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, (IN(2)O), 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 3.1 degrees C in a cylinder, which held 8.7 MPa at 15 degrees C. In a cylinder holding 13.8 MPa, which is the standard filling pressure stipulated by the National Health Service, the condensation temperature was -2.3 degrees C. At the highest cylinder filling pressure investigated (14.15 MPa) the separation temperature was even less, -3.0 degrees C. After exposure of cylinders to -40 degrees C 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 IN(2)O.