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Observations on Mattress Covers: Results of a Pilot Study
Journal of Tissue Viability 1998 Vol 8 No 1
5
OBSERVATIONS ON MATTRESS COVERS: RESULTS OF A PILOT STUDY STEPHEN THOMAS Director, Surgical Materials Testing Laboratory, Bridgend 1. Summary Samples of covers from three commercially available mattresses were examined in the laboratory using test methods originally devised for testing surgical dressings. These revealed that although the covers shared many common features, there were differences in the conformability and tensile properties which may be of some clinical relevance. The study also confirmed that with some minor modifications, the experimental techniques used would be suitable for a future, more comprehensive review of mattress performance. In a separate investigation designed to examine the consequences of a failure of a mattress cover, the bioburden of a foam core removed from a damaged cover revealed the presence of very large numbers of microorganism, well in excess of 10 10 per gram of foam which could act as a recevoir of contamination and thus a source of cross infection.
The aim of this preliminary study was to determine the suitability of these systems for this present application before embarking upon a more detailed and comprehensive programme of comparative testing of the mattresses and mattress covers in current use in the UK. For this reason the three mattresses examined have not been identified by name, although a brief description of the structure of each is provided. a microbiological investigation was undertaken on a different mattress with a damaged cover that had been retrieved from a surgical ward in a hospital elsewhere in the United Kingdom. The aim of this simple study was to identify and quantify the number of microorganisms in the foam using standard laboratory techniques.
In a separate but related study,
3. Materials, Methods and Results 2. Introduction Many articles have been published in the medical and scientific press describing the factors that lead to the development of pressure damage. Most of these publications also make reference to the important role of specialised beds and mattresses and other pressure relieving aids in the prevention of this condition.
3.1 Mattress covers examined Mattress cover 'A' consists of a weft knitted polyester fabric coated with polyurethane
Despite the rapid growth in the use of alternating pressure relieving devices, many patients who are at risk of developing pressure damage are still nursed upon conventional mattresses consisting essentially of a block of foam enclosed in a waterproof plastic sleeve.
Mattress cover 'C' is composed of two different fabrics. The upper and lower surfaces consist of a knitted polyester fabric coated with polyurethane. The edges are made from a microporous fabric that is designed to permit the passage of water vapour but prevent penetration by bacteria.
In a recent review, Rithalia 1 made particular reference to the important contribution made to the performance of a foam mattress by the properties of the cover. He suggested that this should form an effective bacterial barrier, be water vapour permeable and possess adequate elastic properties to prevent the hammocking effect caused by a tightly fitting inelastic cover. He concluded that 'very little work has been carried out on these aspects of support systems and reliable information is scarce. Recognising this problem within our laboratory, we have recently conducted a pilot study in which we compared the key aspects of the performance of three mattress covers using test systems previously used for evaluating wound management materials.
Mattress cover 'B' consists of a weft knitted nylon fabric coated with polyurethane
3.2 Moisture Vapour Transmission Rate (MVTR) It is generally accepted that the accumulation of moisture between a mattress and the patient's skin can cause discomfort and may contribute to maceration leading in tum to tissue damage and infection. A cover which is permeable to water vapour but impermeable to liquids and bacteria should help to alleviate this problem.
The moisture vapour transmission rate of the three covers was determined using a test method that has been described previously for examining the performance of film dressings and hydrocolloids 2 . In this test, a section of the cover is applied to a Paddington
6
Journal of Tissue Viability 1998 Vol 8 No 1
cup (a modified Payne cup) the opening of which has a cross sectional area of 10 cm 2• To the cup is added 20 ml of a standard test solution consisting of sodium and calcium chloride containing 142mmol of Na+ ions and 2.5mmol of Ca2+ ions, values which are typical of those found in human serum. The cup is placed with the solution in contact with the cover in 'iln incubator at 37T upon the pan of a top loading balance.
Figure 1. Moisture vapour transmission through cover of Mattress A.
§ Cl
'O
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c: 'iii .c .£
"'
Q)
The results are summarised in table 1 and expressed graphically in figures 1 to 4.
-0.4 -0.5
Cl
c:
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;;: ()
-0.6 -0.7
At the end of the test the recorded data are down-loaded for analysis. Initially three samples of each cover were examined by this method but because of unexpected variability in the results obtained with Mattress C, three additional tests were performed with this sample.
-0.3
Q)
u
The balance is connected to an electronic data logging device which records over a 24 hour period, any changes in the weight of the cup caused by the loss of moisture vapour through the cover.
