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Low temperature steam and formaldehyde sterilization
Journal
of Hospital
Infection
(1983)
4, 305-314
EQUIPMENT
Low temperature
REPORT
steam and formaldehyde sterilization R. G. Robertshaw
Department
of Pathology, Pinderfields General Hospital, WakeJield, West Yorkshire WFl 4DG
Aberford
Road,
Summary:
A standard low temperature steam/formaldehyde autoclave was tested according to the manufacturer’s instructions, using a range of test pieces containing Bacillus stearothermophilus spores as the challenge organism. There were failures in killing the challenge organism and the reasons for these are discussed. A description of modifications made to the autoclave is given together with details of an improved operating cycle. The performance of the modified autoclave was greatly improved and conditions were established for reliable and consistent sterilization of all the test pieces. A commercially produced prototype autoclave similarly modified also showed effective sterilization.
Introduction
In the earlier part of the century, methods of sterilizing heat sensitive medical materials by formaldehyde and water vapour at low temperatures and reduced pressures were described by various authors (Esmarch, 1902; Ecker and Pillemer, 1938; Nordgren, 1939). In recent years, modern steam sterilizing technology has enabled the principles of the earlier work to be applied to modifications of existing high temperature steam sterilizers to enable them to function at the lower temperatures required for use with low temperature steam and formaldehyde (LTSF). Early studies in this country were made in Bristol by Alder and his colleagues (Alder, Brown and Gillespie, 1966; Alder, 1968; Alder, Gingell and Mitchell, 1971; Mitchell and Alder, 1975), but their system was never developed commercially. Other workers (Dahlstrom, 1967; Gibson, 1977; Nystrom, 1973; Pickerill, 1975; Weymes, White and Harris, 1975; Deverill and Cripps, 1981), confirmed the sterilizing potential of this process, but doubts about the reliability and consistency of the existing commercially produced autoclaves were being expressed (Cripps, Deverill and Ayliffe, 1976). The conventional type of autoclave, adapted for LTSF sterilization, is only partially jacketed, at the top, sides and base of the chamber, but not usually in the door or the rear. The uneven heating from the partial jacketing may be of little consequence in high temperature sterilization, but in 0195%670,,83~040305
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t” 1983 The Hospnal
305
Infection
Somty
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R. G. Robertshaw
LTSF sterilization, the resulting condensation of steam from the cooler door and the rear areas of the chamber can hinder sterilization because the excessive moisture produces polymers of formaldehyde and failure to sterilize challenge cultures. Formaldehyde polymer residues may be found on the articles processed. It has also been recognized for some years that an autoclave with a jacket covering all areas of the chamber wall was necessary to reduce temperature fluctuations inside the chamber (Alder and Simpson, 1982; Weymes, 1977). In August 1976, the Department of Health and Social Security (DHSS) initiated an evaluation of the Hospital Sterilizing and Disinfection Unit (HSDU) at Pinderfields General Hospital, Wakefield, West Yorkshire. As part of this work, microbiological evaluation of the LTSF process was undertaken to establish conditions for consistent and reliable operation. Materials
and
methods
A standard ‘Swingclave’ autoclave (British Sterilizer Company Ltd, Hainault, Essex) was used. The autoclave consisted of a rectangular-shaped stainless steel chamber 0.66 m high, 0.66 m wide and 0.99 m deep with a capacity of 0.431 m3. The side walls, ceiling and floor of the chamber were steam jacketed but not the rear wall and door. The autoclave can operate with either a low temperature steam cycle alone or with formaldehyde. Both cycles are controlled automatically by means of timing devices, each of which can be set to any desired standard. The work described in this paper is concerned only with modifications to the LTSF process and the improved results obtained. The formaldehyde solution in a reservoir was drawn into the chamber under vacuum, through a vaporizer tube connected to the chamber steam supply pipe. This process was found to be inefficient because of inadequate vaporization of the formaldehyde solution. A more efficient vaporizing system was, therefore, introduced which allowed controlled quantities of formaldehyde solution to flow from a reservoir through a steamheated by steam from the mains supply at jacketed heat exchanger, approximately 35 psi, before being injected into the chamber. The process is similar in principle to that described by Alder (1968) and Alder and Simpson (1982). Other modifications to the autoclave were made as follows. All sub-atmosphere steam supply lines were fitted with electrical trace heating. The electrical heating was adjusted to maintain the pipe line contents at 80°C. ‘Dead-legs’ in the steam lines to the autoclave were eliminated. The chamber door was fitted with electrical trace heating between the back of the door and the door insulation. The electrical heating was adjusted to maintain the door temperature at 80°C. A Spirax non-return sight glass was fitted between the vaporizer and chamber entry point to check on possible entry of condensate to the chamber. A chamber pressure recorder was fitted.
Steam and formaldehyde
sterilization
307
The diameter of the jet in the formaldehyde solution reservoir was restricted to 0.8 mm, and the time during which the vacuum was applied to the formaldehyde solution transfer tube was extended from 1.5 to 20 s thus allowing a slower and more precise flow of fluid and ensuring that a predetermined volume could be injected at each pulse. Details
of the revised
operating
(i)
cycle are shown
(ii)
(iii)
in Figures
(iv)
1 and 2.
(VI
(vi)
Stages of LTSF cycle
Figure 1. Diagram showing the pressure changes for one complete low temperature steam and formaldehyde cycle. Stages of cycle and approximate duration of each stage are as follows: (i) air removal aided by steam flushing, 15 min; (ii) injection of formaldehyde followed by steam at 73°C (see Figure 2), 5-7 min each pulse; (iii) sterilizing (holding) period, 60 min; (iv) steam flush to remove formaldehyde, 15 min; (v) airing period, 1.5 min; (vi) vacuum break.
