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 (IN2O), 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 -15.5degreesC 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.