-0.2
c:
-0.8
:---~~~~~~~~~'o-'-~-'--'---'-~_l_-'----'~~
0
2
4
6
8
10
12
14
16
18
20
22
24
Time (hours)
Figure 2. Moisture vapour transmission through cover of Mattress B. 0.0
111IT:rr.-..--.-.~,-,-,--~.-.-.~.-...-,-~.-~~~
-0.1
§ Cl
-0.2
c:
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Table 1. Moisture vapour transmission through mattress cover samples.
.,uc:
'iii .c .£
Gl Cl
c:
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Mattress Mattress A Mean value Mattress B Mean value Matress C (Top)
Mean value Matress C (Side) Mean
Sample
MVTR (g/1 Ocm2/24hrs)
1 2 3
0.76 0.58 0.56 0.63
1 2 3
0.58 0.62 0.48 0.56
1 2 3 4 5 6 1 2
l.50t 2.65t 0.76 0.65 0.43 0.61 0.61 17.41 17.50 17.45
-0.4 -0.5 -0.6 -0.7 -0.8
~~~~~~~~~~'"o--'-~-'--'----'~-..L---'-'~-'------'
0
2
4
6
8
10
12
14
16
18
20
22
24
Time (hours)
Figure 3. Moisture vapour transmission through cover of Mattress C (top).
-0.5
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~
-1.0
Q)
u ~
'iii .c .£
-1.5
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c:
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-2.5 -3.0
'-'--'--~.L___,._L~J_,---'--~-'--.,_.L~--'--.__l~--'--'----'~~
0
2
4
6
8
10
12
14
Time (hours)
16
18
20
22
24
Total Care
The alternating alternative ......... .
24 hour Pressure Relief The Eclipse Full Seat Cushion providing comfort whilst featuring alternating pressure relief available with either mains or battery powered pump.
The Genesis is our Paediatric Alternating Mattress Replacement System. It is made of durable materials and is esigned d to fit a standard cot.
The Eclipse Alternating Mattress Overlay System for patients at d medium risk of eveloping pressure sores.
PI:RIIII. DYNAMIC PRESSUR E RELIEF by Park Ho"""
Park House, Blackburn Road, Birstall Batley. WF17 9PL Tel: 01924 441881 Fax: 01924 440631
Journal of Tissue Viability 1998 Vol 8 No 1
8
Figure 4. Moisture vapour transmission through cover of Mattress C (side). 0.0
o;-r--.--.---,~--,-....,.-.-..,...,,_,..~,_,.-.-..,...,,......,...~~~~
-2.0
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-8.0
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sample is clamped into position using a retaining ring and the outer surface covered with a dry filter. A hydrostatic head of 500 mm water gauge is then generated in the chamber and maintained for 18 hours, after which time the filter paper is examined for the presence of moisture.
.£
-12.0
Cl
-14.0
Three samples of each mattress were examined in this way (Table 2).
Table 2. VVaterproofness of mattress cover samples
CD
c: OS
()
Number tested
Number failed
Complies/ Does not comply
Mattress A
3
0
Complies
Mattress B
3
0
Complies
Mattress C (top)
3
0
Complies
Mattress C (side)
3
3
Does not comply
Sample
.s:::
-16.0 -18.0 -20.0
~~~~~~~~__.__._,_.___.__,__.__,~_,_.-..1.~-'--'
0
2
.4
6
8
10
12
14
16
18
20
22
24
Time (hours)
In table 1, two anomalous results (marked t) have been
excluded from the calculation of the mean value for the MVTR of Mattress C (see figure 3) as it was suspected that these were due to small holes in the fabric. Further support for this theory was obtained from the results of the Bacterial Barrier test performed subsequently.
The results suggest that in general, the permeability of the patient contact surfaces of the three covers are broadly similar although the results obtained with Mattress C appeared to be more variable, possibly due to variation in the thickness of the polyurethane coating applied to the fabric substrate. Further tests on larger numbers of samples would be required to confirm this. The side panels of Mattress C, which are designed to allow air to permeate through the mattress core, are approximately 30 times more permeable than the upper and lower surfaces. To put these results into context, semipermeable film dressings such as Opsite™ and Tegaderm™ give results in the order of 0.8g/10cm2/24hrs or 800g/m2/24hrs when tested by this method. 3.3 VVaterproofness The ability of the cover to resist the passage of liquid was determined using a test based upon a method described in the British Pharmacopoeia 1993, Appendix XX K. The apparatus consists of a chamber, open at one end, bearing a flange with an internal diameter of 50 mm. A retaining ring with the same internal diameter as the hole in the flange is mounted over the open end of the chamber.