Ttme (mm)
Figure 2. Detail of a formaldehyde admission of steam.
and steam pulse from
Stage 2, Figure
1 showing
delay in
308
R. G. Robertshaw
The delay period between formaldehyde entry and steam injection serves two main purposes. Complete vaporization of the formaldehyde solution is assured and it allows formaldehyde gas to penetrate before steam entry to ensure effective sterilization. A record of the vaporization occurs on the pressure recorder thereby confirming that the formaldehyde has entered the chamber, also it gives a warning of a blocked jet in the reservoir or similar fault. Figures 1 and 2 demonstrate the small rise in formaldehyde vapour pressure at the commencement of each pulse in stage 2 before the steam is injected.
Challenge organism Paper strips impregnated with viable spores of Bacillus stearothermophilus (Oxoid, code BR23) were used throughout the work, except on one occasion when Southern Group Laboratories B. stearothermophilus spore discs were used. The strips or discs contained approximately lo6 spores.
Test pieces Bags and envelopes. (a) Paper bags normally
used in hospital sterilization units. (b) ‘Sterilpeel’ bags manufactured by C. R. Band Ltd, Pennywell Industrial Estate, Sunderland. (c) Glassine envelopes. Helix. A Helix (Line and Pickerill, 1973) consisting of 4.55 cm of 3 mm diameter stainless steel tubing giving a length/bore ratio of 1500: 1 was used. The terminal chamber was 1 ml capacity. The helix was placed in a paper bag for test purposes. ‘Alder’ type test piece (Alder, 1968; Alder et al., 1971). Three test pieces were made and tests were done in triplicate. In use the test pieces were laid in a cardboard box which in turn was placed in a paper bag. ‘Bowie and Dick’ type towel pack (Bowie, Kelsey and Thompson, 1963). Test packs were used containing 30 huckaback towels with a spore strip situated at the top and bottom and between every third towel making a total of 11 strips per test. The towels were freshly laundered for each test. Pouches. Pouches were made of one to ten layers of fabric (140 mm by 64 mm) folded in half and stitched leaving an opening on one side for insertion of a spore strip. The open end was then sealed with autoclave tape. The pouch materials used were dressing towels, crepe, ‘Sterifield’ paper and ‘Sterilpeel’. Fifty pouches each containing a spore strip were suspended evenly throughout the autoclave in each test. Two special tests were carried out using pouches made from both halves of ‘Sterilpeel’ bags. One set was prepared by joining together both paper halves of the bag and a second set by joining together the corresponding film halves. foam is widely used for Polyurethane foam test pieces. Polyurethane packaging delicate equipment. Simple test pieces were used to assess the ability of gaseous steam and formaldehyde to penetrate the packing and sterilize objects contained within.
Steam and formaldehyde
sterilization
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1. A cardboard tray, 30 cm x 45 cm, previously sterilized at 134°C to remove volatile tar products (Alder and Mitchell, 1970), was lined with a green dressing towel, and two layers of polyurethane foam (12 mm thick) were placed inside. Spore strips were placed at the following levels, and the tray was wrapped in a dressing towel, (a) Between the dressing towel and the bottom of the tray; (b) Between the bottom of the tray and the lower foam layer; (c) Between the two foam layers; (d) Between the upper foam layer and the dressing towel wrapping. Five spore strips were placed in each layer-one at each corner and one in the middle making a total of 20 spore strips per test. 2. A ‘Wolf’ telescope and fibre optic cable were laid out between two layers of polyurethane foam contained in a cardboard tray which in turn was double-wrapped inside a dressing towel bag. Spore strips were placed in seven positions beneath the telescope and cable, respectively. 3. A ‘Stortz’ fibre optic cable was placed in a cardboard tray as described in 2 above. The ‘Stortz’ telescope was placed in a hard polyurethane foam mould contained within an aluminium box possessing a tight-fitting lid. The base and lid of the box were each perforated with 209 holes, 4 mm in diameter. Spore strips were placed beneath the cable (four positions) and telescope (three positions). Homogeneity test. Spore strips (27 in number) were suspended on cotton threads evenly throughout the chamber. Additional spore strips were attached to the walls and placed on the floor. Incubation of spore strips. After each test the spore strips were transferred to bottles containing 18 ml ‘Oxoid’ tryptone soya broth (ref. CM 129) and incubated at 56°C for 21 days. Each culture was inspected daily. Control tests. For each fresh batch of broth these were: incubation of five untreated spore strips when growth must occur within 24 h; incubation of five bottles of broth without spore strips during the duration of the test period when no growth must occur. To eliminate the possibility of viable spores failing to grow due to some inhibitory property of the medium, one bottle was chosen at random from each test after completion of the 21 day period and a fresh spore strip added. Growth should occur within 24 h.
