Come on baby, light my fire
We've all heard the stories about the football team running around with their pants in flames. But why should intestinal gas be combustible? Is it a danger and if so how can we make it safer? Anjali Nataraja presents everything you ever wanted to know about flammable flatus but were too afraid to ask
"Turn it off!" was my greeting as I walked into the darkened living room one evening: I'd turned on the lights. I was surprised to see my three (male) flatmates dancing about with a box of matches. Apparently, as the only woman in the flat, I had been oblivious to the art that my dear friends had been perfecting.
I later discovered that they had been training for months. This night was the result of careful preparation with large amounts of curry and beer providing excellent substrate, I was informed. Unimpressed at the thought of any further delay to my latest (video taped) instalment of Eastenders, I was assured that I was "in for a real treat."
Balanced on the edge of the sofa, my three flatmates carefully leaned back and lifted their legs up in the air. Then each excitedly lit a match and held it to the seat of their trousers. We were still all in the dark, and, after a protracted countdown, I witnessed the greatest firework display I'd ever seen. Jets of blue flame (bigger and bluer than the flame on your grandmother's brandy soaked Christmas pudding) emanated from each of them in a perfectly synchronised performance.
This was more than two years ago, at the start of my career as a clinical medical student. Days later, I started my general surgery attachment with the colorectal surgeons. With this clear demonstration of the explosive nature of "bowel gas" in mind, I secretly began to worry about the possibility of ignition from sparks from the surgeon's diathermy, especially in the enclosed space provided by laparoscopic and endoscopic techniques. Trying to make a good impression on a new firm, I decided that this worry was probably not the first thing to mention when asked if I had "any questions." Since then, strangely enough, I haven't had the opportunity to air this concern.
I was relieved to discover, in the Christmas edition of the BMJ, that I was not the only one to be concerned with such "hot issues."1 Satisfied that I was not unnaturally obsessed, I decided to do a literature search--a little odd, I admit, but a distraction from the essay I was supposed to be researching. I discovered that explosion of colonic gas is a very real and serious, if little discussed, complication of bowel (particularly lower bowel) surgery.
Combustible gases (mainly hydrogen and methane) are produced throughout the gastrointestinal tract, and many reports describe explosion of these gases during surgery using electrocoagulation; one of the first cases was in 1944.2 Injuries sustained included ecchymoses and minor charring of organs at best; multiple perforations then sepsis and death at worst.
The presence of combustible gases in the digestive system is the result of digestive processes, bacterial fermentation, diffusion of gas into bowel from the blood stream, and swallowed air.3 Diet, digestion, and metabolism alter the quantities of these gases. Methane production increases after a meal of meat or bean; hydrogen production increases after milk is ingested.3 In their millennium edition of this experiment, my flatmates had carefully chosen a variety of different foods to establish which would best "improve" their performance.
The large bowel often contains more than 40% combustible gases by volume, with the maximum recorded concentrations 69% hydrogen and 56% methane. Kirk showed that human flatus may contain 44% hydrogen and 30% methane.4 The 12 hour fast before surgery and preoperative bowel cleansing help to reduce the quantities of these gases. The high protein, high carbohydrate milk drinks available on wards may have exactly the undesired effect.
Combustible gases, when mixed with oxygen (or air) in certain proportions will explode when ignited. The lower explosive limit for a gas is the lowest concentration of gas that will explode when mixed with air (4% for hydrogen; 5% for methane). A sufficient increase in the concentration of combustible gas renders the mixture non-explosive. The upper explosive limit is just below this point (72% for hydrogen; 15% for methane).
Neither hydrogen or methane is combustible in less than 5% oxygen. The use of oxygen and nitrous oxide in anaesthesia increase the concentration of these gases in the gastrointestinal tract. The insufflation of an inert gas, such as carbon dioxide (widely recommended in the articles, but seldom practised), narrows the explosive range and is used in three ways: (a) the combustible gas is completely displaced with carbon dioxide, (b) the inert gas is mixed with the combustible flatus in an amount to prevent an explosive mixture, or (c) placing a barrier of inert gas between inflammatory flatus and the ignition source (diathermy).
The overgrowth of bowel flora (in particular E coli) increases the combustibility of colonic flatus. Flatus is increased by the use of mannitol in bowel preparation. Mannitol is not digested or absorbed by the human small bowel, but colonic bacteria readily ferment it: the use of mannitol increases bacterial liberation of hydrogen fivefold.5 Considering this and the large number of explosions during surgery after manitol was used, it seems that mannitol is entirely unsuitable for bowel preparation where diathermy is to be used in an enclosed space.
Colonic combustibility may seem ridiculous, but it is a well recognised, if seldom mentioned, complication of certain procedures. This applies particularly to endoscopy and laparoscopy involving diathermy. High protein diets, mannitol as a bowel preparation agent, use of oxygen and nitrous oxide in anaesthetics, and the use of high voltage, high frequency diathermy may all increase the likelihood of an explosion. Preoperative fasting, bowel cleansing, antibiotics (to remove some of the bacteria that produce gas), and the insufflation of carbon dioxide during the operation may decrease this risk.
Humans produce a mere 200 ml of combustible gases a day (though given the radiance of the display I was treated to, I don't doubt that much more is possible). If you thought this was a lot, consider this: cows produce around 30 litres of such gases a day, making bovine endoscopy a very dangerous occupation.
Anjali Nataraja, final year medical student, University of Oxford
Email: anjali.nataraja@jesus.ox.ac.uk
studentBMJ 2002;10:131-170 May ISSN 0966-6494
- Chapman S. Hot air? BMJ 2001;323:1449.
- Lamblin A, Truffert L. L'explosion des gaz intestinaux au sigmoidienne mortelle. Arch Mal Appar Dig Nutr 1994;33:148-52.
- Levy E. Explosions during lower bowel electrosurgery. Am J Surg 1954;88:754-8.
- Kirk E. The quantity and quality of human colonic flatus. Gastroenterology 1949;12:782-94.
- Bond JH, Levitt MD. Colonic gas explosion: is a fire extinguisher necessary? Gastroenterology 1979;77:1349-50.