The chamber is filled completely with water at 19 to 21°C and a sample of the material under test is slid horizontally over the cell with its facing layer in contact with the water in such a way as to avoid the inclusion of air between the surface of the water and the lower surface of the sample. The
The results confirm that the large surfaces of all three mattresses form an effective barrier to water. The fabric on the side of Mattress C, however, does not prevent the passage of liquid when subjected to pressure. 3.4 Comformability The ability of the cover to stretch and conform under pressure was determined using a test method first used to examine the conformability of semipermeable films 3. The apparatus is similar to that used to measure waterproofness, but instead of filling the test chamber with water, air is slowly forced into the chamber by means of a large syringe. The pressure generated within the chamber causes the cover to stretch and form a hemisphere which gradually increases in size until the upper surface comes into contact with a marker placed 20 mm above its surface at the start of the test. The pressure within the test system at this point is then recorded by means of a transducer connected to the syringe via a three-way tap.
Under the conditions of the test, the conformability of the fabric is inversely proportional to the pressure required to distort it by a predetermined amount. The results of this test are summarised in table 3. Within the table each result represents the mean of 5 determinations and the figures in brackets represent standard deviations. As the side cover of Mattress C is permeable to air, it was not possible to undertake the test on this material. The ability of the cover to deform is limited by the structure of the base fabric. Under the conditions of test, the knitted nylon substrate of Mattress B deforms more easily than that of the other covers tested. The pressure required to produce a
Journal of Tissue Viability 1998 Vol 8 No 1 Table 3. Conformability of mattress cover samples.
Dressing
Inflation Pressure (mmHg)
Mattress A
>950
Mattress B
535 (24.7)
Mattress C (top)
>950
Mattress C (side)
no result*
deformation of 20 mm in Mattresses A and C exceeded the measuring range of the test instrument. In retrospect, the arbitrarily chosen deformation of 20 mm although appropriate to film dressings, may be too high for the current application. If, for example, a deformation of 10 mm had been chosen as the end point, it might have been possible to obtain numerical values for all the products tested. In future studies, the possibility of using a value more appropriate to the intended use of the fabric will be considered. 3 .5 Tensile Properties The tensile properties of the cover were examined using an Instron, a tensile testing machine which can be used to measure the relationship between applied force and the extension (or compression) of the material under examination. For the purpose of this investigation, samples 25 mm wide and 75 mm long were cut from each mattress cover in two directions at right angles to each other and the force and elongation at break determined with a cross-head speed (rate of extension) of 200 mm/min (table 4). Within the table each result represents the mean of 10 determinations and figures in brackets represent standard deviation.
The differences in the load at break of the three covers although large, are unlikely to be clinically relevant, as all probably exceed the forces that are likely to be applied to the Table 4. Tensile properties of mattress cover.
Mattress cover
Elongation at break (mm)
Maximum load at break (N)
Mattress A
63.7 (6.70)
154.6 (22.3)
Mattress B
128.1 (15.68)
247.1 (25.0)
Mattress C (top)
46.8 (7.05)
133.2 (17.9)
Mattress C (side)
22.5 (6.00)
103.0 (17.11)
9
mattress under normal conditions of use. Perhaps more relevant is the difference in the values for elongation at break which also provides an indication of the extensibility and therefore the conformability of the mattress cover fabric. 3 .6 Bacterial Barrier Properties The bacterial barrier properties of each sample were determined using a laboratory method first used to test semipermeable film dressings. Portions of each cover are firmly clamped between two hemispherical flanged glass vessels, each having an internal diameter of 75 mm so that they effectively form two separate chambers.
To one chamber is added 50 ml of sterile tryptone soya broth and to the other 50 ml of a broth culture of Pseudomonas aeruginosa. The whole system is incubated at 37 ± OSC in a horizontal position for 48 hours after which the chamber that originally contained sterile broth is examined for the presence of the test organism. This test was carried out in triplicate and the results are summarised in table 5. Within the table, (+) denotes penetration of sample by bacteria and (-) indicates that penetration did not take place.
Table 5. Bacterial barrier properties.
Test sample
Passage of micro-organisms through film (+/-)
Mattress A Mattress B Mattress C (top)
--+
Mattress C (side)
not tested
Two of the covers tested proved to form an effective bacterial barrier in each of the three tests. Mattress C, however, failed to prevent the passage of microorganisms in one test. This could be due to a defect in the polyurethane which could provide a possible explanation for the variability in the moisture vapour transmission results obtained previously. The material used in the construction of the side panels was not tested.