Results
The following section contains details of results obtained with the revised cycle and compares them with those obtained with the previous cycle. The data summarized in Table I is taken from approximately 1500 tests (using approximately 22,000 spore strips) and shows the conditions required for consistent sterilization of all spore strips within each test piece. Using test pieces 14 (Table I) with spore strips in bags and helices, the ‘revised’ cycles
Paper bags Glassine envelope ‘Sterilpeel’ bags Helix ‘Bow&Dick’ pack Telescopes and cable in foam Pouches (paper and towel) ‘Alder’ Homogeneity test
spore strip) 40-50 40-50 50 40 40-50 40 60-70 determined
12 11-12 7 :; 8 10 Not
Formaldehyde usage mg/l chamber volume/ pulse
cycle
11-12
No. of pulses
Previous
I. Comparison of old and new cycles showing conditions for sterilization
Test piece (containing
Table
ii 40 60
it 40
60
Holding time (min) No. of pulses
of B. stearothermophilus
cycle
Formaldehyde usage mg/l chamber volume/ pulse
Revised
Holding time (min)
spore strips in the test pieces
Steam and formaldehyde
sterilization
311
sterilized the spore strips using less than half of the pulses and about half the quantity of formaldehyde solution/pulse, without a holding period, when compared with the previous cycle. In test pieces 5-8 with the spore strips packed in porous materials, the number of pulses required to sterilize them using the same quantity of formaldehyde solution/pulse were fewer with the revised cycle, also the quantity of formaldehyde/pulse required and the holding period of 30 min was much less than was required by the previous cycle. The holding period was necessary because evidence was obtained experimentally to show that a temperature gradient existed across the packing material used and depending upon the nature of the packing material the time taken for all parts of the pack to reach chamber temperature ranged from 5 to 12 min during the last steam injection. The effect of the holding period was clearly shown in two sets of experiments with the revised method using Bow&Dick type towel packs. Twenty tests were performed using 4-pulses with the revised cycle without a holding period, and compared with 20 similar tests which included a 30-min holding period. With no holding period 18 failures (90 per cent) out of 20 tests were recorded but when a holding period was included only two (10 per cent) failures were recorded out of the 20 tests. Using the revised cycle, spore strips placed within all the materials tested were sterilized, except those contained within pouches prepared from the film portions of a ‘Sterilpeel’ bag, where there was always growth after two days incubation. This experiment was prompted by the observations that spore strips in ‘Sterilpeel’ bags were frequently not sterilized by the revised cycle when placed film-side upwards on the chamber tray whereas those contained in freely-suspended bags were always sterilized. Materials in ‘Sterilpeel’ bags must be positioned correctly in the sterilizer chamber. Application of jindings to prototype autoclave In October 1979, Charles F. Thackray Limited, Leeds, was commissioned by the Yorkshire Regional Health Authority to build a LTSF autoclave based upon the findings of the research carried out in the HSDU at Pinderfields General Hospital. The prototype autoclave consisted of a chamber of 0.415 m3 capacity and was equipped with the following features. All chamber surfaces, including the door, were provided with a jacket containing circulating hot water. A sight glass was fitted in the line between vaporizer and chamber. All sub-atmospheric steam lines were properly lagged and trapped. A steam-jacketed baffled vaporizer was fitted. Instrumentation to allow variable control of delay period between formaldehyde entry and steam pulse was fitted. A pulse control device to control the number of pulses given during the cycle was incorporated, thus making the pulsing stage independent of time. Improved instrumentation. A three-pen recorder was fitted to provide a
312
R. G. Robertshaw
continuous record of pressure, temperature and formaldehyde consumption. A low level cut-off was fitted to the formaldehyde reservoir to ensure that the cycle cannot be started unless there is sufficient formaldehyde solution present to permit the cycle to be completed normally. The autoclave was tested using the revised cycle previously described (Figure 1). Time did not permit a detailed study to be made and hence only one set of revised cycle operating conditions was studied, but the opportunity was taken to use both Oxoid and Southern Group B. stearothermophilus spore preparations as the challenge. Ten tests with Bow&Dick type packs each containing 11 spore strips, and five homogeneity tests each of which used 27 spore strips were carried out. The conditions in each of the tests consisted of eight pulses, 27 mg/l formaldehyde/pulse and a holding time of 40 min. No growth was obtained from any of the spore strips. Similar tests were carried out using ‘Southern Group’ spore discs in three helices and three Alder test pieces. No growth was obtained from any of the spore strips. Discussion
The advantage of the revised cycle compared with the previous one is probably due to the delay period between formaldehyde entry and steam injection (Figures 1 and 2). This procedure allows time for the formaldehyde vapour to penetrate the load before the steam entry, whereas a simultaneous injection of formaldehyde vapour and steam may, depending upon the method of vaporization, result in the penetration of the load by steam before the.formaldehyde has had time to vaporize. Figure 2 clearly shows that the formaldehyde vapour does require time to vaporize to the maximum extent. If the steam is allowed to enter too soon after the formaldehyde injection this may result in a reduced concentration of gas in the load, particularly when equipment is contained in a porous packing. Where porous packing was used to contain test pieces experimental evidence showed that a holding period following the last pulse was necessary to ensure that all parts of the packing reached chamber temperature. Condensate in the chamber is a cause of failure in LTSF autoclaves. Not only is the formaldehyde concentration reduced (Weymes, White and Harris, 1975; Marcos and Wiseman, 1979), but also the load may be wetted thus reducing further the chances of sterilization (Nordgren, 1939). The primary objective should therefore be to eliminate the possibility of condensate being transferred to the chamber and to ensure complete vaporization of the formaldehyde. To eliminate the problem of condensation and to ensure adequate penetration of formaldehyde gas the following design recommendations are suggested. 1. The chamber
