Physical properties of inhalational anaesthetics

Critical temperature

• Critical temperature is the temperature above which a compound cannot be liquefied by pressure alone.
• When a compound is above its critical temperature
  --> Gaseous form = gas
• When a compound is below its critical temperature
  --> Gaseous form = vapour

Thus, at room temperature

• Halothane, enflurane, isoflurane, desflurane, sevoflurane, and N2O are all vapours

Vapour pressure

• Vapour pressure is pressure exerted by vapour
• Saturated vapour pressure is the maximal pressure exerted by vapour at equilibrium

Dalton's law

• Total pressure exerted by a mixture of gas/vapour
  = sum of partial pressure exerted by each of the component gases if present alone and occupying the same volume as the mixture

Tension

• Tension is the pressure exerted by a gas/vapour within a solution
• The state of anaesthesia is related to the tension (i.e. partial pressure) of inhalational anaesthetics in the brain
  * NOT concentration