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Special Articles

STERILISATION BY STEAM UNDER

INCREASED PRESSURE

A THIRD COMMUNICATION TO THE MEDICAL RESEARCH COUNCIL

BY THE WORKING PARTY ON PRESSURE-STEAM STERILISERS *

* The members of the working party are: Dr. J. W. HOWIE, chair-man, Dr. V. D. ALLISON, Dr. J. H. BOWIE, Mr. E. A. BRUGES,PH.D., Dr. E. M. DARMADY, Dr. R. J. FALLON, Prof. ROBERTKNOX, M.D., Dr. J. A. V. SHONE, Mr. G. SYKES, M.SC., Prof.C. A. WELLS, F.R.C.S., Mr. G. A. P. WYLLIE, PH.D., Dr. J. C.KELSEY, secretary. Address: Disinfection Reference Labora-tory, Central Public Health Laboratory, Colindale Avenue,London, N.W.9.

IN 1957 the Council set up a working party to definethe performance required of steam sterilisers, and toconsider how this could be obtained. Its first com-munication (1959)1 examined the requirements forefficient sterilisation by steam, described the principlesand technical procedures involved, and discussed their

practical application; the second communication (1960)2commented briefly on four matters that had arisen

during the intervening year. In this third report, the

working party comments on advances made in the past3 years, describes the present situation, and discusses ingeneral terms the future of steam sterilisation.

Sterilisation of Fabrics by High-vacuum TechniquesAfter the publication of the working party’s recommenda-

tions and of British Standard Specifications,3 numerous high-vacuum sterilisers were manufactured. Reports on their efficacywere slow to appear, but when tests could be made it wasfound that many sterilisers were not satisfactory. 4 5 comparisonof the temperatures recorded by thermocouples placed free inthe chamber and inside a test pack showed that penetration bysteam was, in certain circumstances, far from instantaneous.Particular difficulties arose when a small load was sterilisedin a large chamber 4 6; this has come to be called the " small-load effect". It now appears that the important factors arethe amount of air actually left in the chamber and the load atthe end of the prevacuum stage. This quantity will be excessiveif the pumping down is inadequate, or if air can leak in throughfaulty door seals or valves between the end of the prevacuumstage and the attainment of atmospheric pressure after theadmission of steam.4 5 The amount of air which may be leftwithout preventing the effective penetration by steam of aparticular load will depend upon the fraction of the totalvolume of the chamber which is occupied by that load. Thesmall-load effect is probably caused by the concentration withinthe load of a proportion of the residual air by entrainmentwith the steam as the latter enters the load to heat it up (theinterior of the load receives very little heat by conductionbecause of its low thermal conductivity). The mixture of airand steam will be carried into the load, and when the steamcondenses the air will be left within it.4 The smaller the sizeof the load relative to the chamber the greater will be the con-centration of the air trapped within it, and this can only escapeby the slow and uncertain process of downward displacement.Recent work by Knox and Pickerill,8 in which the technique ofAlder and Gillespie 9 was used to determine by direct measure-ment the amount of residual air, tends to confirm this inter-pretation of the small-load effect.

1. Lancet, 1959, i, 425.2. ibid. 1960, ii, 1243.3. B.S. 3219: 1960, Horizontal Cylindrical Hospital Sterilisers; B.S. 3220:

1960, Horizontal Rectangular Hospital Sterilisers.4. Bowie, J. H. in Symposium on Recent Developments in the Sterilisation

of Surgical Materials; p. 109. London, 1961.5. Darmady, E. M., Drewett, S. E., Hughes, K. E. A. J. clin. Path. 1964,

17, 126.6. Harris, H. F., Allison, V. D. Lancet, 1961, ii, 603.7. Fallon, R. J. J. clin. Path. 1961, 14, 666.8. Knox, R., Pickerill, S. K. Lancet, 1964, i, 1318.9. Alder, V. G., Gillespie, W. A. J. clin. Path. 1957, 10, 299.