must be free from ‘cold spots’. The chamber
walls, door
Steam and formaldehyde
sterilization
313
and ceiling should be jacketed and provided with circulating hot water. Hot water is preferred to steam on the grounds of easier temperature control, fewer engineering problems and safety in use. 2. Attention should be paid to the steam supply pipework. All pipework should be as short as possible, properly lagged and with the traps correctly sited. A trap must be included in the line immediately before the mains supply becomes sub-atmospheric. Sub-atmospheric steam pipes should be lagged and water-jacketed, or fitted with electrical trace heating to maintain a temperature of 80°C. 3. A delay period should be allowed between the formaldehyde vaporization and the steam injection. The vaporizer should be capable of producing a vapour free of entrained liquid or ‘carry-over’. The vaporizer should be jacketed and served with mains steam (approximately 30 psi) to ensure rapid vaporization. 4. Good instrumentation is one of the keys to reliable operation. Too often equipment is supplied with standard gauges with unsuitable ranges or the gauges may be badly sited or possibly a combination of both. Reliable and consistent sterilization depends upon the establishment of correct conditions within the chamber and hence the instrumentation must supply clear and precise information regarding these conditions. The following information is considered to be essential. (a) Temperature. A knowledge of the chamber temperature is important because sensitive equipment must not be damaged by exposure to excessive heat. The present recommended temperature range for the LTSF process is 73”C( - 2 + 5) and a high temperature cut-off should be provided to operate at 5°C above the maximum temperature to prevent damage. (b) Pressure. An accurate record of pressure is essential since sterilization is dependent upon the correct sequence of pressure changes occurring within the autoclave chamber during the cycle. In addition to visual confirmation of correct pulsing, the entry of formaldehyde vapour in the chamber is shown by a small but distinct increase in pressure before that of the subsequent steam pulse. (c) Formaldehyde consumption. Instrumentation should include a recorder to provide a permanent record of the changes occurring in the formaldehyde reservoir level during the cycle and thus provide a record of formaldehyde consumption. The reservoir should also be fitted with a low-level alarm cut-off device which prevents the cycle from starting if the level of solution in the reservoir is insufficient for completion. (d) Control of formaldehyde usage should by means of a pulse control device to regulate accurately the number of pulses used in a cycle. (e) Correct loading of the chamber especially when using ‘Sterilpeel’ bags. The results obtained with the revised cycle show it to be a reliable process capable of sterilizing consistently all the spore strips in each test piece
314
R. G. Robertshaw
studied. It has been standard practice in the HSDU at Pinderfields Hospital since May 1978 and no failures have been recorded with and Pickerill helix, included in every load since that date.
General the Line
I am indebted to the Department of Health and Social Security for funding this research. I am grateful to Mr L. Jones and to Mr J. Walker of the Hospital Engineers Department for their advice during the work and for carrying out modifications to the autoclave.
References Alder,
V. G. (1968). Sterilisation by low-temperature steam and formaldehyde under subatmospheric pressure at 80°C. COSPAR (Committee of Space Research) Technique Manual Series No. 4 (Sneath, P. M. A., Ed.), pp. 151-155. COSPAR Secretariat, Paris. Alder, V. G. & Mitchell, J. P. (1970). Recent developments on the use of sub-atmospheric steam and formaldehyde at 80°C for the disinfection of cystoscopes. British Hospital (3 October). Journal and Social Service Review 19441946 Alder, V. G. & Simpson, R. A. (1982). In Principles and Practice of Disinfection, Preservation and Sterlisation (Russell, A. D., Ayliffe, G. A. J. & Hugo, W. B., Eds), Blackwell Scientific Publications Ltd, London. Alder, V. G., Brown, A. M. & Gillespie, W. A. (1966). Disinfection of heat-sensitive material by low-temperature steam and formaldehyde. Journal of Clinical Pathology 19, 83-89. Alder, V. G., Gingell, J. C. & Mitchell, J. P. (1971). Disinfection of cytoscopes by subatmospheric steam and steam and formaldehyde at 80°C. British Medical Journal 3, 677-680. Bowie, J. H., Kelsey, J. C. & Thompson, G. R. (1963). The Bowie and Dick Autoclave Tape Test. Lancet 1, 586. Cripps, N., Deverill, C. E. A. & Ayliffe, G. A. J. (1976). Problems with low temperature steam and formaldehyde sterilizers. Hospital Engineering 19, 9-10. Dahlstrom, H. (1967). Disinfection by low temperature steam. British HospitalJournal and Social Science Review 1208-I 2 15. Deverill, C. E. A. & Cripps, N. F. (1981). Tests on a low temperature steam and formaldehyde autoclave: The Miniclave 80. Journal of Hospital Infection 2, 175-190. Ecker, E. E. & Pillemer, L. (1938). Pressure cooker sterilizer for urologic instruments. Modern Hospital 50, 8687 (May). Esmarch, E. (1902). Die Wirking von Formalinwasser/dampen in Disinfectionsapparat. Hygenische Rundschau 12, 961-970. Gibson, G. L. (1977). Processing urinary endoscopes in a low temperature steam and formaldehyde autoclave. Journal of Clinical Pathology 30, 269. Line, S. J. & Pickerill, J. K. (1973). Testing a steam-formaldehyde sterilizer for gas penetration efficiency. Journal of Clinical Pathology 26, 716-720. Marcos, D. & Wiseman, D. (1979). Measurement of formaldehyde concentrations in a subatmospheric steam-formaldehyde autoclave. Journal of Clinical Pathology 32, 567-575. Mitchell, J. P. & Alder, V. G. (1975). The disinfection of urological endoscopes. British Journal of Urology 47, 571-575. Nordgren, G. (1939). Investigations on the sterilization efficacy of gaseous formaldehyde.
Acta Pathologica et Microbiologica Scandinavica, Supplement 40. B. (1973). Central Sterilising Club, 15th Meeting, Reading, pp. 1 I-12. J. K. (1975). A practical system for steam formaldehyde. Sterilizing Laboratory Practice 401-404 (June).
