Phase II: Maintenance

•  After the induction phase comes the maintenance phase, where the goal is to sustain the anesthetic effects. Agents used during the induction phase have a brief duration of action, so additional agents are used during the maintenance phase.

• During maintenance, a volatile sedative-hypnotic agent and an analgesic are used. An IV anesthetic can also be added.

1.   Blood:Gas Partition Coefficient

• The blood:gas coefficient of a volatile anesthetic tells you the speed of induction. This coefficient describes the concentration of the drug dissolved in the blood compared to the concentration of the drug in the gaseous phase inside the blood vessel. Low blood:gas coefficient = fast speed of induction.

2.  Oil:Gas Partition Coefficient 

• The coefficient compares how much of the drug dissolves in oil/fat compared to in the gaseous phase, which tells you about its potency. High oil:gas coefficient = high potency.

3.  Minimal alveolar concentration (MAC)

• MAC is defined as the smallest concentration (in 100% oxygen, in the absence of other IV agents) of volatile anesthetic required in the alveoli to prevent a response to painful stimuli in 50% of patients. 

• The MAC of a volatile anesthetic can be reduced by combining it with an IV anesthetic, opioids, nitrous oxide, and/or regional/peripheral nerve blocks.