4. Microbiological Examination of Damaged Mattress
Samples through the foam core measuring approximately 150 mm x 150 mm were cut from 8 different sites (figure 5). These were weighed and placed in sealed plastic bags together with 100 ml of quarter strength Ringer solution. The
10
Journal of Tissue Viability 1998 Vol 8 No 1
bags were then placed in a stomacher for 10 minutes after which a series of dilutions were made from the resultant extract. Aliquots of these extracts were applied in duplicate to tryptone soya agar (incubated at 30-35°C) to promote the growth of aerobic bacteria, and sabouraud dextrose agar containing chloramphenicol (incubated at 20-25°C) to prqmote the growth of fungi. Additional tryptone soya plates were incubated in the absence of oxygen to detect the presence of any anaerobes present. Figure 5. Mattress sampling positions.
0
0
G
From the number of colonies present on these plates, the number of colony forming present in each foam core was calculated (table 6). Cultures from the most heavily contaminated samples were also sent to the pathology department for identification (table 7). Table 6. Number of colony forming units at different locations on damaged mattress.
Sample site
Total count (no per gram) Yeasts/Fungi Anaerobes Aerobes
1
1.4 x 102
<1x102
<1x102
2
4.3 x 107
>1.3 x 104 *
<1x102
3
<1.0 x 102
<1x102
<1x102
4
<1.0 x 102
<1x102
<1x102
5
<1.0 x 102
<1x102
<1x102
6
<1.0 x 102
<1x102
<1x102
7
2.1 x 102
6.4 x 102
<1x102
8t
8.9 x 107
>1.8 x 104 *
2.5 x 103
*Colonies too numerous to count (ie >200) t Damaged area Table 7. Identification of microorganisms.
Figure 6. Test pieces taken from sample 8.
A
Sample site
Organism no
Identity
2
1
Scopulariopsis breuicaulis
2
2
Staphylococcus simulans
8
3
Staphylococcus epidermidis
8
4
Pseudomonas testosteranii
B
c D
A foam core taken from the most heavily contaminated portion of the mattress was cut into four sections and total viable counts carried out on each section. This was done to determine the depth of penetration of the contamination through the foam (table 8).
5. Discussion
This preliminary study has identified potentially important differences in the extensibility and conformability of the samples examined. It has also highlighted a degree of
Journal of Tissue Viability 1998 Vol 8 No 1 variability in the permeability and possibly the surface integrity of one the mattresses. Table 8. Distribution of microorganisms throughout mattress core.
Sample
Total count (No aerobes per gram)
11
The microbiological examination of the mattress with the damaged cover although not directly relevant to the products examined in the first part of the study, has confirmed once again the findings of others4 that the foam interior can act as a reservoir for bacteria and as such represents a serious potential source of infection. This gives further weight to the requirement that the cover should form an effective bacterial barrier.
Address for Correspondence
A
4.0 x 103
B
<1.3 x 102
c
9.7 x 102
D
5
Dr S Thomas, SMTL, Bridgend General Hospital, Quarella Road, Bridgend, Mid Glamorgan CF31 lJP.
References 1.
1.6 x 10
2.
The study has further demonstrated that, with the exception of the conformability test, which requires some minor modifications to make it suitable for use with relatively inelastic fabrics, the methods used are appropriate for evaluating mattress covers and as such will be used in the next phase of the testing programme.
3.
4.
Rithalia S. Pressure sores: which foam mattress and why? Journal of Tissue Viability 1996; 6(4): 115-119. Thomas S, Loveless P. A comparative study of the properties of six hydrocolloid dressings, Pharmaceutical Journal 1991; 247: 672-675. Thomas S, Loveless P. Comparative review of the properties of six semipermeable film dressings, Pharmaceutical Journal 1988; 240: 785-789. Loomes S. Is it safe to lie down in hospital? The Journal of Infection Control Nursing. Nursing Times 1988; 84(149): 63-65.
