The sterilization of dressings.

The application of moist heat in an autoclave is the best method of sterilizing dressings. In order to kill the most resistant pathogenic spores they must be exposed for at least 20 minutes to saturated steam at 1210 C. (Ministry of Health, 1955). At this temperature pure steam exerts a pressure of 15 lb./sq. in. The steam should be dry but at the phase boundary between liquid and vapour, so that it condenses readily and transfers its latent heat to cooler objects. The efficient removal of air from the autoclave chamber and its contents is essential for satisfactory sterilization. Air trapped inside dressings may hinder penetration by steam, and also may cause, temporarily, a condition of unsaturation (Savage, 1937). Unsaturation may also be caused in other ways, for example, by allowing the temperature of the autoclave jacket to rise much above that of the chamber. Air may be removed from the chamber and its contents before sterilization either by forced evacuation or by downward displacement. Evacuation is usually carried out by means of a steam ejector which creates a partial vacuum of 15 to 20 in. Hg, once or twice, the air being replaced each time by steam. A more efficientand more costly-pump can be employed to give a partial vacuum of 26 to 28 in. Hg. In the downward displacement method the air, being heavier than steam, gravitates to the bottom of the chamber and thence escapes through a vent. A thermometer in the escape line shows when the steam in the bottom of the chamber is free from air, since the presence of air reduces the temperature below that of pure steam at the operatingpressure. Evacuation cannot remove all the air from a mass of dressings, since the vacuum is not complete. Even if the evacuation is repeated, there will still be residual air which may be compressed finally by the steam to a position at the centre of the mass (Savage, 1937). Downward displacement is preferred by some authorities (Walter, 1948; Bowie, 1955), but, as Walter points out, its efficiency must depend in part on the method of packing and loading the dressings. The authors have felt that reliance on the thermometer in the steam escape line to show that the dressings are free from air might lead to error, since air might escape so slowly from the dressings that the thermometer would not be sensitive enough to detect its presence. This possibility would be greater if the dressings were badly packed and the packages or drums few. For the commercial sterilization of dressings Perkins (1956) states that an efficient pump, capable of evacuating up to 26 to 28 in. Hg, provides the best means of removing air. Some manufacturers of autoclaves recommend a combined method consisting of double evacuation up to 18 to 20 in. Hg, followed by continuous venting during sterilization. The use of metal drums to hold dressings, towels, etc., is open to criticism. Heat penetration is slower than in fabric-wrapped packages (Walter, 1948). The use of drums encourages overpacking (Howie and Timbury, 1956). Although a good deal has been written about methods of autoclaving dressings, comparatively little has been done to evaluate them experimentally. In particular, there is a lack of evidence to support some of the statements which have been made about the merits of different ways of removing air. In the work reported here residual air trapped inside dressings has been measured and temperatures recorded at points inside the autoclave and its contents, and the efficiency of steriliz. ation has been assessed under various conditions by means of heat-resistant spores and chemical tests. The objects were to compare the efficiency of sterilization after the removal of air by double evacuation with that by downward displacement, to compare drums with packages, and chemical with bacteriological tests of sterilization.