Nystrom, Pickerill,
Weymes, C., White, J. D. & Harris, C. (1975). Studies in the use of low concentrations of formaldehyde with steam at sub-atmospheric pressures as a method of sterilizing nonporous heat sensitive items. In Note 4, Greater Glasgow Health Board Sterilisation Centre. Weymes, C. (1977). Low temperature steam and formalin. (A paper given to the Sterilizing Club Meeting, Bristol, March 1977). Association of Sterile Supply ManagersJournal 6 (2), 8-10 (November).
of Hospital
Infection
(1983)
4, 305-314
EQUIPMENT
Low temperature
REPORT
steam and formaldehyde sterilization R. G. Robertshaw
Department
of Pathology, Pinderfields General Hospital, WakeJield, West Yorkshire WFl 4DG
Aberford
Road,
Summary:
A standard low temperature steam/formaldehyde autoclave was tested according to the manufacturer’s instructions, using a range of test pieces containing Bacillus stearothermophilus spores as the challenge organism. There were failures in killing the challenge organism and the reasons for these are discussed. A description of modifications made to the autoclave is given together with details of an improved operating cycle. The performance of the modified autoclave was greatly improved and conditions were established for reliable and consistent sterilization of all the test pieces. A commercially produced prototype autoclave similarly modified also showed effective sterilization.
Introduction
In the earlier part of the century, methods of sterilizing heat sensitive medical materials by formaldehyde and water vapour at low temperatures and reduced pressures were described by various authors (Esmarch, 1902; Ecker and Pillemer, 1938; Nordgren, 1939). In recent years, modern steam sterilizing technology has enabled the principles of the earlier work to be applied to modifications of existing high temperature steam sterilizers to enable them to function at the lower temperatures required for use with low temperature steam and formaldehyde (LTSF). Early studies in this country were made in Bristol by Alder and his colleagues (Alder, Brown and Gillespie, 1966; Alder, 1968; Alder, Gingell and Mitchell, 1971; Mitchell and Alder, 1975), but their system was never developed commercially. Other workers (Dahlstrom, 1967; Gibson, 1977; Nystrom, 1973; Pickerill, 1975; Weymes, White and Harris, 1975; Deverill and Cripps, 1981), confirmed the sterilizing potential of this process, but doubts about the reliability and consistency of the existing commercially produced autoclaves were being expressed (Cripps, Deverill and Ayliffe, 1976). The conventional type of autoclave, adapted for LTSF sterilization, is only partially jacketed, at the top, sides and base of the chamber, but not usually in the door or the rear. The uneven heating from the partial jacketing may be of little consequence in high temperature sterilization, but in 0195%670,,83~040305
+ 10 102.00:0
t” 1983 The Hospnal
305
Infection
Somty
306
R. G. Robertshaw
LTSF sterilization, the resulting condensation of steam from the cooler door and the rear areas of the chamber can hinder sterilization because the excessive moisture produces polymers of formaldehyde and failure to sterilize challenge cultures. Formaldehyde polymer residues may be found on the articles processed. It has also been recognized for some years that an autoclave with a jacket covering all areas of the chamber wall was necessary to reduce temperature fluctuations inside the chamber (Alder and Simpson, 1982; Weymes, 1977). In August 1976, the Department of Health and Social Security (DHSS) initiated an evaluation of the Hospital Sterilizing and Disinfection Unit (HSDU) at Pinderfields General Hospital, Wakefield, West Yorkshire. As part of this work, microbiological evaluation of the LTSF process was undertaken to establish conditions for consistent and reliable operation. Materials
and
methods
A standard ‘Swingclave’ autoclave (British Sterilizer Company Ltd, Hainault, Essex) was used. The autoclave consisted of a rectangular-shaped stainless steel chamber 0.66 m high, 0.66 m wide and 0.99 m deep with a capacity of 0.431 m3. The side walls, ceiling and floor of the chamber were steam jacketed but not the rear wall and door. The autoclave can operate with either a low temperature steam cycle alone or with formaldehyde. Both cycles are controlled automatically by means of timing devices, each of which can be set to any desired standard. The work described in this paper is concerned only with modifications to the LTSF process and the improved results obtained. The formaldehyde solution in a reservoir was drawn into the chamber under vacuum, through a vaporizer tube connected to the chamber steam supply pipe. This process was found to be inefficient because of inadequate vaporization of the formaldehyde solution. A more efficient vaporizing system was, therefore, introduced which allowed controlled quantities of formaldehyde solution to flow from a reservoir through a steamheated by steam from the mains supply at jacketed heat exchanger, approximately 35 psi, before being injected into the chamber. The process is similar in principle to that described by Alder (1968) and Alder and Simpson (1982). Other modifications to the autoclave were made as follows. All sub-atmosphere steam supply lines were fitted with electrical trace heating. The electrical heating was adjusted to maintain the pipe line contents at 80°C. ‘Dead-legs’ in the steam lines to the autoclave were eliminated. The chamber door was fitted with electrical trace heating between the back of the door and the door insulation. The electrical heating was adjusted to maintain the door temperature at 80°C. A Spirax non-return sight glass was fitted between the vaporizer and chamber entry point to check on possible entry of condensate to the chamber. A chamber pressure recorder was fitted.
Steam and formaldehyde
sterilization
307
The diameter of the jet in the formaldehyde solution reservoir was restricted to 0.8 mm, and the time during which the vacuum was applied to the formaldehyde solution transfer tube was extended from 1.5 to 20 s thus allowing a slower and more precise flow of fluid and ensuring that a predetermined volume could be injected at each pulse. Details
of the revised
operating
(i)
cycle are shown
(ii)
(iii)
in Figures
(iv)
1 and 2.
(VI
(vi)
Stages of LTSF cycle
Figure 1. Diagram showing the pressure changes for one complete low temperature steam and formaldehyde cycle. Stages of cycle and approximate duration of each stage are as follows: (i) air removal aided by steam flushing, 15 min; (ii) injection of formaldehyde followed by steam at 73°C (see Figure 2), 5-7 min each pulse; (iii) sterilizing (holding) period, 60 min; (iv) steam flush to remove formaldehyde, 15 min; (v) airing period, 1.5 min; (vi) vacuum break.