8th European Conference on Advances in Wound Managetn.ent 26-28 April 1998 Palacio de Congressos de Madrid, Spain The theme for the conference will be
"Acute and Chronic Wounds: is there a difference?" For further details, please contact: The Conference Office, 8th European Conference on Advances in Wound Management Emap Healthcare Ltd Porters South Crinan Street London Nl 9XW Tel: 01718434942
5
OBSERVATIONS ON MATTRESS COVERS: RESULTS OF A PILOT STUDY STEPHEN THOMAS Director, Surgical Materials Testing Laboratory, Bridgend 1. Summary Samples of covers from three commercially available mattresses were examined in the laboratory using test methods originally devised for testing surgical dressings. These revealed that although the covers shared many common features, there were differences in the conformability and tensile properties which may be of some clinical relevance. The study also confirmed that with some minor modifications, the experimental techniques used would be suitable for a future, more comprehensive review of mattress performance. In a separate investigation designed to examine the consequences of a failure of a mattress cover, the bioburden of a foam core removed from a damaged cover revealed the presence of very large numbers of microorganism, well in excess of 10 10 per gram of foam which could act as a recevoir of contamination and thus a source of cross infection.
The aim of this preliminary study was to determine the suitability of these systems for this present application before embarking upon a more detailed and comprehensive programme of comparative testing of the mattresses and mattress covers in current use in the UK. For this reason the three mattresses examined have not been identified by name, although a brief description of the structure of each is provided. a microbiological investigation was undertaken on a different mattress with a damaged cover that had been retrieved from a surgical ward in a hospital elsewhere in the United Kingdom. The aim of this simple study was to identify and quantify the number of microorganisms in the foam using standard laboratory techniques.
In a separate but related study,
3. Materials, Methods and Results 2. Introduction Many articles have been published in the medical and scientific press describing the factors that lead to the development of pressure damage. Most of these publications also make reference to the important role of specialised beds and mattresses and other pressure relieving aids in the prevention of this condition.
3.1 Mattress covers examined Mattress cover 'A' consists of a weft knitted polyester fabric coated with polyurethane
Despite the rapid growth in the use of alternating pressure relieving devices, many patients who are at risk of developing pressure damage are still nursed upon conventional mattresses consisting essentially of a block of foam enclosed in a waterproof plastic sleeve.
Mattress cover 'C' is composed of two different fabrics. The upper and lower surfaces consist of a knitted polyester fabric coated with polyurethane. The edges are made from a microporous fabric that is designed to permit the passage of water vapour but prevent penetration by bacteria.
In a recent review, Rithalia 1 made particular reference to the important contribution made to the performance of a foam mattress by the properties of the cover. He suggested that this should form an effective bacterial barrier, be water vapour permeable and possess adequate elastic properties to prevent the hammocking effect caused by a tightly fitting inelastic cover. He concluded that 'very little work has been carried out on these aspects of support systems and reliable information is scarce. Recognising this problem within our laboratory, we have recently conducted a pilot study in which we compared the key aspects of the performance of three mattress covers using test systems previously used for evaluating wound management materials.
Mattress cover 'B' consists of a weft knitted nylon fabric coated with polyurethane
3.2 Moisture Vapour Transmission Rate (MVTR) It is generally accepted that the accumulation of moisture between a mattress and the patient's skin can cause discomfort and may contribute to maceration leading in tum to tissue damage and infection. A cover which is permeable to water vapour but impermeable to liquids and bacteria should help to alleviate this problem.
The moisture vapour transmission rate of the three covers was determined using a test method that has been described previously for examining the performance of film dressings and hydrocolloids 2 . In this test, a section of the cover is applied to a Paddington
6
Journal of Tissue Viability 1998 Vol 8 No 1
cup (a modified Payne cup) the opening of which has a cross sectional area of 10 cm 2• To the cup is added 20 ml of a standard test solution consisting of sodium and calcium chloride containing 142mmol of Na+ ions and 2.5mmol of Ca2+ ions, values which are typical of those found in human serum. The cup is placed with the solution in contact with the cover in 'iln incubator at 37T upon the pan of a top loading balance.
Figure 1. Moisture vapour transmission through cover of Mattress A.
§ Cl
'O
"'l!?
c: 'iii .c .£
"'
Q)
The results are summarised in table 1 and expressed graphically in figures 1 to 4.
-0.4 -0.5
Cl
c:
"'
;;: ()
-0.6 -0.7
At the end of the test the recorded data are down-loaded for analysis. Initially three samples of each cover were examined by this method but because of unexpected variability in the results obtained with Mattress C, three additional tests were performed with this sample.
-0.3
Q)
u
The balance is connected to an electronic data logging device which records over a 24 hour period, any changes in the weight of the cup caused by the loss of moisture vapour through the cover.
