlled trials to examine clinical outcome are almost prohibitively large undertakings for this category of product and therefore simple efficacy measures are appropriate as long as the limitations of the study are understood the assumptions made in using this performance index are as follows 1 it is unimportant how high the interface pressure is on an area of tissue during the loaded part of the cycle we assume for these purposes that occlusion to blood is total while loaded and that higher loading will not produce greater occlusion during the unloaded part of the cycle longer duration at lower pressure is better no attempt is made to accommodate second-order effects such as reperfusion injury in the index the performance of the system as a whole is determined by that region on the pressure map showing the worst performance throughout the cycle 2 3 4 considering a hypothetical loading cycle on a single body location as shown in figure 1 interface pressure hypothetically measured in mmhg is plotted against time in minutes giving an idealised sinusoidal waveform we can see from this graph the cycle time of the pressure profile as illustrated 3 threshold levels 10mmhg 20mmhg and 30mmh are shown on the graph and we can see the regions of the loading cycle where the pressure falls below these thresholds one option for creating an index of performance is to cite the time duration during which the pressure is measured to be lower than a particular threshold eg 30mmhg however this approach fails to identify benefits of dropping far below the threshold value as opposed to dropping just below the threshold value for example the profile shown in figure 2 having a similar duration below the threshold would be seen to perform as well as that in figure 1 which falls well below the threshold for much of that duration one means of addressing this shortcoming is to cite durations at several different thresholds and summating them or weighting them to give a compound value thus values would be cited for time below 30mmhg time below 20mmhg and time below 10mmhg 4

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commercial in confidence alternatively a compound value may be calculated by taking the area of the loading cycle beneath the threshold value shown as the shaded area a30 in the figures this area takes into account both the duration width of the shape and the degree of pressure reduction below the threshold height of the shape these are expressed in mmhg x minutes may then be divided by the cycle time to give a value of pressure relief below threshold expressed in mmhg so we define api api mmhg p=30 pressure ­30 dt p=30 tc 140 cycle time tc 120 interface pressure/mmhg 100 80 60 40 20 0 0 10 20 30 40 50 60 a30 pressure threshold 10 threshold20 threshold 30 70 80 time/mins figure 1 hypothetical loading cycle at a single body location 5

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commercial in confidence 140 cycle time tc 120 interface pressure/mmhg 100 80 60 40 20 0 0 10 20 30 40 50 60 pressure threshold 10 threshold20 threshold 30 a30 70 80 time/mins figure 2 alternative hypothetical loading cycle at same body location instrumentation an xsensor pressure mapping array was used for this study on the basis that it is relatively flexible compared to most pressure mapping systems one concern with pressure mappers is that the presence of the mat in the system will introduce mechanical artefacts to the system being measured and these concerns are mitigated somewhat by the flexibility of the mat subjects 3 healthy experimental subjects were chosen each giving informed consent table 1 subject group anthropometrics id code id01 id02 id03 age 39 63 34 height 1.80m 1.68m 1.58m weight 74 kg 95kg 52kg procedure comparison with nimbus 3 1 2 the apam was set up on the bed as with a flat sheet and nominally inflated the xsensor calibrated daily was placed on the mattress 6

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commercial in confidence 3 4 5 6 7 8 9 the subject lay supine on the mattress with the xsensor array aligned with the pelvic area showing the lower back to the upper thigh pressure was set as instructed by the manufacturer according to body weight after 5 equilibration cycles pressure was mapped over 3 pressure cycles average api was calculated over the 3 cycles for every point on the map the point with the lowest value of api averaged over 3 cycles was recorded this was repeated 3 times to give a median and range value for each subject this was repeated for all 3 subjects procedure examination of alternating function with elevated backrest 1 using the inflation pressures as specified each subject was pressure mapped with the bed profiled so that the backrest was at 45 degrees and the thigh section at approximately 15 degrees api was calculated in the new configuration 2 7

