DEFLAGRATION occurring in a respiratory circuit can be fatal to the patient directly connected to the system. 1Although the circuit is often filled with oxygen at a high concentration, lack of an igniting source and fuel make combustion impossible. Therefore, it is extremely rare to see combustion around an anesthetic machine in the absence of flammable anesthetics. 2,3Here we report a combustive accident, in which an expiration valve in an anesthetic machine was destroyed and flame was observed inside the expiratory limb of the circuit.
An 18-yr-old man was anesthetized with sevoflurane-nitrous oxide during orthopedic surgery on his left clavicle. Anesthesia was induced with propofol (2 mg/kg), and tracheal intubation was facilitated with vecuronium (120 μg/kg). Anesthesia was maintained with 1.5–2.0% sevoflurane, nitrous oxide, and oxygen. The lungs were ventilated using the ventilator incorporated in the anesthetic machine (Cato; Draeger Inc., Lubeck, Germany).
Immediately after the completion of the surgery, administration of sevoflurane and nitrous oxide was discontinued. Oxygen was delivered to the respiratory circuit at 6 l/min. Seven minutes after the start of ventilation with 100% oxygen, exudate trapped in the endotracheal tube was suctioned. At that time, the anesthetist sprayed Xylocaine® pump spray (AstraZeneca, London, UK) twice (approximately 0.2–0.3 ml) to lubricate the internal surface of the tube. The patient was ventilated again with 100% oxygen under intermittent positive pressure ventilation mode. Three minutes after suctioning, the patient opened his eyes in response to verbal command and started breathing spontaneously. The ventilation mode of the anesthetic machine was then changed from intermittent positive pressure ventilation to “MAN./SPONT.”
Immediately after this action, the anesthetist and a circulating nurse heard a moderately loud explosive sound. They looked at the origin of the sound, a connecting port between the anesthetic machine and the respiratory circuit, and found that a disc valve in the expiratory limb had been destroyed (fig. 1). The dome cover was intact, and there was no gas leakage from the circuit. Approximately 10 s after the explosive sound, an orange, ring-shaped flame ran up from the destroyed valve toward the endotracheal tube for approximately 15 cm. The flame stopped 10 cm from the air filter (ARNK F001; Draeger Inc.) inserted between the anesthetic machine and the respiratory circuit and disappeared. The anesthetist disconnected the circuit from the endotracheal tube immediately. The anesthetist and the nurse smelled a scorched odor originating from the circuit. Since the patient was awake and breathing normally, the endotracheal tube was extubated. The patient had no airway problems and did not complain of dyspnea or chest discomfort. The machine and the circuit were left as they were to be subjected to a technical inspection by the manufacturer.
Draeger Inc., Lubeck, Germany) was destroyed. Although the valve was not functional, the dome cover was intact, and there was no gas leakage from the circuit. (A ) A positive end-expiratory pressure (PEEP) valve scorched and deformed by high temperature (small inset shows intact PEEP valve). (B ) Schematic illustrations of the whole anesthesia circuit and the assembly of expiratory valve unit. Line with arrowheads indicates the parts that were deformed by high temperature.
Inspection by the Manufacturer
Three components are required for fires and explosions to occur: an oxidizing gas to support combustion, a flammable material, and a source of ignition. In the current case, the gas that supported combustion was unquestionably the 100% oxygen flowing in the respiratory circuit. The laboratory of the manufacturer scrutinized the respiratory circuit for a possible source of ignition and the material that was burned in this accident. In the respiratory circuit, there was only one part that was acting at high temperature, a platinum wire in the flow sensor. During normal operation, the temperature of this sensor is 140°C. When the wire becomes worn out after long use, it is possible for it to reach temperatures greater than 900°C; however, the wire itself will not start burning even in an atmosphere of 100% oxygen. Although the fact that the wire is incombustible was confirmed in an experiment in a Draeger laboratory, the condition of the wire before the accident could not be revealed by examination of the machine.
The burnt parts taken from the machine were investigated. Flow sensor and air filter set in the expiratory limb were deformed, probably by high temperature. The inside of the air filter was scorched and tainted with ash. Examinations with gas chromatography and mass spectrography revealed silicone oil (maybe used on O-ring) and di-t-butyl-phenol (an antioxidant for plastics). However, no residue of the burnt material could be identified. It was considered possible that the material was completely burned out, leaving no residue. Analyzing the sequence of events, inspectors suspected that Xylocaine pump spray was the inflammable material present at the scene of this accident. The solution in Xylocaine pump spray contains 241 mg/ml ethanol. Explosive limits for ethanol in air at 1 atm is 3.5–15%, and the ignition temperature is 363°C. Flash point is 13°C. During an oxygen-rich condition, ethanol is more flammable than under air atmosphere. Platinum can act as a catalyst and decrease the ignition temperature of ethanol. Accordingly, the ethanol contained in Xylocaine pump spray can be ignited when the vapor mixes with oxygen at a concentration between the specified limits and makes contact with an ignition source.
From the analysis of the sequence of events and the results of the manufacturer's laboratory investigations, a possible scenario of the accident was proposed as follows:
Ethanol contained in Xylocaine pump spray was vaporized in the warm expiratory gas from the patient, which consisted mainly of oxygen.
The gas—oxygen-rich and containing ethanol within explosion limits—reached the expiration gas analysis unit equipped with a hot platinum wire as a part of the flow sensor.
At the time of contact, the gas was ignited by the high temperature and catalyzing action of the platinum wire.
High temperature during the deflagration damaged the expiratory port valve made of incombustible ceramic. During the patient's spontaneous breathing, a flame of burning ethanol ran backward to the endotracheal tube because the expiratory valve was not functioning.
Under high-flow gas, temperature decreased quickly, and the damage was limited near the part of ignition.
Although the aforementioned scenario is a reasonable sequence of events, there is some uncertainty. Could the two pumps of Xylocaine pump spray (approximately 0.1 ml × 2) supply enough fuel for the deflagration? How was the concentration of ethanol in the oxygen-rich expiration gas set between the explosion limits? It is plausible that a large volume of expiration gas should quickly dilute the inflammable ethanol to a very low concentration below the lower limit of explosion. However, we could not deny the possibility that a gas deposit containing ethanol (highly volatile and relatively heavy; vapor pressure at 20°C is 5.8 kPa, and relative density of air mixture is 1.03) within the explosive limits (3.5–15%) was composed accidentally in a part of respiratory circuit.
The anesthetic machine presented in this report does not meet the criteria of an APG machine, which can be used with inflammable gas, such as ether or cyclopropane. The User Instruction Manual clearly prohibited use with any inflammable gases. The Xylocaine pump spray, which was speculated as a source of fuel in this accident, contains ethanol at a high concentration. The label attached to the bottle clearly noted that the solution is flammable and prohibited use under flammable conditions. The anesthetist working in this case knew of these warnings; however, he did not consider that Xylocaine pump spray solution sprayed into the endotracheal tube could flow into the machine with minimal dilution and make contact with an ignition source. Although ether or cyclopropane not commonly used in modern operation rooms, there are still several risks of fire relating to anesthesia practice. 3Recently, Barker and Polson 4reported a fire in the operating room, in which isopropyl alcohol used for surgical preparation was suspected as a fuel.
It is impossible to eliminate all sources of ignition and inflammable materials from anesthesia practice. Anesthetists should be cautious of deflagration accidents even in sophisticated modern operating rooms. Although the patient in this case did not suffer an injury from the accident, fires around an anesthetic machine and the respiratory circuit tend to cause serious injury or have mortal consequences.