-0.2
c:
-0.8
:---~~~~~~~~~'o-'-~-'--'---'-~_l_-'----'~~
0
2
4
6
8
10
12
14
16
18
20
22
24
Time (hours)
Figure 2. Moisture vapour transmission through cover of Mattress B. 0.0
111IT:rr.-..--.-.~,-,-,--~.-.-.~.-...-,-~.-~~~
-0.1
§ Cl
-0.2
c:
~ -0.3 l!? Gl
Table 1. Moisture vapour transmission through mattress cover samples.
.,uc:
'iii .c .£
Gl Cl
c:
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;;: ()
Mattress Mattress A Mean value Mattress B Mean value Matress C (Top)
Mean value Matress C (Side) Mean
Sample
MVTR (g/1 Ocm2/24hrs)
1 2 3
0.76 0.58 0.56 0.63
1 2 3
0.58 0.62 0.48 0.56
1 2 3 4 5 6 1 2
l.50t 2.65t 0.76 0.65 0.43 0.61 0.61 17.41 17.50 17.45
-0.4 -0.5 -0.6 -0.7 -0.8
~~~~~~~~~~'"o--'-~-'--'----'~-..L---'-'~-'------'
0
2
4
6
8
10
12
14
16
18
20
22
24
Time (hours)
Figure 3. Moisture vapour transmission through cover of Mattress C (top).
-0.5
§ g> 'O
~
-1.0
Q)
u ~
'iii .c .£
-1.5
2l, -2.0
c:
"'
;;: ()
-2.5 -3.0
'-'--'--~.L___,._L~J_,---'--~-'--.,_.L~--'--.__l~--'--'----'~~
0
2
4
6
8
10
12
14
Time (hours)
16
18
20
22
24
Total Care
The alternating alternative ......... .
24 hour Pressure Relief The Eclipse Full Seat Cushion providing comfort whilst featuring alternating pressure relief available with either mains or battery powered pump.
The Genesis is our Paediatric Alternating Mattress Replacement System. It is made of durable materials and is esigned d to fit a standard cot.
The Eclipse Alternating Mattress Overlay System for patients at d medium risk of eveloping pressure sores.
PI:RIIII. DYNAMIC PRESSUR E RELIEF by Park Ho"""
Park House, Blackburn Road, Birstall Batley. WF17 9PL Tel: 01924 441881 Fax: 01924 440631
Journal of Tissue Viability 1998 Vol 8 No 1
8
Figure 4. Moisture vapour transmission through cover of Mattress C (side). 0.0
o;-r--.--.---,~--,-....,.-.-..,...,,_,..~,_,.-.-..,...,,......,...~~~~
-2.0
§
-4.0
c: '5
-6.0
!I!
-8.0
c:
-10.0
Cl
OS
CD 01
OS
o;
.0
sample is clamped into position using a retaining ring and the outer surface covered with a dry filter. A hydrostatic head of 500 mm water gauge is then generated in the chamber and maintained for 18 hours, after which time the filter paper is examined for the presence of moisture.
.£
-12.0
Cl
-14.0
Three samples of each mattress were examined in this way (Table 2).
Table 2. VVaterproofness of mattress cover samples
CD
c: OS
()
Number tested
Number failed
Complies/ Does not comply
Mattress A
3
0
Complies
Mattress B
3
0
Complies
Mattress C (top)
3
0
Complies
Mattress C (side)
3
3
Does not comply
Sample
.s:::
-16.0 -18.0 -20.0
~~~~~~~~__.__._,_.___.__,__.__,~_,_.-..1.~-'--'
0
2
.4
6
8
10
12
14
16
18
20
22
24
Time (hours)
In table 1, two anomalous results (marked t) have been
excluded from the calculation of the mean value for the MVTR of Mattress C (see figure 3) as it was suspected that these were due to small holes in the fabric. Further support for this theory was obtained from the results of the Bacterial Barrier test performed subsequently.
The results suggest that in general, the permeability of the patient contact surfaces of the three covers are broadly similar although the results obtained with Mattress C appeared to be more variable, possibly due to variation in the thickness of the polyurethane coating applied to the fabric substrate. Further tests on larger numbers of samples would be required to confirm this. The side panels of Mattress C, which are designed to allow air to permeate through the mattress core, are approximately 30 times more permeable than the upper and lower surfaces. To put these results into context, semipermeable film dressings such as Opsite™ and Tegaderm™ give results in the order of 0.8g/10cm2/24hrs or 800g/m2/24hrs when tested by this method. 3.3 VVaterproofness The ability of the cover to resist the passage of liquid was determined using a test based upon a method described in the British Pharmacopoeia 1993, Appendix XX K. The apparatus consists of a chamber, open at one end, bearing a flange with an internal diameter of 50 mm. A retaining ring with the same internal diameter as the hole in the flange is mounted over the open end of the chamber.
