Bioavailability of Ketamine and Thiopental
The bioavailability of ketamine varies significantly by route: intramuscular (IM) is approximately 93%, intranasal (IN) is approximately 50%, and oral is only 17-26% due to extensive first-pass metabolism. Thiopental administered intravenously has 100% bioavailability by definition, as IV administration bypasses absorption barriers 1, 2.
Ketamine Bioavailability by Route
Intramuscular Administration
- Bioavailability: 93% 2
- Absorption is rapid with peak plasma concentrations achieved quickly
- Population pharmacokinetic modeling in children confirms bioavailability of 41% in pediatric populations, though this may reflect different study methodologies 3
- The FDA label indicates IM doses of 6.5-13 mg/kg produce surgical anesthesia within 3-4 minutes 4
Intranasal Administration
- Bioavailability: approximately 50% 5
- Peak plasma concentrations occur at 20-21 minutes after administration
- Significant variability exists between individuals, with some drug being swallowed and undergoing first-pass metabolism
- In dogs (translational model), IN bioavailability reached 147% when co-administered with dexmedetomidine, suggesting enhanced absorption under certain conditions 6
- The high variability and partial swallowing make this route less predictable for anesthesia induction 5
Oral Administration
- Bioavailability: 17-26% due to extensive first-pass hepatic metabolism 2, 7, 8
- Only 20-25% of an oral dose reaches systemic circulation 9
- Peak concentrations occur later (approximately 59 minutes) with high between-subject variability (148%) 7
- The low bioavailability necessitates higher oral doses (typically 3-5 times the IV dose) to achieve therapeutic effects
- S-ketamine oral thin films demonstrate 26% bioavailability with 80% undergoing first-pass metabolism to norketamine 8
Intravenous Administration
- Bioavailability: 100% by definition (direct systemic delivery)
- FDA-approved doses range from 1-4.5 mg/kg for induction, with 2 mg/kg producing 5-10 minutes of surgical anesthesia within 30 seconds 4
Thiopental Bioavailability
Thiopental administered intravenously has 100% bioavailability as it is given directly into the bloodstream 1, 10. The guideline data shows:
- Plasma protein binding: 60-96%
- Half-life: 3-8 hours
- A single IV injection of 30 mg/kg is sufficient for induction and maintenance of anesthesia in rabbits 10
Clinical Implications
For ketamine, the route of administration dramatically affects dosing requirements:
- IV route: 1-4.5 mg/kg (most predictable, 100% bioavailability)
- IM route: 6.5-13 mg/kg (93% bioavailability, compensates for incomplete absorption)
- IN route: Higher doses needed due to 50% bioavailability and high variability
- Oral route: 3-5 times IV dose required (only 17-26% bioavailability)
The extensive first-pass metabolism of oral ketamine produces high levels of norketamine, which has approximately one-third the potency of ketamine but may contribute to analgesic effects 2, 7. This metabolite profile differs significantly from parenteral routes where ketamine predominates initially.
Common pitfall: Assuming equivalent dosing across routes will result in either inadequate effect (underdosing oral/IN) or excessive effect (overdosing when switching from oral to parenteral routes). Always adjust doses based on route-specific bioavailability 4, 2.