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a Reproduced from M. J. Halsey, in General Anaesthesia, 5th edn (ed. J. F. Nunn, J. E. Utting and B. R. Brown), Butterworths, London, 1989, Ch. 2.

a Reproduced from M. J. Halsey, in General Anaesthesia, 5th edn (ed. J. F. Nunn, J. E. Utting and B. R. Brown), Butterworths, London, 1989, Ch. 2.

Influence of body temperature

Temperature also influences anaesthetic solubility; temperature increase leads to a decrease in solubility as expected from section 2.4.1. Table 2.4 shows the temperature coefficients of the both water/gas, Awater/gas, and oil/gas, Aoil/gas, partition coefficients for a range of anaesthetic gases.

These data are relevant clinically because of possible wide variation of body temperature in the surgical patient. Body temperature may be lowered as a result of preoperative sedation, by cutaneous vasodilation, by the infusion of cold fluids and by reduced metabolism under operating conditions. The increase in oil/gas partition coefficient with decreasing temperature means that the effective concentration at the hydrophobic site of action is increasing and hence the apparent potency of the anaesthetic increases.

Summary

• Raoult's law can be used to calculate the partial pressure of a component in the vapour phase above a liquid under conditions of equilibrium if the composition of the liquid is known and if the system is assumed to be ideal. Mixtures of the hydro-fluoroalkane propellants HFA 134a and HFA 227 obey Raoult's law over a wide concentration range, but positive deviations from this law occur when the cosol-vent alcohol is included in the formulation.

• The variation of vapour pressure with temperature is described by the Clausius-Clapeyron equation; an equation, which provides a useful method for the experimental determination of the enthalpy changes accompanying phase transitions.

• The relative lowering of the vapour pressure following the addition of a solute to a solvent is equal to the mole fraction of the solute. A consequence of this change of vapour pressure is that the boiling point of the solution is increased and its freezing point decreased.

• The solubility of a gas in a liquid may be expressed by the Ostwald solubility coefficient, which is the volume of gas dissolved in unit volume of liquid at a given temperature, or as the Bunsen's absorption coefficient, in which the temperature and pressure are reduced to standard conditions.

• The solubility of a gas in a liquid decreases with increase of temperature at constant pressure and is directly proportional to pressure at a constant temperature (Henry's law).

• Application of temperature and pressure relationships in the prediction of the solubility of anaesthetic gases in vivo is complicated by the interaction of these gases with the lipids and proteins in the blood and in tissue fluids.

References

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