It adds ~0.65 MAC so that I can give ~0.2 MAC equivalent of propofol with a little ketamine. Would be happy to do this with pure propofol ketamine but, as I mentioned earlier, I can only get the 20cc vials of propofol in this ASC and the supply of these is limited so I only use sedative dosing of propofol and supplement with something else. Sometime narcotic, this time nitrous.
Theoretically your concern regarding nitrous and fire/ explosion is valid and is supported by the Neuman article as quoted in Barash and Cullen. The paper clearly shows that methane ignition is not a concern, but at the highest reported hydrogen concentrations (from a 1970 paper by Levitt and a 1967 Handbook of Physiology) combustion could be possible. Given that intestinal hydrogen concentration increases rapidly with carbohydrate feeding and decreases just as rapidly in fasting patients, one has to inquire about the characteristics of the patient with the highest concentration recorded (was it after a lactulose challenge which is common in GI papers looking at hydrogen levels) and the characteristics of the other patients, specifically were there any fasted patients included and what was the highest concentration of hydrogen in a fasted patient. Unfortunately, the article is unavailable to me at this time, but I will see if I can ILL it to get a clearer picture.
There are three or four case reports of fires/ explosions when nitrous or oxygen was used as the insufflating agent for laparoscopy. There are a similar number of case reports of intra-abdominal fires/ explosions during laparotomy. I do not believe there is a report of fire/ explosion during N2O based anesthesia for laparoscopy. Not saying it couldn't happen. It would require a pretty unlikely perfect storm of events, and even then the risk is probably lower than during laparotomy.
A simple nick would result in a slow leak of gasses that would be diluted beyond the point of combustibility so you would need to have just the right intestinal gas composition with cautery applied directly to the segment of bowel containing that pocket of gas for combustion to occur.
I have to ask do you reduce your FiO2 during intraperitoneal cases? Baseline intestinal O2 concentration is not high enough to maintain combustion, but oxygen supplementation during anesthesia does increase intestinal O2 concentration to a level that could support combustion of both methane and hydrogen.
One patient that might be at higher risk is a patient with an small bowel obstruction as he/ she could become suddenly unobstructed intraop providing a substrate bolus for hydrogen production in the colon. Surgeon then decides to do a colostomy but doesn't enter the bowel sharply...
Oh, and this particular surgeon doesn't use laparoscopic cautery so I wasn't particularly concerned about the potential for intra-abdominal fire on the day in question.
- pod
