Anesthetic considerations at moderate altitude.

The influence that changes in altitude may have on various aspects of anesthetic practice are discussed. The consequences of alterations in the partial pressure and density of anesthetic gases are described, as well as the problems that may arise as a result of using measuring devices calibrated in percentages that actually measure partial pressure. The performance of vaporizers under conditions of reduced barometric pressure is considered. The behavior of flowmeters at altitude was studied. The increase in flow produced by flowmeters under hypobaric conditions is not a constant factor dependent on density, but is variable at flow rates below 4 L/min. The following suggestions are put forward. Anesthetists should be encouraged to think in terms of the partial pressures of gases that they are using rather than concentrations. Measuring devices and vaporizers that are dependent on partial pressure and not on concentration should be calibrated in the appropriate units. The term MAC could be replaced by MAPP, which would more accurately represent the important factor in the delivery of anesthetic gases. The value of nitrous oxide in anesthetic practice at altitude needs to be reassessed.