The Sterility and Sterilization of Lung Ventilators.

Intermittent positive pressure breathing machines are in use for the treatment of many respiratory and cardiac conditions, but there is no agreement regarding the risk of cross infection and the necessity of sterilizing part, or all, of this equipment. Until recently, complete sterilization has been difficult to effect, and so reliance has been placed on boiling or chemical disinfection of the breathing tubing only. During an outbreak of unexplained post-operative respiratory complications in one respiratory unit amongst patients who had been ventilated on positive pressure breathing machines, the same organisms were found to be common to both the patients and the machines. We therefore investigated this to see whether, if machines become infected by patients, prevention is possible. For the experiments sterile Radcliffe positive negative ventilators were used. Complete sterilization can be achieved by exposing a machine to a mixture of 10% ethylene oxide gas in an atmosphere of carbon dioxide over a period of time, provided that the machine is run continuously during the process so that the gas permeates all parts (Baker and Seaber, 1962). The complete machine, with the breathing tubes detached and the humidifier empty, is placed in a large plastic bag, which is made gas-tight with special zip fasteners, and the electric cable is led to a suitable power point through a sleeve in the bag. Air is exhausted from the bag as far as possible by applying suction from a domestic vacuum cleaner. The bag is filled with the mixture of ethylene oxide and carbon dioxide and sealed, and the machine is run in this atmosphere for 12 hours. The apparatus is then removed and, with the machine still running, flushed through with air for two hours. The efficacy of this method has been checked by deliberately infecting machines with a 24-hr. broth culture of Staphylococcus aureus of known phage type, which was sprayed on to the inner surfaces of all components of the ventilators. After sterilization, swabs were taken from all the infected parts, plated on to a blood agar plate, and inoculated into tubes of nutrient broth. On no occasion was any growth detected, although controls set up at the same time and not exposed to ethylene oxide all showed growth. Flushing with air after sterilization was necessary in order to remove all traces of ethylene oxide. This gas is absorbed on to the surface of rubber and some plastic materials and might be given off into the breathing circuit later. The time of two hours (Fig. 1) was determined by frequent sampling of the effluent and estimating the ethylene oxide by a standard gas detector method (H.M.S.O., 1951).