Effect of nitrous oxide on folate and vitamin B12 metabolism in patients.

Exposure to nitrous oxide (N2O) markedly enhances excretion of formic acid and formiminoglutamic acid (FIGLU) in the urine of rats, suggesting a disruption in the normal pathways of folic acid metabolism secondary to an N2O-induced inactivation of methionine synthase. We tested whether surgical patients (23 having total hip replacements and 26 having resection of acoustic neuromas) exposed to isoflurane alone or combined with N2O responded similarly. We found no increase in urinary formic acid and FIGLU in patients exposed to N2O for hip replacement, but a small, transient increase in the FIGLU-to-creatinine ratio in those undergoing resection of acoustic neuromas (mean duration of anesthesia = 9.3 h). This increase peaked at the end of anesthetic exposure and returned toward control levels by the first day after anesthesia and surgery. Low preoperative levels of red blood cell folate and low-normal levels of serum vitamin B12 did not predict an increase in formic acid or FIGLU in response to N2O. Although an occasional patient may prove highly susceptible to and develop signs of severe vitamin B12 and folic acid deficiency after exposure to N2O, our findings suggest that this is a rare event.