Unsatisfactory experience with some of these high-pressure/high-vacuum sterilisers has led some workers to recommendthat in every cycle of operations the temperature conditionsshould be monitored from within the load 4 7 or that an inte-

grator should be provided to extend the sterilising time ifnecessary.4 Attempts have also been made to facilitate airelimination by improving the degree or the duration of theprevacuum period. Experience has shown that these measuresoften fail, and better results have been obtained by steaminjection, either by drawing a vacuum followed by the admissionof steam and afterwards by drawing a vacuum for a secondtime or by a continuous or pulsed injection of steam into thechamber during the pumping-down process.4 10-12 In this waythe air in the chamber is diluted out with steam until theresidual amount is not enough to interfere with steam penetra-tion into a small load when the pressure is raised during thesterilising stage. The theory of this technique and the physicsof high-vacuum sterilisation generally have been discussed atlength. 12-14A number of workers who realised the practical difficulties

of monitoring each cycle from within the load have soughtto design a

"

dummy load " which would reflect conditionswithin the actual load but which could be left in situ withthe sensing element permanently within it. Most of these

dummy loads have proved unsatisfactory, but a " loadsimulator " has recently been developed (K. M. Henfrey,personal communication). In the experience of members ofthe working party, this can be relied upon, when it is used aspart of the control system of an automatic sterilising cycle,to ensure that satisfactory sterilising conditions are achievedwithin the load.

Test for the Efficacy of High-vacuum SterilisersBecause of these findings and the rapidity with which

techniques are changing, these sterilisers should not be

judged on the actual cycle of operations employed but ratheron their ability to carry out functional tests consistently. Theseare now usually based on the use of the standard test packdescribed by Bowie. Huckaback towels are folded to give acube of approximately 1 ft. in each dimension, and packed in adressing casket (B.s.s. 3281), a box of perforated metal or card-board, or a fabric wrap; temperature readings inside and out-side the test pack are then compared during a standard cycleusing thermocouples (or other thermometric devices). If theremoval of air is adequate and the penetration by steam is

virtually instantaneous, the temperature recorded in thechamber drain becomes a true indication of the conditionswithin the load. These two factors-the temperature attainedand the time for which that temperature is held-provide thebasis for assessing the efficiency of the sterilising process. Thethermocouples and leads must be so designed and arrangedthat no artificial heat path is formed along which steam cantrack into the test pack. This test is essential before purchaseof equipment, and thereafter whenever possible. Furtherdetails appear in an appendix to a Hospital TechnicalMemorandum. 15

When thermocouples are not readily available for routine usethe Bowie-Dick autoclave tape test for steam penetration shouldbe used.16 A cross of heat-sensitive tape on a sheet of paperis placed at the centre of the test pack. Inadequate eliminationof air is indicated by a central zone within which the colour ofthe tape is less intense than that at the periphery. This zone

corresponds to a central bubble of residual air. This is solely atest for penetration by steam; the temperature achieved and thetime for which it is held must be inferred from the record ofthe temperature within the chamber drain. The value of the

10. Henfrey, K. M. Hosp. Engr. 1961, 15, 260.11. Wilkinson, G. R., Peacock, F. G. J. Pharm. Pharmacol. 1961, 13, suppl.

67T.12. Henry, P. S. H., Scott, E. J. appl. Bact. 1963, 26, 234.13. Henry, P. S. H. ibid. 1959, 22, 159.14. Rowe, T. W. G., Kusay, R., Skelton, E. in Transactions of the VIIIth

International Vacuum Symposium and IInd International Congress,1961 (edited by Luther E. Preuss); vol. I. Oxford, 1962.

15. Ministry of Health. Hospital Technical Memorandum no. 10. H.M.Stationery Office, 1963.

16. Bowie, J. H., Kelsey, J. C., Thompson, G. R. Lancet, 1963, i, 586.

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test depends upon careful attention to the details originallydescribed. In particular, two misconceptions have arisen

