EXPLOSION HAZARDS DURING GENERAL ANESTHESIA AND ENDOSCOPIC SURGERIES :
Although the use of inflammable anesthetic agents has declined greatly over the last two or three decades, ether is still used in some countries, the risk of fires or explosions has not been eliminated. However, bowel gas consisting of methane, hydrogen and hydrogen sulfide is highly flammable. Operating room supplies that my be combustible include endotracheal tubes, oxygen catheters, surgical drapes, benzoin aerosol, alcohol cleansing solutions, and even petroleum based ointments.
Fires are produced when fuels undergo combustion. A conflagration differs from a fire in having a more rapid and more violent rate of combustion. A fire becomes an explosion if the combustion is sufficiently rapid to cause pressure waves that, in turn cause sound waves. If these pressure waves possess sufficient energy to ignite adjacent fuel, the combustion is extremely violent and termed a detonation.
Fire requires three ingredients:
The modern volatile anesthetic agents are non-flammable and non-explosive at room temperature in either air or oxygen.
Oils and greases are petroleum-based and form excellent fuels. In the presence of high pressures of oxygen, nitrous oxide or compressed air, these fuels may ignite spontaneously, an event termed dieseling (an analogy with the diesel engine). Thus oil or grease must not be used in compressed air, nitrous oxide or oxygen supplies.
Surgical spirit burns readily in air and the risk is increased in the presence of oxygen or nitrous oxide. Other non-anesthetic inflammable substances include methane in the gut (which may be ignited by diathermy when the gut is opened), paper dressings and plastics found in the operating theatre suite.
Ether burns in air slowly with a blue flame, but mixture of nitrous oxide, oxygen and ether are always explosive. It has been suggested that if administration of ether is discontinued 5 min before exposure to a source of ignition, the patient’s expired gas is unlikely to burn provided that an open circuit has been used after discontinuation of ether.
The stoichiometric concentration of a fuel and oxidizing agent is the concentration at which all combustible vapour and agent and agent are completely utilized. Thus the most violent reactions take place in stoichiometric mixtures, and as the concentration of the fuel moves away from the stoichiometric range, the reaction gradually declines until a point is reached (the flammability limit) at which ignition does not occur.
The inflammability range for ether is 2-82% in oxygen, 2-36%in air, and 1.5-24%in nitrous oxide. The stoichiometric concentration of ether in oxygen is 14% and there is a risk of explosion with ether concentrations of approximately 12-40% in oxygen. In air, the stoichometric concentration of ether is 3.4% and explosion do not occur.
SUPPORT OF COMBUSTION
It should always be remembered that as the concentration of oxygen increases, so does the likelihood of ignition of a fuel and the conversion of the reaction from fire to explosion.
Nitrous oxide supports combustion .During laparoscopy, there is a risk of perforation of the bowel and escape of methane or hydrogen into the peritoneal cavity. Consequently, the use of nitrous oxide to produce a pneumoperitoneum for the procedure is not recommended; carbon di111oxide is to be preferred, as it does not support combustion (and, in addition, has a much greater solubility in blood than nitrous oxide , thereby diminishing the risk of gas embolism)
SOURCES OF IGNITION
The two main sources of ignition in the operating theatre are static electricity and diathermy.
Electrostatic charges are produced on non-conductive material , such as rubber mattresses, plastic pillow cases and sheets, woolen blankets, nylon, terylene, hosiery garments, rubber tops of stools and non-conducting parts of anesthetic machine and breathing systems.
Diathermy equipment has now become an essential element of most surgical practice. However, it should not be used in the presence of inflammable agents.
Other sources of ignition
Prevention of static charges
Where possible, antistatic conducting material should be used in place of non-conductors.The resistance of antistatic material should be between 50k Ohms cm-1.
All material should be allowed to leak static charges through the floor of the operating theater. However, if the conductivity of the floor is too high, there is a risk of electrocution if an individual forms a contact between mains voltage and ground. Consequently, the floor of the operating theater is designed to have a resistance of 25-50 k ohms. When measure between two electrodes place I m apart. This allows the gradual discharge of static electricity to earth. Personnel should wear conducting shoes, each with a resistance of between 0.1 and 1 M Ohms.
Moisture encourages the leakage of static charges along surfaces to the floor. The risk of sparks from accumulated static electricity charges is reduced if the relative humidity of the atmosphere is kept above 50%.
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