Carrier gas composition The composition of the carrier gas may affect vaporizer output by: -
Changes in the viscosity and density of the gas mixture affecting the proportion of the total flow that passes through the vaporizing chamber. The viscosities of air and nitrous oxide are lower than those of oxygen. In the variable bypass vaporizers, the characteristic of the flow splitting valve results in decreased gas flow through the vaporizing chamber, and hence reduced output, when using air and especially nitrous oxide compared with 100% oxygen. This effect is however, not clinically significant.
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Ambient..pressure Saturated vapor pressure is solely a function of temperature. Therefore, if ambient pressure is reduced, the (constant) SVP becomes a greater proportion of the total (reduced) pressure, and the output concentration (in volumes %) rises. The change in the agent concentration in the delivered gas flow can be calculated by: Pcal
Agent %1 = Agent %cal x P1
Where agent %1 is the agent concentration at the present ambient pressure, P1 is the present ambient pressure, Pcal is the atmospheric pressure at which vaporizer was calibrated and agent %cal is the agent % delivered at the calibrated atmospheric pressure. But the partial pressure of the agent will not change with change in ambient pressure and since it is the partial pressure that determines level of anesthesia, the clinical effect of change in ambient pressure is insignificant and the vaporizer can be used in the same way at high altitude, sea level or under hyperbaric conditions. This, however, does not apply to the desflurane Tec 6 vaporizer. The Tec 6 vaporizer is pressurized to 2 atm; there is no compensation for ambient pressure and thus the concentration delivered in the fresh gas flow is stable, regardless of ambient pressure. Thus, the dial setting must be increased to maintain partial pressure of the
agent
at
high
altitudes.