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.
- Boiling point of −88°C, well below ambient temperature!
- Critical temperature of 36.5°C (at and above which vapour cannot be liquefied, no matter how much pressure is applied)
- Critical pressure of 72.6 bar (7260kPa!) (the point at which phase boundaries vanishes)
- Low blood/gas solubility coefficient (0.47 at 37 °C) = rate of equilibration of alveolar with inspired concentrations is very fast.
- Not metabolised and is excreted unchanged.
- Analgesic = inhibits NMDA and has stimulatory activity at dopamine, α1 and α2- adrenergic and opioid receptors.
- Gives us the 'Concentration Effect', 'Second Gas Effect' and 'Diffusion Hypoxia'