Ttme (mm)
Figure 2. Detail of a formaldehyde admission of steam.
and steam pulse from
Stage 2, Figure
1 showing
delay in
308
R. G. Robertshaw
The delay period between formaldehyde entry and steam injection serves two main purposes. Complete vaporization of the formaldehyde solution is assured and it allows formaldehyde gas to penetrate before steam entry to ensure effective sterilization. A record of the vaporization occurs on the pressure recorder thereby confirming that the formaldehyde has entered the chamber, also it gives a warning of a blocked jet in the reservoir or similar fault. Figures 1 and 2 demonstrate the small rise in formaldehyde vapour pressure at the commencement of each pulse in stage 2 before the steam is injected.
Challenge organism Paper strips impregnated with viable spores of Bacillus stearothermophilus (Oxoid, code BR23) were used throughout the work, except on one occasion when Southern Group Laboratories B. stearothermophilus spore discs were used. The strips or discs contained approximately lo6 spores.
Test pieces Bags and envelopes. (a) Paper bags normally
used in hospital sterilization units. (b) ‘Sterilpeel’ bags manufactured by C. R. Band Ltd, Pennywell Industrial Estate, Sunderland. (c) Glassine envelopes. Helix. A Helix (Line and Pickerill, 1973) consisting of 4.55 cm of 3 mm diameter stainless steel tubing giving a length/bore ratio of 1500: 1 was used. The terminal chamber was 1 ml capacity. The helix was placed in a paper bag for test purposes. ‘Alder’ type test piece (Alder, 1968; Alder et al., 1971). Three test pieces were made and tests were done in triplicate. In use the test pieces were laid in a cardboard box which in turn was placed in a paper bag. ‘Bowie and Dick’ type towel pack (Bowie, Kelsey and Thompson, 1963). Test packs were used containing 30 huckaback towels with a spore strip situated at the top and bottom and between every third towel making a total of 11 strips per test. The towels were freshly laundered for each test. Pouches. Pouches were made of one to ten layers of fabric (140 mm by 64 mm) folded in half and stitched leaving an opening on one side for insertion of a spore strip. The open end was then sealed with autoclave tape. The pouch materials used were dressing towels, crepe, ‘Sterifield’ paper and ‘Sterilpeel’. Fifty pouches each containing a spore strip were suspended evenly throughout the autoclave in each test. Two special tests were carried out using pouches made from both halves of ‘Sterilpeel’ bags. One set was prepared by joining together both paper halves of the bag and a second set by joining together the corresponding film halves. foam is widely used for Polyurethane foam test pieces. Polyurethane packaging delicate equipment. Simple test pieces were used to assess the ability of gaseous steam and formaldehyde to penetrate the packing and sterilize objects contained within.
Steam and formaldehyde
sterilization
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1. A cardboard tray, 30 cm x 45 cm, previously sterilized at 134°C to remove volatile tar products (Alder and Mitchell, 1970), was lined with a green dressing towel, and two layers of polyurethane foam (12 mm thick) were placed inside. Spore strips were placed at the following levels, and the tray was wrapped in a dressing towel, (a) Between the dressing towel and the bottom of the tray; (b) Between the bottom of the tray and the lower foam layer; (c) Between the two foam layers; (d) Between the upper foam layer and the dressing towel wrapping. Five spore strips were placed in each layer-one at each corner and one in the middle making a total of 20 spore strips per test. 2. A ‘Wolf’ telescope and fibre optic cable were laid out between two layers of polyurethane foam contained in a cardboard tray which in turn was double-wrapped inside a dressing towel bag. Spore strips were placed in seven positions beneath the telescope and cable, respectively. 3. A ‘Stortz’ fibre optic cable was placed in a cardboard tray as described in 2 above. The ‘Stortz’ telescope was placed in a hard polyurethane foam mould contained within an aluminium box possessing a tight-fitting lid. The base and lid of the box were each perforated with 209 holes, 4 mm in diameter. Spore strips were placed beneath the cable (four positions) and telescope (three positions). Homogeneity test. Spore strips (27 in number) were suspended on cotton threads evenly throughout the chamber. Additional spore strips were attached to the walls and placed on the floor. Incubation of spore strips. After each test the spore strips were transferred to bottles containing 18 ml ‘Oxoid’ tryptone soya broth (ref. CM 129) and incubated at 56°C for 21 days. Each culture was inspected daily. Control tests. For each fresh batch of broth these were: incubation of five untreated spore strips when growth must occur within 24 h; incubation of five bottles of broth without spore strips during the duration of the test period when no growth must occur. To eliminate the possibility of viable spores failing to grow due to some inhibitory property of the medium, one bottle was chosen at random from each test after completion of the 21 day period and a fresh spore strip added. Growth should occur within 24 h.
