Which ultra-short acting barbiturate is used to induce anesthesia and should not be used in very thin animals?

• A. Phenobarbital
• B. Methohexital
• C. Pentobarbital
• D. Thiopental

Answer: (D)

Induction drugs that are ultra-short acting work because they rapidly cross into the brain and produce anesthesia quickly, then are pulled out of the brain as they redistribute to other tissues. Thiopental is the classic ultra-short acting barbiturate used for rapid induction. The reason it’s avoided in very thin animals is its high lipid solubility and the fact that lean animals have little fat to serve as a storage reservoir. With less adipose tissue, thiopental stays in the bloodstream and brain longer, leading to prolonged anesthesia and an increased risk of respiratory and cardiovascular depression. So thiopental is the one to avoid in very thin animals because its pharmacokinetic profile makes recovery more unpredictable in those patients.