Fires and Explosions In OR and ICU By Professor Dr. Mohammed Saeed Abd El Aziz

General principles Combustible agent or fuel + O ₂ or oxidizing agent → reaction products + energy. Activation energy Heat energy → ↑ temp. of the mixture which when exceed certain level → self sustained 2 C ₃ H ₆ + 9 O ₂ → 6 CO ₂ + 6 H ₂ O+ Energy Cyclopropane to O ₂ 18% or 2:9 → Stoichiometric concentration. 2 Vols + 9 Vols → 11 Vols

It is the concentration at which all the combustible vapor and oxidizing agent are completely used up → Stoichiometric concentration. Stoichiometric mixtures→ the most suitable mixture for ignition. Different mixtures → less violent reaction or no ignition. Flammability limit is the limit outside which the mixture will not burn.

Cyclopropane Lower flammability limit upper flammability limit 2.5% 2:9 63% 18% In O ₂ 2.5% 4.3% 100% In air Lower flammability limit upper flammability limit limit stoichiometric limit concentration No ignition Fire Explosion Fire No ignition

Fire Explosion Speed low high Pressure 1 bar 25 bar Temp C ⁰ 3000 C Light Less More Sound waves minimal maximal In normal anesthetic practice, cyclopropane burns but not explodes in air. Flammability range is reduced and stoichiometric conc. Is altered. The inert nitrogen molecules absorb energy+ does not take part in the reaction; reaction is less violent.

2C ₃ H ₆ + 9O ₂ +36 N ₂ → 6 CO ₂ + 6 H ₂ O+ 36 N ₂ + Energy _________ 9( O ₂ + 4 N ₂ ) Nitrous oxide is an oxidizing agent, rapidly breaks down in the presence of heat of fire or explosion to release O ₂ →33 % O ₂ mixture→ more energy 2 N ₂ O = 2 N ₂ + O ₂ +Energy ie, more violent reaction in the presence of N ₂ O and release more energy.

Other Flammable Agent Ether Lower flammability limit upper flammability limit Stoichiometric conc. 2% 14% 82% 100% in O ₂ 2% 34% 100% in air Ether does not explode in air but burns. The conc. Of ether in oxygen used in anesthesia→ within limits for explosion.

If liquid ether split→ high conc. Of ether vapour near the floor→ approach high flammability limit in air → burn→ cool flames, it is invisible and at lower temp. level. Ether→ CO ₂ + H ₂ O, this in normal reaction, but in Cool flames → oxidation is partial→ acetic acid and acetaldehyde. Cool flames→ transmit risk of ignition to anesthetic circuit (breathing system) Flame : is a combustion associated with emission of light and heat, a- static (confined) b- self- propagated. The Fire Triad a-Fuel b- a source of ignition or heat. C- an oxidizer

Other Flammable Agents (Fuel) Ethyl Chloride Ethylene Vinyl ether, they are less commonly used. Halothane and trichloroethylene → suitable mixture and suitable temp.→ ignition but not in concentration used in anesthetic practice. Anesthetic agents now are not flammable but other agent, like ethyl alcohol, are flammable. Saturated vapour of alcohol in air in normal room temp. 6% mixtures

Stoichiometric conc 0% 3% 6% 19% alcohol in ↓ air Fire risk of saturated vapour at room temp. Surgical spirit Methane- hydrogen in patient’s gut with diathermy→ fire. Ethyl chloride→ anesthetic spray. Paper, plastic drapes, dressings. Pure O ₂ + pressure +high speed → high fire and explosion risk, oil and grease must be avoided in any O ₂ supply system.

Activation Energy and Ignition Temperature Combustible agent + O ₂, they are apart by repulsive forces and in order to burn molecules should be close enough to react → ↑ the speed of collision →↑ temp. by spark or some source of heat ( enough heat to reach activation energy). Ignition temperature:- the temperature at which a mixture must be raised for combustion to start. It varies with components and proportion of potentially explosive mixture.

Stoichiometric conc. Of many explosive mixture temperature is about 400 C ⁰ but ether-O ₂ mixture is low as 150 C ⁰. Spirit lamb, diathermy, Lasers→ ignite flammable materials. Sparks→ activation energy→ activate an ether O ₂ explosion when present in stoichiometric conc. (spark is around 1 microjoule in value but it has a high voltage, it is more risky than hot wire or open flame)

Static Electricity Important source of sparks Conductive floor, low resistance → ↑ electric risk, high resistance↑ build up of static electricity. Antistatic rubber. Soled shoes contain carbon. ETT conduction by humidified gases passing through it.

Equipments→ sparks or static electricity. 2 types of flammable mixtures -mixtures with oxygen or nitrous oxide as oxidizing agent. -mixtures with air, activation energy is less in the first one. Flammable mixtures near anesthetic machines 5cm-25cm. Category of equipment →APG (avoid ignition with O ₂ or nitrous oxide- green band) AP (avoid ignition with air- green circle)

Types of operating room fires A -Fires in the patient 1- Laser a- upper airway surgery b- lower airway surgery 2-Electrocautry a- during tonsillectomy b- during laparoscopic surgery c- during tracheostomy. B- Fires on (outside) the patient 1- During monitored anesthesia care(MAC) for head and neck surgery 2- During skin preparation using isopropyl alcohol.

Safety precaution to prevent fires and explosions 1- Avoidance of the use of flammable anesthetic agents ( C-F bond) decrease flammability. The use of sources of ignition such as diathermy or laser under controlled conditions. Use the Iow-flow or closed circuits. Allow good ventilation of OR. Use a well functioning scavenging system. 2-Classification of equipments according to the zones of risk: 5 cm zone (anesthetic mixtures is high and oxidizing agent O2 or N2O), 25 cm zone (anesthetic mixtures is low and oxidizing agent is air), APG( 5cm) and AP (25 cm) equipment

3-Prevention of electric sparks PPM (periodic preventive maintenance) Spark –proof mercury switches. Dry battery. 4-Prevention of static electricity (antistatic materials) Antistatic floors, antistatic breathing circuits, antistatic cloths, antistatic wheels, antistatic rubber-soled shoes, relative humidity less than 50% and grounding or earthing. 5-Fire-fighting equipments (fire extinguishers) Should be available all the time, class A –B- C e g, Halon fire extinguisher, CO2 extinguishers and pressurized water. 6- Precaution during laser surgeries.

Extinguishing a fire 1-Remove oxidizing agents. 2-Remove burning drapes. 3-Use a fire blanket. 4-Sprinkler system. 5-Halton fire extinguishers are used.

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