Results
The following section contains details of results obtained with the revised cycle and compares them with those obtained with the previous cycle. The data summarized in Table I is taken from approximately 1500 tests (using approximately 22,000 spore strips) and shows the conditions required for consistent sterilization of all spore strips within each test piece. Using test pieces 14 (Table I) with spore strips in bags and helices, the ‘revised’ cycles
Paper bags Glassine envelope ‘Sterilpeel’ bags Helix ‘Bow&Dick’ pack Telescopes and cable in foam Pouches (paper and towel) ‘Alder’ Homogeneity test
spore strip) 40-50 40-50 50 40 40-50 40 60-70 determined
12 11-12 7 :; 8 10 Not
Formaldehyde usage mg/l chamber volume/ pulse
cycle
11-12
No. of pulses
Previous
I. Comparison of old and new cycles showing conditions for sterilization
Test piece (containing
Table
ii 40 60
it 40
60
Holding time (min) No. of pulses
of B. stearothermophilus
cycle
Formaldehyde usage mg/l chamber volume/ pulse
Revised
Holding time (min)
spore strips in the test pieces
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sterilized the spore strips using less than half of the pulses and about half the quantity of formaldehyde solution/pulse, without a holding period, when compared with the previous cycle. In test pieces 5-8 with the spore strips packed in porous materials, the number of pulses required to sterilize them using the same quantity of formaldehyde solution/pulse were fewer with the revised cycle, also the quantity of formaldehyde/pulse required and the holding period of 30 min was much less than was required by the previous cycle. The holding period was necessary because evidence was obtained experimentally to show that a temperature gradient existed across the packing material used and depending upon the nature of the packing material the time taken for all parts of the pack to reach chamber temperature ranged from 5 to 12 min during the last steam injection. The effect of the holding period was clearly shown in two sets of experiments with the revised method using Bow&Dick type towel packs. Twenty tests were performed using 4-pulses with the revised cycle without a holding period, and compared with 20 similar tests which included a 30-min holding period. With no holding period 18 failures (90 per cent) out of 20 tests were recorded but when a holding period was included only two (10 per cent) failures were recorded out of the 20 tests. Using the revised cycle, spore strips placed within all the materials tested were sterilized, except those contained within pouches prepared from the film portions of a ‘Sterilpeel’ bag, where there was always growth after two days incubation. This experiment was prompted by the observations that spore strips in ‘Sterilpeel’ bags were frequently not sterilized by the revised cycle when placed film-side upwards on the chamber tray whereas those contained in freely-suspended bags were always sterilized. Materials in ‘Sterilpeel’ bags must be positioned correctly in the sterilizer chamber. Application of jindings to prototype autoclave In October 1979, Charles F. Thackray Limited, Leeds, was commissioned by the Yorkshire Regional Health Authority to build a LTSF autoclave based upon the findings of the research carried out in the HSDU at Pinderfields General Hospital. The prototype autoclave consisted of a chamber of 0.415 m3 capacity and was equipped with the following features. All chamber surfaces, including the door, were provided with a jacket containing circulating hot water. A sight glass was fitted in the line between vaporizer and chamber. All sub-atmospheric steam lines were properly lagged and trapped. A steam-jacketed baffled vaporizer was fitted. Instrumentation to allow variable control of delay period between formaldehyde entry and steam pulse was fitted. A pulse control device to control the number of pulses given during the cycle was incorporated, thus making the pulsing stage independent of time. Improved instrumentation. A three-pen recorder was fitted to provide a
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R. G. Robertshaw
continuous record of pressure, temperature and formaldehyde consumption. A low level cut-off was fitted to the formaldehyde reservoir to ensure that the cycle cannot be started unless there is sufficient formaldehyde solution present to permit the cycle to be completed normally. The autoclave was tested using the revised cycle previously described (Figure 1). Time did not permit a detailed study to be made and hence only one set of revised cycle operating conditions was studied, but the opportunity was taken to use both Oxoid and Southern Group B. stearothermophilus spore preparations as the challenge. Ten tests with Bow&Dick type packs each containing 11 spore strips, and five homogeneity tests each of which used 27 spore strips were carried out. The conditions in each of the tests consisted of eight pulses, 27 mg/l formaldehyde/pulse and a holding time of 40 min. No growth was obtained from any of the spore strips. Similar tests were carried out using ‘Southern Group’ spore discs in three helices and three Alder test pieces. No growth was obtained from any of the spore strips. Discussion
The advantage of the revised cycle compared with the previous one is probably due to the delay period between formaldehyde entry and steam injection (Figures 1 and 2). This procedure allows time for the formaldehyde vapour to penetrate the load before the steam entry, whereas a simultaneous injection of formaldehyde vapour and steam may, depending upon the method of vaporization, result in the penetration of the load by steam before the.formaldehyde has had time to vaporize. Figure 2 clearly shows that the formaldehyde vapour does require time to vaporize to the maximum extent. If the steam is allowed to enter too soon after the formaldehyde injection this may result in a reduced concentration of gas in the load, particularly when equipment is contained in a porous packing. Where porous packing was used to contain test pieces experimental evidence showed that a holding period following the last pulse was necessary to ensure that all parts of the packing reached chamber temperature. Condensate in the chamber is a cause of failure in LTSF autoclaves. Not only is the formaldehyde concentration reduced (Weymes, White and Harris, 1975; Marcos and Wiseman, 1979), but also the load may be wetted thus reducing further the chances of sterilization (Nordgren, 1939). The primary objective should therefore be to eliminate the possibility of condensate being transferred to the chamber and to ensure complete vaporization of the formaldehyde. To eliminate the problem of condensation and to ensure adequate penetration of formaldehyde gas the following design recommendations are suggested. 1. The chamber
