What is the recommended dose and dilution of thiopentone for induction of anesthesia?

Thiopentone Dosing and Dilution for Anesthesia Induction

For induction of anesthesia, thiopentone should be administered at a dose of 4-5 mg/kg intravenously, prepared as a 2.5% solution (25 mg/mL). 1

Recommended Dosing

• The standard induction dose for adult patients is 4-5 mg/kg IV 1
• Lower doses (3.5 mg/kg for males and 3.3 mg/kg for females) may be appropriate in some populations 2
• Doses below 4 mg/kg are associated with increased risk of awareness during anesthesia 1
• Ensure adequate dosing initially, with additional doses readily available if intubation proves difficult 1

Patient-Specific Dosing Considerations

• Women require significantly lower doses than men (approximately 10-15% reduction) 3
• Elderly patients require dose reduction due to altered pharmacokinetics 3
• ASA III-IV patients require significantly lower doses than ASA I-II patients 3
• Obese patients require lower mg/kg dosing based on total body weight 3
• Patients with alcohol consumption habits may require higher doses 3

Dilution and Preparation

• Standard preparation is as a 2.5% solution (25 mg/mL) 1
• For pediatric or precise dosing, can be further diluted to 1% solution (10 mg/mL) 1
• Must be prepared in a fresh solution immediately before use 1
• Alkaline solutions will precipitate thiopentone; avoid mixing with acidic drugs 1

Drug Interactions and Adjuncts

• Pretreatment with opioids significantly reduces thiopentone requirements:
  • Fentanyl 5 μg/kg reduces thiopentone ED50 from 4.0 mg/kg to 2.2 mg/kg 4
  • Opioid premedication with phenothiazine or hyoscine markedly reduces induction dose 3
• Small doses of benzodiazepines have less effect on thiopentone dosing requirements 3

Clinical Considerations

• Thiopentone has largely been replaced by propofol in many settings due to:
  • Better suppression of airway reflexes with propofol 1
  • Fewer drug errors and greater familiarity with propofol 1
  • Easier preparation of propofol 1
• Thiopentone may cause more hypotension than other induction agents in hypovolemic patients 5
• The liver is the primary organ responsible for thiopentone elimination 6

Safety Precautions

• Have additional doses available should difficulty with intubation be encountered 1
• Consider gentle bag-mask ventilation (pressure <20 cmH2O) after induction to prevent oxygen desaturation 1
• Be prepared to reduce or remove cricoid pressure if intubation or mask ventilation proves difficult 1
• Monitor for hypotension, particularly in hypovolemic or hemodynamically unstable patients 5

Alternative Agents

• Propofol (2-2.5 mg/kg) is an alternative with better airway reflex suppression 1
• Etomidate (0.3 mg/kg) may provide greater hemodynamic stability in traumatized patients 1
• Ketamine may be beneficial in patients with asthma or chronic obstructive pulmonary disease due to bronchodilation effects 1