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.
Tunstall
M E
ME
Department of Environmental and Occupational Medicine, University of Aberdeen, University Medical School, Foresterhill, Aberdeen AB25 2ZD, UK.
Ross
J A
JA
D000776:Anesthesiology / Q000295:instrumentation; D000777:Anesthetics / Q000737:chemistry*; D019162:Anesthetics, Combined; D007530:Isoflurane / Q000737:chemistry*; D009609:Nitrous Oxide / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D011312:Pressure
D000777:Anesthetics; D019162:Anesthetics, Combined; D007530:Isoflurane; D009609:Nitrous Oxide; D010100:Oxygen