FRCA Notes

The Effect of Altitude on Inhaled Anaesthesia

  • The output of a vaporiser is calibrated to 1atm pressure
  • 1% on the dial gives a vapour pressure of 1% at 1atm i.e. 1% of 101.3kPa = 1kPa

Sevoflurane at altitude

  • The SVP at 20ºC of sevoflurane is 22.7kPa
  • I.e. 22.7/101.3 = 22.7% of atmospheric pressure at sea level

  • If altitude is increased, the atmospheric pressured drops
  • If you ascend to (say) 5,000m, atmospheric pressure is 50kPa i.e. half of that at sea level

  • At 5,000m:
    • The SVP of sevoflurane is unchanged (SVP unaffected by ambient pressure)
    • Therefore the concentration of sevoflurane has doubled (now 22.7/50.5 = 45.4% of atmospheric pressure at 5,000m)
    • Therefore the output from the vaporiser when it is dialled at 1% is actually doubled i.e. 2%
    • 2% of the atmospheric pressure (50kPa), however, is 1kPa
  • Therefore, even though the concentration of sevoflurane has doubled, its partial pressure is unchanged
  • As it is the partial pressure of the agent that dictates clinical effect, the clinical effect is unchanged

In general

  • In general, the agent concentration can be calculated:

    • %delivered (concentration) = %dialled x (Pcalibrated) / (Paltitude)


  • Using the example of 1% sevoflurane at sea level:
    • %delivered = 1% x (101.3) / (101.3)
    • %delivered = 1%
    • Partial pressure = concentration x atmospheric pressure
    • Partial pressure = 1% x 101.3 = 1kPa

  • Now using the example of 1% sevoflurane at 5,000m:
    • %delivered = 1% x (101.3) / (50.5)
    • %delivered = 2%
    • Partial pressure = concentration x atmospheric pressure
    • Partial pressure = 2% x 50.5 = 1kPa

  • The Tec6 vaporiser is pressurised to 2atm
  • There is no compensation for ambient pressure
  • The concentration delivered in the fresh gas flow is constant regardless of changes in ambient pressure

  • 2% desflurane at sea level:
    • % delivered = % dialled = 2%
    • Partial pressure = 2% of 101.3kPa = 2kPa

  • 2% desflurane at 5,000m:
    • % delivered = % dialled = 2%
    • Partial pressure = 2% of 50.5kPa = 1kPa

  • Therefore for desflurane at altitude, one must increase the dialled percentage to maintain the same partial pressure of anaesthetic agent

  • Temperature decreases with altitude - the 'lapse rate'
  • There is a 6.5ºC reduction in temperature for every 1,000m ascent from 0 - 11,000m
  • This does reduce the SVP, therefore reducing the clinical effect of an agent
  • Modern vaporisers are temperature-compensated
    • They can be used in the same way at altitude, sea-level or hyperbaric conditions without temperature correct (except at extremes e.g. outer space)
    • This doesn’t apply to the Tec6 (desflurane) vaporiser