must be free from ‘cold spots’. The chamber
walls, door
Steam and formaldehyde
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313
and ceiling should be jacketed and provided with circulating hot water. Hot water is preferred to steam on the grounds of easier temperature control, fewer engineering problems and safety in use. 2. Attention should be paid to the steam supply pipework. All pipework should be as short as possible, properly lagged and with the traps correctly sited. A trap must be included in the line immediately before the mains supply becomes sub-atmospheric. Sub-atmospheric steam pipes should be lagged and water-jacketed, or fitted with electrical trace heating to maintain a temperature of 80°C. 3. A delay period should be allowed between the formaldehyde vaporization and the steam injection. The vaporizer should be capable of producing a vapour free of entrained liquid or ‘carry-over’. The vaporizer should be jacketed and served with mains steam (approximately 30 psi) to ensure rapid vaporization. 4. Good instrumentation is one of the keys to reliable operation. Too often equipment is supplied with standard gauges with unsuitable ranges or the gauges may be badly sited or possibly a combination of both. Reliable and consistent sterilization depends upon the establishment of correct conditions within the chamber and hence the instrumentation must supply clear and precise information regarding these conditions. The following information is considered to be essential. (a) Temperature. A knowledge of the chamber temperature is important because sensitive equipment must not be damaged by exposure to excessive heat. The present recommended temperature range for the LTSF process is 73”C( - 2 + 5) and a high temperature cut-off should be provided to operate at 5°C above the maximum temperature to prevent damage. (b) Pressure. An accurate record of pressure is essential since sterilization is dependent upon the correct sequence of pressure changes occurring within the autoclave chamber during the cycle. In addition to visual confirmation of correct pulsing, the entry of formaldehyde vapour in the chamber is shown by a small but distinct increase in pressure before that of the subsequent steam pulse. (c) Formaldehyde consumption. Instrumentation should include a recorder to provide a permanent record of the changes occurring in the formaldehyde reservoir level during the cycle and thus provide a record of formaldehyde consumption. The reservoir should also be fitted with a low-level alarm cut-off device which prevents the cycle from starting if the level of solution in the reservoir is insufficient for completion. (d) Control of formaldehyde usage should by means of a pulse control device to regulate accurately the number of pulses used in a cycle. (e) Correct loading of the chamber especially when using ‘Sterilpeel’ bags. The results obtained with the revised cycle show it to be a reliable process capable of sterilizing consistently all the spore strips in each test piece
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studied. It has been standard practice in the HSDU at Pinderfields Hospital since May 1978 and no failures have been recorded with and Pickerill helix, included in every load since that date.
General the Line
I am indebted to the Department of Health and Social Security for funding this research. I am grateful to Mr L. Jones and to Mr J. Walker of the Hospital Engineers Department for their advice during the work and for carrying out modifications to the autoclave.
References Alder,
V. G. (1968). Sterilisation by low-temperature steam and formaldehyde under subatmospheric pressure at 80°C. COSPAR (Committee of Space Research) Technique Manual Series No. 4 (Sneath, P. M. A., Ed.), pp. 151-155. COSPAR Secretariat, Paris. Alder, V. G. & Mitchell, J. P. (1970). Recent developments on the use of sub-atmospheric steam and formaldehyde at 80°C for the disinfection of cystoscopes. British Hospital (3 October). Journal and Social Service Review 19441946 Alder, V. G. & Simpson, R. A. (1982). In Principles and Practice of Disinfection, Preservation and Sterlisation (Russell, A. D., Ayliffe, G. A. J. & Hugo, W. B., Eds), Blackwell Scientific Publications Ltd, London. Alder, V. G., Brown, A. M. & Gillespie, W. A. (1966). Disinfection of heat-sensitive material by low-temperature steam and formaldehyde. Journal of Clinical Pathology 19, 83-89. Alder, V. G., Gingell, J. C. & Mitchell, J. P. (1971). Disinfection of cytoscopes by subatmospheric steam and steam and formaldehyde at 80°C. British Medical Journal 3, 677-680. Bowie, J. H., Kelsey, J. C. & Thompson, G. R. (1963). The Bowie and Dick Autoclave Tape Test. Lancet 1, 586. Cripps, N., Deverill, C. E. A. & Ayliffe, G. A. J. (1976). Problems with low temperature steam and formaldehyde sterilizers. Hospital Engineering 19, 9-10. Dahlstrom, H. (1967). Disinfection by low temperature steam. British HospitalJournal and Social Science Review 1208-I 2 15. Deverill, C. E. A. & Cripps, N. F. (1981). Tests on a low temperature steam and formaldehyde autoclave: The Miniclave 80. Journal of Hospital Infection 2, 175-190. Ecker, E. E. & Pillemer, L. (1938). Pressure cooker sterilizer for urologic instruments. Modern Hospital 50, 8687 (May). Esmarch, E. (1902). Die Wirking von Formalinwasser/dampen in Disinfectionsapparat. Hygenische Rundschau 12, 961-970. Gibson, G. L. (1977). Processing urinary endoscopes in a low temperature steam and formaldehyde autoclave. Journal of Clinical Pathology 30, 269. Line, S. J. & Pickerill, J. K. (1973). Testing a steam-formaldehyde sterilizer for gas penetration efficiency. Journal of Clinical Pathology 26, 716-720. Marcos, D. & Wiseman, D. (1979). Measurement of formaldehyde concentrations in a subatmospheric steam-formaldehyde autoclave. Journal of Clinical Pathology 32, 567-575. Mitchell, J. P. & Alder, V. G. (1975). The disinfection of urological endoscopes. British Journal of Urology 47, 571-575. Nordgren, G. (1939). Investigations on the sterilization efficacy of gaseous formaldehyde.
Acta Pathologica et Microbiologica Scandinavica, Supplement 40. B. (1973). Central Sterilising Club, 15th Meeting, Reading, pp. 1 I-12. J. K. (1975). A practical system for steam formaldehyde. Sterilizing Laboratory Practice 401-404 (June).
Nystrom, Pickerill,
Weymes, C., White, J. D. & Harris, C. (1975). Studies in the use of low concentrations of formaldehyde with steam at sub-atmospheric pressures as a method of sterilizing nonporous heat sensitive items. In Note 4, Greater Glasgow Health Board Sterilisation Centre. Weymes, C. (1977). Low temperature steam and formalin. (A paper given to the Sterilizing Club Meeting, Bristol, March 1977). Association of Sterile Supply ManagersJournal 6 (2), 8-10 (November).