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commercial in confidence 4 results figure 3a figure 3b 8

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commercial in confidence figure 3c figure 3d figure 3 a-d example snapshots during cycle elite 5550 figure 3 shows 4 snapshots of the pressure distribution at various times throughout the cycle with the bed in the flat position it can be seen that the loci of highest 9

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commercial in confidence pressures do move throughout the cycle starting at 3a the highest pressure region is in the region of row 20 by mid-cycle 3c this region has separated into two high regions with high points at columns 16 and 34 respectively a largely off-loaded area now appears at column row 20 accordingly a trace of a similar nature to those shown in figures 1 2 could be drawn for a single sensor cell in column 20 and the api for that cell calculated similarly api can be calculated for any cell on the map figure 4 pressure profile at sample points elite 5550 74kg subject an example pressure vs time trace taken at various points on the map is shown in figure 4 each different coloured line represents a different point on the surface of the mattress for each different coloured line api can be calculated as the integral of pressure.time below 30mmhg note that the pink and red traces will give a higher less effective value of api than the blue or green traces in this evaluation the value for the green trace least effective would give the api recorded for this case note that in the sensor locations shown pressure at no time approaches zero,or total pressure relief 10

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commercial in confidence the nimbus 3 mattress was set up with pressure settings according to the following rules subject 1 subject 2 subject 3 soft-medium medium-hard soft the same procedure was followed to map interface pressure throughout the cycle and for each subject api was calculated according to the same formula figure 5 pressure profile at sample points nimbus 3 74kg subject referring to figure 5 it can be seen that the nimbus 3 gives a relatively smooth pressure profile with sensor cells falling into 2 easily recognisable groups on opposite cycles in the case of this subject although total pressure relief was observed by some of the cells in one part of the cycle red and pink traces those cells that were inflated in that part of the cycle do not fully deflate in the opposite part of the cycle blue and green traces this means that for at least some body regions total pressure relief does not occur the effectiveness of this product clinically has been demonstrated 11

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commercial in confidence convincingly in randomised controlled trials this suggests that total relief is not a prerequisite for effectiveness figure 6 gives an example of the incompletely deflated cells between inflated cells in mid-cycle figure 6 nimbus 3 74kg subject mid cycle 12

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commercial in confidence api performance compared for nimbus and airflex 4 3 nimbus api elite 5550 2 1 0 40 50 60 70 body weight/kg 80 90 100 figure 7 api performances compared for 3 subjects figure 7 shows the performance of the nimbus 3 compared to that of the elite for all subjects in this graph for each subject the api value given for the nimbus 3 appears in red showing the median value with whiskers to indicate range the elite 5550 is shown in blue again with median and range marked no statistically significant difference is found using non-parametric tests and it can be seen that there is considerable overlap it must be stressed that for such a small number of subjects only gross differences would be likely to be demonstrated with statistical significance this reflects problems with reproducibility that are inherent in pressure measurements on live human subjects 13

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commercial in confidence results examination of alternating function with elevated backrest figure 8 elite 5550 74kg subject elevated backrest and gatch mid-cycle figure 9 elite 5550 74kg subject elevated backrest and gatch opposite cycle 14

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commercial in confidence on both apam systems the api value was zero for all three subjects in the profiled position example pressure maps showing different stages of the inflation cycle are shown referring to figures 8 and 9 it can be seen that the pressurised regions are brought much closer together and cause some encroachment of the deflated areas this encroachment was sufficient to give zero api values in all cases at the predicted pressure setting however it can be seen that although alternating behaviour is greatly impaired by the squeezing together of sacks some pressure differential is maintained between inflated and deflated regions so some alternating behaviour is maintained it may be that the 30mmhg threshold is unreasonable for the sitting position since more of the body weight is being applied to the pelvis figure 10 nimbus 3 74kg subject elevated backrest and gatch mid cycle 15

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