British Journal of Anaesthesia, 2002, Vol. 89, No. 6 820-824
© 2002 The Board of Management and Trustees of the British Journal of Anaesthesia
Clinical Investigations |
Simulated use of premixed 0.25% isoflurane in 50% nitrous oxide and 50% oxygen
Department of Environmental and Occupational Medicine, University of Aberdeen, University Medical School, Foresterhill, Aberdeen AB25 2ZD, UK j.a.ross@abdn.ac.uk
Declaration of interest. Patented by the University of Aberdeen. If commercial production were to occur, the university and authors would benefit.
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 –15.5°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.
Br J Anaesth 2002; 89: 820–4