concerning the test. First, it is the uniformity of colour acrossthe tape and not the intensity that is significant; penetrationafter a satisfactory prevacuum can be shown by a faint butuniform colour change following an exposure to steam for aslittle as 10 seconds. Secondly, if the exposure period isextended beyond 31 ’1, minutes a false-positive test result will beobtained: the air bubble can be displaced by gravity or, if itremains, long exposure to a lower temperature than 134°C willcause the colour of the tape to change. If the Bowie-Dicktest is unsatisfactory at 31/2 minutes, there can be no doubtthat the elimination of air is inadequate and that the steriliseris operating, if at all, by downward displacement. This slow anduncertain process, if accepted, will require a greatly extendedexposure period; it must be recognised that the necessaryextension of time even if it results in a uniformly colouredtape indicates the inadequacy of the apparatus. If sterilisersmust be so operated for a limited period pending repair or up-grading, the sterilising process should be constantly checkedby thermocouple measurements or by the liberal use of Browne’s stubes, as described in the first report.The Bowie-Dick test has also been found useful by engineers

as a rapid means of detecting air leaks or pump failures, and inassessing the success of any corrective measures they mayapply.

In addition to these tests for steam penetration, the sterilisershould be tested for air leaks by drawing and holding a vacuumwithin the chamber, without steam injection and noting anyrise in the absolute pressure.4 5 7 This test is probably bestcarried out under engineering supervision; it too is describedin the Hospital Technical Memorandum.15

Damage to Fabrics and RubberAs foreshadowed in the second report, it has now been

shown that high-vacuum, high-temperature sterilisation

probably causes less damage to rubber gloves, anaestheticrubberware, and fabrics than did the classical proceduresof downward displacement or partial evacuation. It now

appears that it is the oxidation associated with lengthy dryingprocesses rather than exposure to steam that is the cause ofdeterioration. 17 18 a

Sterilisation of Unwrapped Instruments and BowlsThere is now abundant evidence that these items can be

quickly and safely sterilised by high-speed downward dis-placement techniques in comparatively simple apparatus. It isdesirable, however, that instruments should be clean and open,and that those with a long narrow lumen should have thisflushed out with water just before processing.

It has recently been suggested that unwrapped instrumentsand bowls could be sterilised by an ultra-rapid high-tempera-ture process 19: if this is borne out by subsequent experiencesuch sterilisers would combine speed with simplicity andreliability.An interesting technique is being developed whereby all the

instruments and fabrics needed for an operation are pre-setand pre-sterilised in special trays which are ready for instantuse as required; the trays then become the tops of surgicaltrolleys for dispensing the instruments to the surgical team.2OThese trays contain fabrics, and therefore a high-vacuumsteriliser is required. If this technique is adopted, arrange-ments for providing sterile instruments for operating-theatreswill need to be reconsidered.

Sterile Water for Operating-theatre UseIn its first report the working party recommended the use of

water autoclaved in separate bottles. This recommendationhas been confirmed by experience. Some of these sterilisers

17. Fallon, R. J., Pyne, J. R. ibid. p. 1200.18. Henry, P. S. H. J. appl. Bact. 1964 (in the press).19. Lane, C. R., Cook, G. T., Kelsey, J. C., Beeby, M. M. Lancet, 1964, i,

358.20. Bowie, J. H., Campbell, I. D., Gillingham, F. J., Gordon, A. R. Brit.

med. J. 1963, ii, 1322.

can be made to work satisfactorily, but only for limited periodsand only with the expenditure of much effort in bacteriologicaltesting and continual maintenance. In a survey of over 150such devices, up to 40% of the classical type of tank steriliserup to 31 % of a more sophisticated design, and up to 42% ofpiped systems failed bacteriologically at least once during three3-month periods of weekly examination by a quantitativemethod designed to distinguish genuine failure from chancecontamination. These episodes could usually, but not always,be attributed to inadequate or inept maintenance. There wasno obvious indication that the apparatus was faulty, and in theabsence of such warning even daily testing may thus fail toreveal the fact that contaminated water has been supplied duringthe 24-hour period during which the test cultures were beingincubated.2

Operational experience has shown that bottled sterile wateris safe to use and simple to provide, and that requirements arenot as great as was once thought.22-27 The working party hasno hesitation, therefore, in reaffirming its previous recom-mendation that sterile water for use in operating-theatres shouldbe sterilised in bottles, and that reliance should not be placedon tanks or piped systems for dispensing water in bulk,although there is room for improvement in the design ofbottles used for this purpose.

