When using low-flow circle absorber techniques, the uptake of nitrous oxide must be considered. In a healthy 70 kg adult the expected uptake of nitrous oxide, with a 70% inspired concentration, after 1.5 hours would be about

uptake of N2O (in mls/min) is equal to 1000 divided by the square root of the time elapsed (in mins) * the time elapsed is 100 minutes. * the square root of this is 10 * 1000 divided by 10 is 100 so the N2O uptake at 100 mins into the case would be about 100mls/min.