R e s e a r c h A r t i c l e T h e e f f i c a c y o f t h e S i m p l if ie d A ir C u s h i o n ( S A C ) COMPARED TO THE POLYURETHANE FOAM CUSHION WITH REGARD TO ISCHIAL TUBEROSITY INTERFACE PRESSURE RELIEF A B S T R A C T : This stu d y w as in itia ted in an a tte m p t to co m p a re the S im p lified a ir cushion (SAC ) w ith the P olyurethane F oam (P U F oam ) w h e e lc h a ir c u sh io n w ith reg a rd to e ffe c tiv e p r e ssu re r e lie f T he stu d y fo c u s e d esp e cia lly on the w h e e lc h a ir b o u n d sp in a l co rd injured (SC I) p atient. Thirty ulcer-free sp in a l cord injured (SC I) p a tie n ts p a rtic ip a te d in this study. P a tien ts w ere tested on a novice w h e e l­ c h a ir cushion, n a m ely the (SAC ) as w ell as on the hig h -d en sity P U F oam cushion, currently u sed in g o ve rn m en t su b sid ise d hospitals. Isc h ia l tuberosity interface p ressu res were m ea su red u n d er b oth isch ia l tuberosities by m ea n s o f the O xford Pressure M o n ito r M K 11 (O P M M K 11). M a xim u m a verage p ressu res on the S A C w ere 73.60 m m H g a n d 8 2 .9 7 m m H g on the P U F oam cushion. The results su g g est that the S A C p ro vid es a d equate p ressu re r e lie f i f com pared to the P U F oam cushion. D u e to the f a c t that the S A C is d esig n ed on the sam e p rin c ip le s as o th e r a ir cushions, it p ro v id e s a m a xim u m su p p o rt area w hich distributes the w eig h t o f the p a tie n t evenly, thereby low ering p e a k pressures. The S A C also has the a b ility to a llo w a ir circulation a t the body interface, w hich aids in tem perature control. K EYW O RD S: SP IN A L CORD INJURY, P R E SSU R E ULCERS, ISC H IA L T U B E R O SIT Y IN TERFAC E PRESSU RE, SAC. VISSER C, M Phys T (research) (UP) EKSTEEN CA, PhD (UP)' ’ Department of Physiotherapy, University of Pretoria 1 INTRODUCTION P re ssu re u lcers are d o cu m e n te d in Biblical passages and medical literature from as early as 2500 B.C (Jiricka et al 1995). Although there are m ultiple fac­ tors contributing to the form ation of pressure ulcers, many authors agree that the cause of a pressure ulcer is mainly attributed to prolonged pressure over an area o f soft tissue covering a bony prom inence such as the ischial tuberosi­ ties, greater trochanters, sacrum and heels (Kernozek et al 1998). O f all the m entioned areas, the ischial tuberosities and sacrum are the m ost dangerous because o f the potential o f infection o f the pelvic bones. T he w heelchair-bound spinal cord injured (SCI) patient is especially sus­ ceptible to pressure ulcer form ation, due to prolonged pressure in the above- m entioned areas. Kosiak (1961) and Res wick (1976) are two of the pioneer researchers on the role o f prolonged pressure in pressure ulcer formation. These two authors stated that pressure higher than 35 m m H g applied to the body surface results in closure o f the capillary circulation and the developm ent o f a pressure ulcer. D insdale (1973) and later Daniel (1981) inv estig ated the am ount o f pressure needed to cause soft tissue dam age, resulting in pressure ulcers in experi­ mental animals. Daniel (1981) concluded from his study that m uscle dam age occurs at high pressure-short duration (500m m H g, 4 hours) w hereas skin destruction requires high pressure-long d uratio n (800m m H g, 8 hours). T he author also stated that a single pro­ longed episode is not the only cause o f a pressure ulcer b ut that shear forces betw een the supporting surface and the skin and underlying tissue, m oisture on the supporting surface as well as the tem perature betw een the patient and the supporting surface, contribute to pres­ sure ulcer formation. In the latest research on the effect on prolonged pressure on skin, Salcido (1993, 1994) indicated that 45 m m H g is needed to result in closure o f the capil­ lary circulation with