Sterilisers for Bottled FluidsIn its first report, the working party suggested that dis-

carded downward-displacement dressing sterilisers could becheaply converted for the sterilisation of bottled fluids, so

that costly new sterilisers need not be purchased.One significant advance has been the development of

sterilisers in which the cycle is not only automatic but can alsobe monitored by a thermocouple placed within one of thebottles of which the load is made up, or from an equivalentbottle permanently mounted inside the chamber. Thispractice is to be recommended and should be used for allmajor new installations.

Several devices are available for rapidly cooling the load,thus saving time and avoiding the possible danger arising fromthe premature removal of hot bottles.28 29

Use of Steam at Sub-atmospheric Pressures

Although the working party is concerned with steam underincreased pressure, reference must be made to a low-tempera-ture/high-vacuum technique now being developed.3O Thisconsists of a normal high-vacuum cycle, but with steam

admitted after the prevacuum only to subatmospheric pressuresso that the holding temperature is between 80° and 90°C. Bymeans of this technique, rapid penetration can be obtained intofabrics or tubular apparatus without causing significant damage.An exposure of 30 minutes is sufficient to disinfect blankets,clothing, rubberware, or plastic articles. Not only are vegeta-tive organisms killed but spores are significantly reduced innumber, for reasons that are not yet clear. Addition of formalinto the process results in the killing of spores in substantialnumbers within 1 to 3 hours.31 The necessary procedures canbe specified as an alternative cycle of operations for a high-vacuum steriliser, which in this way can be used for dis-

infecting bedding or for sterilising heart-lung machines andother heat-sensitive items.

STERILISATION POLICY

In its first report the working party drew attention tothe lack of understanding of the technical requirements21. Kelsey, J. C., Beeby, M. M. Lancet, 1964, ii, 82.22. Blacow, N. W. J. Hosp. Pharm. 1963, 20, 170.23. Behr, G., Tallett, E. R. Lancet, 1963, i, 489.24. Helliwell, P. J., Newman, T. H. ibid. 1962, ii, 1121.25. Hibbard, B. M., Hesketh, G. ibid. 1963, i, 57.26. Howie, J. W., Kelsey, J. C. ibid. p. 165.27. Wells, C., Lynch, P. F. ibid. p. 57.28. Bowie, J. H. in Symposium on the Operation of Sterilising Autoclaves;

p. 28. London, 1959.29. Wilkinson, G. R., Peacock, F. G., Robins, E. L. J. Pharm. Pharmacol.

1960, 12, suppl. 197T.30. Alder, V. G., Gillespie, W. A. J. clin. Path. 1961, 14, 515.31. Alder, V. G. ibid. 1964, 17, 198 (abstract).

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involved, the need for the final responsibility to be givento a specified member of the medical staff, and the

necessity for regular routine maintenance. All thesematters continue to be of the utmost importance. Itmust now be stressed that the purchase of steam sterilisersis not to be taken lightly but should be preceded by acareful reappraisal of sterilising needs, staff available, andengineering services. In its second report the workingparty drew attention to the potentially greater outputof high-vacuum sterilisers as compared with downward-displacement apparatus, and suggested that plannersshould take into account the possibility of concentratingsterilising capacity within a central sterile supply service.One c.s.s.D. now supplies 15,000 beds with ward dressingsand 85 operating-theatres with linen, by using a totalautoclave capacity of only 56 c. ft.; if required it could

supply 20,000 beds and 100 operating-theatres (Weymes 3zand personal communication). By such centralisation,not only is steriliser capacity used economically butmaintenance, testing, and supervision are greatly simpli-fied. It would also allow full advantage to be takenof automation, which cannot be long delayed fromthe hospital scene. In the opinion of the workingparty, serious consideration should be given to this

approach.When the purchase of steam sterilisers is contem-

plated, reference should be made to the appropriateBritish Standard Specifications 3 and to HospitalTechnical Memorandum no. 10.15

CONCLUSION

The working party was set up to define the performancerequired of steam sterilisers, and to consider how thiscould be obtained. Its members now consider that boththese aims have been achieved, and that responsibilityfor action must pass to the administrative field. It isnot for the working party to say what form this actionshould take, but in its absence patients will continue tobe subjected to unnecessary risk, and much valuabletime will be wasted because pathologists and surgeonshave to worry about matters of sterility which should bebeyond question in a well-conducted hospital.

fhe members of the working party would like to express thanks toall who have helped in their deliberations, particularly to Dr. P. S. H.Henry for advice about physical matters, and also to the representa-tives of steriliser manufacturers for making available much relevantinformation.

