Simulated use 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. Possible phase separation of the mixture was studied during simulated administration.


METHODS
A sinusoidal pump set at stroke volume of 2 litres and a rate of 20-22 bpm and cycling for 1 min in three was used to simulate breathing during the painful contractions of labour.


RESULTS
The temperature inside a 10-litre capacity cylinder did not drecrease sufficiently to cause separation of the gas mixture. Temperature in the demand valve decreased to -15.5 degrees C and this caused a small amount of liquid formation within the valve. Accordingly, the inspired concentration during the first breath of mixture in a cycle could be transiently as high as 0.55%. The concentration observed at the patient connection after the first breath varied between 0.17 and 0.28%.


CONCLUSIONS
The system delivered a clinically acceptable performance although further development to avoid liquid condensation is needed.
Ross
J A
JA
Department of Environmental and Occupational Medicine, University of Aberdeen, University Medical School, Foresterhill, Aberdeen AB25 2ZD, UK. j.a.ross@abdn.ac.uk
Tunstall
M E
ME
D000776:Anesthesiology / Q000295:instrumentation; D019162:Anesthetics, Combined; D003080:Cold Temperature; D006801:Humans; D007530:Isoflurane; D009609:Nitrous Oxide; D010100:Oxygen
D019162:Anesthetics, Combined; D007530:Isoflurane; D009609:Nitrous Oxide; D010100:Oxygen