Emergence: Induction Agents

First, we must reverse the effects of our induction and maintenance agents by discontinuing them. 

The discontinuation of each drug needs to be timed so that it aligns with the end of the surgery.

IV induction agents are cleared via drug redistribution from the brain and into fatty tissues. Agents with greater lipophilicity will redistribute faster.

Bolus Dose: After a bolus dose, the duration of action is very short for most anesthetic agents. For instance, the duration of action after a bolus dose of propofol is ~10 min, as it rapidly redistributes from the brain to body tissues and fat.

Continuous Infusions: Clearance depends on the context-sensitive half-time (i.e., time required for plasma drug concentration to decrease by 50% after discontinuing the drug infusion). The recovery time for propofol, regardless of the duration of the infusion, is relatively short. Conversely, the context-sensitive half-time of dexmedetomidine varies (ex. 4 minutes after a 10-minute infusion vs. 250 minutes after an 8-hour infusion). 

Adjuvant agents (such as opioids or benzodiazepines) act synergistically with induction agents, thereby increasing recovery time. Dose reductions of these agents during the maintenance phase of anesthesia are necessary. 

Volatile anesthetics are cleared by exhalation of the agent. 

Properties that allow for faster clearance:

• Lower blood:gas and oil:gas partition coefficient: Greater solubility in gas means more agent in the alveoli where the agent is breathed out

• Higher minute ventilation: Faster clearance with higher minute ventilation (mL/min), which depends on tidal volume (mL/breath) and respiratory rate (breaths/min). Opioids can decrease minute ventilation and slow the clearance of volatile anesthetics.

• Shorter duration of administration: Prolonged administration will increase time required for clearance, as more of the agent is dissolved in the tissues. 

Refer to the Maintenance page for descriptions of partition coefficients.