The chamber is filled completely with water at 19 to 21°C and a sample of the material under test is slid horizontally over the cell with its facing layer in contact with the water in such a way as to avoid the inclusion of air between the surface of the water and the lower surface of the sample. The
The results confirm that the large surfaces of all three mattresses form an effective barrier to water. The fabric on the side of Mattress C, however, does not prevent the passage of liquid when subjected to pressure. 3.4 Comformability The ability of the cover to stretch and conform under pressure was determined using a test method first used to examine the conformability of semipermeable films 3. The apparatus is similar to that used to measure waterproofness, but instead of filling the test chamber with water, air is slowly forced into the chamber by means of a large syringe. The pressure generated within the chamber causes the cover to stretch and form a hemisphere which gradually increases in size until the upper surface comes into contact with a marker placed 20 mm above its surface at the start of the test. The pressure within the test system at this point is then recorded by means of a transducer connected to the syringe via a three-way tap.
Under the conditions of the test, the conformability of the fabric is inversely proportional to the pressure required to distort it by a predetermined amount. The results of this test are summarised in table 3. Within the table each result represents the mean of 5 determinations and the figures in brackets represent standard deviations. As the side cover of Mattress C is permeable to air, it was not possible to undertake the test on this material. The ability of the cover to deform is limited by the structure of the base fabric. Under the conditions of test, the knitted nylon substrate of Mattress B deforms more easily than that of the other covers tested. The pressure required to produce a
Journal of Tissue Viability 1998 Vol 8 No 1 Table 3. Conformability of mattress cover samples.
Dressing
Inflation Pressure (mmHg)
Mattress A
>950
Mattress B
535 (24.7)
Mattress C (top)
>950
Mattress C (side)
no result*
deformation of 20 mm in Mattresses A and C exceeded the measuring range of the test instrument. In retrospect, the arbitrarily chosen deformation of 20 mm although appropriate to film dressings, may be too high for the current application. If, for example, a deformation of 10 mm had been chosen as the end point, it might have been possible to obtain numerical values for all the products tested. In future studies, the possibility of using a value more appropriate to the intended use of the fabric will be considered. 3 .5 Tensile Properties The tensile properties of the cover were examined using an Instron, a tensile testing machine which can be used to measure the relationship between applied force and the extension (or compression) of the material under examination. For the purpose of this investigation, samples 25 mm wide and 75 mm long were cut from each mattress cover in two directions at right angles to each other and the force and elongation at break determined with a cross-head speed (rate of extension) of 200 mm/min (table 4). Within the table each result represents the mean of 10 determinations and figures in brackets represent standard deviation.
The differences in the load at break of the three covers although large, are unlikely to be clinically relevant, as all probably exceed the forces that are likely to be applied to the Table 4. Tensile properties of mattress cover.
Mattress cover
Elongation at break (mm)
Maximum load at break (N)
Mattress A
63.7 (6.70)
154.6 (22.3)
Mattress B
128.1 (15.68)
247.1 (25.0)
Mattress C (top)
46.8 (7.05)
133.2 (17.9)
Mattress C (side)
22.5 (6.00)
103.0 (17.11)
9
mattress under normal conditions of use. Perhaps more relevant is the difference in the values for elongation at break which also provides an indication of the extensibility and therefore the conformability of the mattress cover fabric. 3 .6 Bacterial Barrier Properties The bacterial barrier properties of each sample were determined using a laboratory method first used to test semipermeable film dressings. Portions of each cover are firmly clamped between two hemispherical flanged glass vessels, each having an internal diameter of 75 mm so that they effectively form two separate chambers.
To one chamber is added 50 ml of sterile tryptone soya broth and to the other 50 ml of a broth culture of Pseudomonas aeruginosa. The whole system is incubated at 37 ± OSC in a horizontal position for 48 hours after which the chamber that originally contained sterile broth is examined for the presence of the test organism. This test was carried out in triplicate and the results are summarised in table 5. Within the table, (+) denotes penetration of sample by bacteria and (-) indicates that penetration did not take place.
Table 5. Bacterial barrier properties.
Test sample
Passage of micro-organisms through film (+/-)
Mattress A Mattress B Mattress C (top)
--+
Mattress C (side)
not tested
Two of the covers tested proved to form an effective bacterial barrier in each of the three tests. Mattress C, however, failed to prevent the passage of microorganisms in one test. This could be due to a defect in the polyurethane which could provide a possible explanation for the variability in the moisture vapour transmission results obtained previously. The material used in the construction of the side panels was not tested.
