[Dysfunction of vaporizers and nitrous oxide: solubility or viscosity?].

Sudden switch of carrier gas from O2 to the N2O/O2 mixture results in an acute decrease in vapour output for a variable delay of time. This has been attributed to the dissolution of N2O in the volatile anaesthetic liquid. The final vapour concentration at steady state had been previously reported to be lower than the concentration dial setting: this was attributed to vaporizer design and to viscosity of carrier gas. When N2O/O2 flow is replaced by O2, the reverse changes occur. To test whether solubility or viscosity is the main factor of the acute changes in vaporizer output when changing the carrier gas, two experiments have been designed. Two types of vaporizers, Cyprane Mark 3 and Dräger Vapor 19, were used to deliver either halothane, or enflurane, or isoflurane. The delivered concentrations were measured in triplicate with an infrared analyzer, zero corrected for N2O, Datex Capnomac and recorded on a Gould polygraph. The analyzer was calibrated before each measure. All measures were obtained at the same dial setting and the same gas flow. To test the solubility hypothesis, the same enflurane Dräger Vapor 19 vaporizer was emptied, dried, saturated with enflurane, emptied and then filled with either 10 or 100 mL of liquid enflurane. To test the viscosity hypothesis, a model of a vaporizer was devised, in which the gas movements are described with the Poiseuille's law. In this model, the delivered concentration F depends on the vapour pressure, the input pressure of the gas flow, the viscosity of the carrier gas and the viscosity of the agent saturated gas.(ABSTRACT TRUNCATED AT 250 WORDS)