necrosis o f the skin and the form ation o f a pressure ulcer. T he wheelchair-cushion perform s a vital role in the prevention o f pressure ul­ cers as one o f the m any devices reducing interface pressure between the patient and the supporting surface. A wide va­ riety o f cushions are currently available on the m arket such as air, foam , water, gel and foam cushions. Som e o f these cushions are very expensive and there­ fore State subsidised institutions are forced to m ake use o f cheaper, and not necessarily the m ost effective pressure relieving cushion. D espite advances in m edicine and research, pressure ulcers rem ain a major cause o f m orbidity and mortality in the SCI patient. During the last decade, South A frica experienced one o f the highest levels o f crim inal violence and m otor vehicle accidents in the world. These factors have generated a significant CORRESPONDENCE: C Visser, CA Eksteen Faculty o f M edicine D epartm ent o f Physiotherapy University o f Pretoria PO Box 667 Pretoria 0001 South Africa Tel (012) 354-2023 Fax (012) 354 1226 SA J o u r n a l o f Ph y s io t h e r a p y 1999 V o l 55 No 4 3 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) sector o f our society w hich is disabled and wheelchair-bound. It is estimated that there are more than five million physical disabled South Africans, m ost o f whom com e from a historically disadvantaged background (Bell 1997). The financial im plication o f recurring pressure ulcers, as well as the high cost o f wheelchair cushions available (exclud­ ing the P olyurethane Foam [PU Foam] cushion) leads to the question of whether it is possible to invent a locally designed and manufactured cushion which is inex­ pensive, easy-to-m ake and operate, and w hich will m atch or exceed the pressure relieving properties o f the Polyurethane Foam cushion. A sim plified air cushion (SAC) was invented to meet this need. The purpose o f this study was to investigate the efficacy o f the SAC, com pared to the PU Foam cushion with regard to the relief o f ischial tuberosity interface pressure. 2 MATERIALS AND METHODS. 2.1 M aterials 2.1.1 Description of the SAC. (Figure 1) The SAC consists o f four em pty vaco- litre-bags (1000 ml.) and a pressure unit. The bags are inflated and arranged in p a­ rallel. The vacolitre-bags are intercon­ nected by a com m on header o f plastic flexible tubing on the nozzle’s side o f the bags, w hich in turn, is connected to the pressure unit. There is a free flow o f air between the airbags w hich causes it to alternately inflate and deflate partially as the patient m oves on them until some state o f equilibrium is reached. The SAC w orks on the same principle as other air cushions namely that by increasing the area o f support and distributing the body w eight more evenly over the seating sur­ face, interface pressure is relieved more effectively. The pressure unit by w hich the vaco- litre bags are inflated, consists o f a m a­ nual pum p, sim ilar to that o f a sphygm o­ manometer, with a unidirectional flow control valve and a small pressure indi­ cator that indicates the am ount o f inter­ nal pressure (inside the cushion) needed to provide effective pressure relief. The unidirectional flow control valve pre­ vents the air escaping from the vacolitre bags once it is inflated. The cushion cover is m ade of a cotton m aterial, divided into 4 separate com ­ partm ents for the vacolitre-bags to fit into. The com partm ents of the cotton cover, acts as pillow cases for the vaco­ litre bags. The cover is not padded. 