THE DECLINE OF PELLAGRA IN THE

SOUTHERN UNITED STATES

J. N. P. DAVIESM.D. Brist.

PROFESSOR OF PATHOLOGY,ALBANY MEDICAL COLLEGE, ALBANY, NEW YORK

THE first recorded outbreak of pellagra in the southernUnited States was in 1907 33; and by 1922, when regularrecords were started, the disease was common and wide-spread throughout the region. Between 1930 and 1933,however, the frequency of pellagra declined sharply, andtoday the condition is seen only in food faddists andalcoholics. The question is, however, why a deficiencydisease, classically associated with ignorance, poverty,and malnutrition, should have declined most dramatically

32. Weymes, C. Hospital, 1963, 59, 135.33. Searcy, G. H. J. Amer. med. Ass. 1907, 49, 37.

just when economic circumstances were at their worst.For in 1930 the United States was in the grip of the mostsevere economic, industrial, and agricultural depressionit had ever known.

KNOWLEDGE OF THE DISEASE

The first national conference on pellagra was held atColumbia, South Carolina, in 1912 when the U.S. PublicHealth Service began investigating the disease. In 1915

pellagra was induced in convict volunteers fed a restricteddiet; and by 1922 the black-tongue syndrome had beensimilarly induced in dogs and cured by supplementingtheir diet with meat, milk, and yeast. Brewer’s yeast insufficient amounts was then found to cure human pellagra,and from 1928 was provided, free of charge, by Stateand county health agencies and the American Red Cross.But the more potent liver extracts were not developeduntil 1932; and the pellagra-preventing factor, nicotinicacid, was identified only in 1937. Thus there seems to beno direct connection between medical and biochemicaladvance and the sudden rapid decline of the disease inthe South.

ECONOMIC AND SOCIAL BACKGROUND

The whole economy of the Southern States was basedon cotton, and ever since the Civil War the South hadbeen a more or less economically depressed area. Wheneconomic depression became general and cotton pricesfell, farmers tended to increase the acreage under cottonand hope that higher sales would compensate for lowerprices. In 1927, however, the southern farmer was worseoff than he had been in 1922, and his net average incomefell from about$874 in 1929 to$342 in 1931. A tenantshare-cropping system was general. Tenant farmers

mortgaged their prospective crop yields and, if cropsfailed or prices fell, could not repay the loans. Wageswere low and food prices high. Local stores could carryonly a narrow range of provisions and merchandise.Whenever food prices rose pellagra became more

widespread.Through poverty and force of habit, the populace

consumed a poor diet based on the three Ms-meal, meat,and molasses. The meal was a cornmeal, and the meatsalt pork and lard. In better times these were supple-mented by wheat meal, rice and dried beans. Very fewpeople cultivated a garden.

Before 1900 the corn was generally ground by smalllocal water-driven mills which produced a coarsely boltedor unbolted meal retaining most of the germ and hull ofthe grain. But, as large milling concerns took over, thiswas replaced by a finely bolted meal which looked better,tasted better, and kept better than the coarser meal butlacked the germ and its vitamins.

The pellagra situation deteriorated steadily in the1920s reaching its nadir in the years 1929-31. By 1932-34, however, there had been great improvement: thenumber of cases reported from the southern States fellby 58°,,0. The frequency of the disease remained at thislower level until, in the 1940s, pellagra virtually dis-

appeared. Mortality reached a peak in 1930 and hasfallen ever since. Thus the decline of pellagra in thesouthern States began and was most rapid in the years1930-33, the depths of the great depression. Sincethe development of pellagra takes time, the cause ofthis decline must have been in operation in 1930 orbefore.