An investigation into the preparation, high-pressure behaviour, and stability of pre-mixed volatile liquid anaesthetics in entonox
In this study the solvent power of Entonox, which has not been studied before, was explored at 67, 81 and 101 bar pressures to realise pre-mixed cylinder mixtures of volatile liquid anaesthetics in Entonox. The enhancement effect due to the pressure in Entonox at 101 bar was found remarkable (halothane, 157%; isoflurane, 206%; enflurane 172%;) and sufficient to provide clinically useful gas-phase concentrations for the volatile anaesthetics, higher than their MAC (minimum anaesthetic concentration) values. The concentration stability of the mixtures, under the conditions studied, showed that the prepared mixtures produced steady concentrations at even the extreme discharging flow-rate of 2 litre per minute without remarkable fluctuations, while the thermal stability of the volatile anaesthetics in Entonox allows a thermal treatment of the valve assembly up to 250°C without any decomposition provided there is no contact with aluminium metal. These mixtures could offer a solution for the portability needed in anaesthetic practice in situations such as, in war and natural disasters, on expeditions, on board ship, domicilary practice, in ambulances and emergency services, and in situations where electricity, and other facilities are not available. In these studies, in addition, two gas chromatographic methods (using single and dual column systems respectively) have been developed for the determination of air, carbon dioxide, nitrous oxide and a volatile anaesthetic (halothane or isoflurane or enflurane) at both low and high concentration levels in mixtures. FTIR spectrophotometry has been used to analyse multi-component mixtures of anaesthetic gases at low concentrations.