2.1.2 The Oxford Pressure Monitor M K 11 (OPM M K 11) (Picture 1) Ischial tuberosity interface pressures were measured by means o f the Oxford pres­ sure monitor MK11, which is a measur­ ing device for m ulti-point assessm ent of interface pressure. It consists o f two 3 by 4 cell-matrixes containing 12 cells each. The unit for m easurem ent is mmHg. 2.2 Testing procedure Thirty SCI patients participated in the study. The group consisted o f 27 males and 3 fem ales with a mean age o f 28 years and a range from 19 to 57 years. M ean body w eight was 65,2 kg with a range from 50 to 100 kg. None o f the patients had any pressure ulcers and all o f the patients were diagnosed to have com plete lesions. Patients were tested using both cushions. Each cushion was positioned in a standard w heelchair for the interface pressure to be tested. The patients w ere positioned in a neu­ tral position in the wheelchair. The neutral position is know n as the position where FIGURE 1: DESCRIPTION OF THE SAC. PROTOTYPE ONE (1) SALINE - BAGS PICTURE 1: THE OXFORD PRESSURE MONITOR MK11 (OPM M K11) 4 SA J o u r n a l o f Ph y sio t h e r a p y 1999 V o l 55 No 4 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) o the hips and knees are flexed to 90 with the feet supported on foot plates. The matrixes o f the OPM M K 1 1 were fixed onto the surface o f the cushion being tested, so that their positions rela­ tive to a referred axis would remain con­ stant. Each cell matrix was placed on the cushion in such a way that the ischial tuberosities o f all patients would fall on the back two row o f cells. The testing time for each cushion was as follows: Total testing time fo r the SAC consisted of: • An initial 2-m inute test on each ischial tuberosity, to determ ine the optimal intra-apparatus pressure (pressure in­ side the cushion) where the ischial tuberosity interface pressure would be at its lowest. • A second 2 - minute test during which the maximum ischial tuberosity inter­ face pressure reading was determined. During this test the intra-apparatus pressure is constant, The testing time fo r the PU Foam was as follows: • A 2 - minute test was perform ed to determ ine the ischial tuberosity inter­ face pressure. All these tests were perform ed on both ischial tuberosities o f every patient. The highest ischial tuberosity inter­ face pressure readings, irrespective o f side, were used for analysis. 3. RESULTS T he m ean(S D ) pressures under the ischial tuberosity were 73.60 (14.63) for the SAC and 82.97 (33.12) for the PU Foam cushion. See Table I. The differ­ ence in interface pressure between the SAC and the PU Foam was not statisti­ cally significant (p>0.05). The Wilcoxon matched pairs signed ranks test was used for the statistical analysis. A more exact com parison o f the two sets o f data can be seen in Graph I In this graph each subject num ber corresponds to the num ber allocated to him /her at the beginning o f the study. From this graph it can be seen that, o f the results o f 30 patients, the SAC had higher interface pressures than the PU Foam in only 11 o f the 30 (36.6% ) patients. 4. DISCUSSION The difference in average interface pres­ sure betw een the SAC and PU Foam cushions was not statistically signifi­ cant. This should be very carefully con­ sidered, however, since a problem o f accurately m easuring pressures over bony prominences has been documented in the literature (Patterson et al 1979). The Oxford pressure monitor, however, seems to be a reliable and durable device that provides reasonably accurate results within known limitations. The PU Foam cushion provided less effective pressure relief o f the two cush­ ions tested, with an average pressure of 82.97 mmHg. This result corresponds with the findings o f B ar (1991) who recorded 87,6 m mHg and Koo (1996) who m easured a value o f 76mmHg. From the results in graph 1, the p ro ­ p erties o f Foam are clearly seen. The TABLE 1: SUMMARY OF DATA SAC PU FOAM Average (mean) mmHG 73.60 82.97 Std Deviation 14.63 33.12 Minimum 45.00 40.00 Maximum 98.00 193.00 Median 72.50 77.00 S.E. 2.67 6.05 95% Confidence limits Upper 78.94 95.06 Lower 68.26 7 0.87 Difference 10.68 24.19 GRAPH 1: MAXIMUM AVERAGE INTERFACE PRESSURE VS. PATIENT NUMBER 4- ED E *R o 2