Start your One-Week Free Trial
Already subscribed? Log in »
General Anesthesia Principles
Lipophilicity
Definition
- The lipid solubility of a drug directly relates to its potency.
Lipophilicity & Potency
- Drugs with highly lipophilicity more readily cross neuronal membranes, and thus have increased potency – they reach their target receptors more quickly than drugs with poor lipophilicity.
- For details regarding lipophilicity and membrane permeability, see our Pharmacokinetics: Absorption & Elimination tutorial.
Mean Alveolar Concentration (MAC)
Definition
- MAC is the alveolar concentration of a drug that prevents movement in 50% of patients.
- Note that the prevention of movement is in response to a standardized stimulus.
Mean Alveolar Concentration & Potency
- MAC and potency are inversely proportional: Potency is 1/MAC.
- As the MAC decreases (the smaller the MAC), the potency increases (the greater the potency).
Blood/Gas Partition Coefficient
Definition
- Defines the solubility of a gas in blood and the rate of induction and recovery.
- It compares the amount of drug in 1 mL of blood to the amount of drug in 1 mL of alveolar gas.
Blood/Gas Partition Coefficient & Speed of Induction & Recovery
Poorly soluble gas
- In the case of a poorly soluble gas, where there is a low blood/gas partition coefficient, the drug travels into the alveoli, enters the blood where it is poorly soluble, thus increasing the partial pressure of the drug (because it doesn’t dissolve in the blood), and this forces it out of the blood and into the brain more easily.
- Poor solubility results in increased pressure, which speeds brain saturation and decreases the onset time of the gas.
Highly soluble
- In the case of a highly soluble drug, where there is a high blood/gas partition coefficient, the drug passes into the alveoli, and then dissolves into the blood (it has a high solubility), so it takes longer for the partial pressure to build, which forces the drug to move into the brain more slowly.
- High solubility results in decreased pressure, which slows brain saturation and delays onset time of the gas.
Summary
- The B/G PC is used clinically to help us understand and define the rate of induction and recovery of anesthesia.
- The lower the blood solubility the more rapid induction and recovery.