Physicochemical properties and why they influence anaesthesia
Dr. David Lyness
- Molecular Weight = low molecular weight and non-polar structure produce low boiling point, high vapour pressure
- Boiling Point = a BP above ambient temperature allows it to exist as a liquid at room temperature, used to calibrate vaporiser
- Saturated Vapour Pressure = allows us to generate known concentrations of vapour. (Dalton’s law)*
- SVP = Evaporation in a closed container will proceed until there are as many molecules returning to the liquid as there are escaping.
- The vapour is now saturated, and is called the saturated vapour pressure (mmHg)
- Biotransformation of volatile anaesthetics assesses the untoward toxic effect of these agents to their metabolism. Low = better
- Blood Gas Solubility = The lower = faster onset and offset of anaesthesia
- Oil Gas Solubility = Potency = Higher number = highly soluble in cerebral tisssue and a lower MAC
- MAC in N2O is important when you consider the carrier effect and the fact that N20 has it's own MAC
*Because the molecules in gaseous form have kinetic energy, they bounce off the walls of the container and exert a pressure that can be measured.
MAC is the minimum alveolar concentration of a vapour in the lungs that will cause loss of motor response to surgical stimulus in 50% of the population under standard conditions and at 1atm.
NITROUS OXIDE is a sweet-smelling, non-irritant colourless gas:
- Molecular weight of 44.