Phase behaviour of premixed 0.25% isoflurane in 50% nitrous oxide and 50% oxygen

Background. Isoflurane (0.25%) in premixed nitrous oxide and oxygen 50/50, v/v, (IN2O), has been suggested for pain relief in labour. Methods. Possible phase separation of this mixture was studied by analysis of samples from pre-filled cylinders as they were cooled. Results. Condensation of isoflurane was found at 3.1degreesC in a cylinder, which held 8.7 MPa at 15degreesC. In a cylinder holding 13.8 MPa, which is the standard filling pressure stipulated by the National Health Service, the condensation temperature was -2.3degreesC. At the highest cylinder filling pressure investigated (14.15 MPa) the separation temperature was even less, -3.0degreesC. After exposure of cylinders to -40degreesC and complete phase separation of the mixture, complete mixing was achieved by 24 h storage in the horizontal position at room temperature and, either three complete inversions of the cylinder or mechanical rolling at 30 r.p.m. Conclusions. These findings should assist the use and storage of IN2O.