Thiopentone Dose for Anesthesia Induction in Adults
The recommended induction dose of thiopentone for adults is 4-5 mg/kg IV, administered as a 2.5% solution (25 mg/mL). 1
Standard Dosing Protocol
- Administer 4-5 mg/kg IV as the standard induction dose for adult patients 1
- Doses below 4 mg/kg are associated with increased risk of intraoperative awareness and should be avoided 1
- The drug should be prepared as a 2.5% solution (25 mg/mL) for standard adult use 1
- For more precise dosing requirements, the solution can be diluted to 1% (10 mg/mL) 1
Critical Dosing Considerations
Elderly patients require significantly lower doses than younger adults, with typical requirements ranging from 3.09-5.06 mg/kg compared to 4.88-8.11 mg/kg in younger patients 2. The reduced dose requirement in elderly patients is due to altered pharmacokinetics, including increased volume of distribution and prolonged elimination half-life 2.
When using slower infusion rates (300 ml/h vs 1200 ml/h), the required dose decreases substantially to approximately 2.8 mg/kg compared to 5.0 mg/kg with rapid injection, though this slower administration prolongs induction time to approximately 91 seconds 3.
Factors Affecting Dose Requirements
The following patient factors significantly influence thiopentone dosing 4:
- ASA physical status: Patients in ASA grades 3-4 require significantly less thiopentone than those in grades 1-2 4
- Gender: Women require lower average doses than men 4
- Body habitus: Obese patients require less drug per kilogram than non-obese patients 4
- Premedication: Opiates combined with phenothiazines or hyoscine markedly reduce the induction dose 4
- Alcohol consumption: Moderate to heavy drinking increases the required induction dose 4
Administration Technique
- Prepare fresh solution immediately before use, as thiopentone is unstable in solution 1
- Avoid mixing with acidic drugs, as alkaline solutions will precipitate thiopentone 1
- Have additional doses readily available in case intubation proves difficult 1
- Consider gentle bag-mask ventilation (pressure <20 cmH₂O) after induction to prevent oxygen desaturation 1
Important Safety Precautions
Bolus administration of high doses in cachectic patients carries risk of immediate cardiovascular collapse and should be avoided 5. In patients with significantly reduced body mass, consider adjusting the dose based on actual body weight rather than using a fixed 2000 mg dose 5.
The plasma concentration typically reaches 60-80 μg/mL during standard induction, with elderly patients showing approximately 10% higher concentrations due to altered distribution 6. Plasma protein binding averages 85% and does not vary significantly with age 6.
Clinical Context
Thiopentone has largely been replaced by propofol in many clinical settings due to propofol's superior suppression of airway reflexes, fewer drug preparation errors, and greater clinician familiarity 1. Alternative induction agents include propofol (2-2.5 mg/kg), etomidate (0.3 mg/kg) for hemodynamically unstable patients, or ketamine for patients with reactive airway disease 1.