4. Microbiological Examination of Damaged Mattress
Samples through the foam core measuring approximately 150 mm x 150 mm were cut from 8 different sites (figure 5). These were weighed and placed in sealed plastic bags together with 100 ml of quarter strength Ringer solution. The
10
Journal of Tissue Viability 1998 Vol 8 No 1
bags were then placed in a stomacher for 10 minutes after which a series of dilutions were made from the resultant extract. Aliquots of these extracts were applied in duplicate to tryptone soya agar (incubated at 30-35°C) to promote the growth of aerobic bacteria, and sabouraud dextrose agar containing chloramphenicol (incubated at 20-25°C) to prqmote the growth of fungi. Additional tryptone soya plates were incubated in the absence of oxygen to detect the presence of any anaerobes present. Figure 5. Mattress sampling positions.
0
0
G
From the number of colonies present on these plates, the number of colony forming present in each foam core was calculated (table 6). Cultures from the most heavily contaminated samples were also sent to the pathology department for identification (table 7). Table 6. Number of colony forming units at different locations on damaged mattress.
Sample site
Total count (no per gram) Yeasts/Fungi Anaerobes Aerobes
1
1.4 x 102
<1x102
<1x102
2
4.3 x 107
>1.3 x 104 *
<1x102
3
<1.0 x 102
<1x102
<1x102
4
<1.0 x 102
<1x102
<1x102
5
<1.0 x 102
<1x102
<1x102
6
<1.0 x 102
<1x102
<1x102
7
2.1 x 102
6.4 x 102
<1x102
8t
8.9 x 107
>1.8 x 104 *
2.5 x 103
*Colonies too numerous to count (ie >200) t Damaged area Table 7. Identification of microorganisms.
Figure 6. Test pieces taken from sample 8.
A
Sample site
Organism no
Identity
2
1
Scopulariopsis breuicaulis
2
2
Staphylococcus simulans
8
3
Staphylococcus epidermidis
8
4
Pseudomonas testosteranii
B
c D
A foam core taken from the most heavily contaminated portion of the mattress was cut into four sections and total viable counts carried out on each section. This was done to determine the depth of penetration of the contamination through the foam (table 8).
5. Discussion
This preliminary study has identified potentially important differences in the extensibility and conformability of the samples examined. It has also highlighted a degree of
Journal of Tissue Viability 1998 Vol 8 No 1 variability in the permeability and possibly the surface integrity of one the mattresses. Table 8. Distribution of microorganisms throughout mattress core.
Sample
Total count (No aerobes per gram)
11
The microbiological examination of the mattress with the damaged cover although not directly relevant to the products examined in the first part of the study, has confirmed once again the findings of others4 that the foam interior can act as a reservoir for bacteria and as such represents a serious potential source of infection. This gives further weight to the requirement that the cover should form an effective bacterial barrier.
Address for Correspondence
A
4.0 x 103
B
<1.3 x 102
c
9.7 x 102
D
5
Dr S Thomas, SMTL, Bridgend General Hospital, Quarella Road, Bridgend, Mid Glamorgan CF31 lJP.
References 1.
1.6 x 10
2.
The study has further demonstrated that, with the exception of the conformability test, which requires some minor modifications to make it suitable for use with relatively inelastic fabrics, the methods used are appropriate for evaluating mattress covers and as such will be used in the next phase of the testing programme.
3.
4.
Rithalia S. Pressure sores: which foam mattress and why? Journal of Tissue Viability 1996; 6(4): 115-119. Thomas S, Loveless P. A comparative study of the properties of six hydrocolloid dressings, Pharmaceutical Journal 1991; 247: 672-675. Thomas S, Loveless P. Comparative review of the properties of six semipermeable film dressings, Pharmaceutical Journal 1988; 240: 785-789. Loomes S. Is it safe to lie down in hospital? The Journal of Infection Control Nursing. Nursing Times 1988; 84(149): 63-65.
8th European Conference on Advances in Wound Managetn.ent 26-28 April 1998 Palacio de Congressos de Madrid, Spain The theme for the conference will be
"Acute and Chronic Wounds: is there a difference?" For further details, please contact: The Conference Office, 8th European Conference on Advances in Wound Management Emap Healthcare Ltd Porters South Crinan Street London Nl 9XW Tel: 01718434942