What are the guidelines for using thiopental for induction of general anesthesia?

Thiopental for Induction of General Anesthesia

Thiopental should be administered at 4-5 mg/kg IV for anesthetic induction in adults, prepared as a 2.5% solution (25 mg/mL), with doses below 4 mg/kg associated with increased awareness risk. 1

Standard Dosing Guidelines

• Adult induction dose: 4-5 mg/kg IV administered at approximately 50 mg every 15 seconds until loss of eyelash reflex 1, 2
• Doses below 4 mg/kg carry increased risk of intraoperative awareness and should be avoided 1
• Additional doses must be readily available if intubation proves difficult 1

Preparation and Administration

• Standard concentration: 2.5% solution (25 mg/mL) for routine adult use 1
• For pediatric patients or precise dosing requirements, dilute to 1% solution (10 mg/mL) 1
• Must be prepared fresh immediately before use; alkaline solutions will precipitate thiopental 1
• Avoid mixing with acidic drugs due to precipitation risk 1

Special Population Considerations

Spinal Cord Injury Patients

• Significantly reduced dose requirements: approximately 3.9 mg/kg total body weight (versus 5.3 mg/kg in controls) 2
• Consider lean body weight calculations: 5.2 mg/kg lean body weight in SCI patients versus 6.6 mg/kg in controls 2
• These patients demonstrate attenuated cardiovascular responses to intubation 2

Breastfeeding Women

• No waiting period required before resuming breastfeeding after thiopental use 3
• Amounts transferred to breast milk are very small 3
• Compatible with breastfeeding due to poor bioavailability and short half-life 3

Cardiovascular and Hemodynamic Effects

Thiopental causes direct myocardial depression and peripheral vasodilation, requiring cautious use in hemodynamically compromised patients. 3

• Median arterial pressure typically decreases from baseline (e.g., 89 mmHg to 74 mmHg) after induction 4
• Cardiac output decreases significantly (e.g., 6.17 L/min to 4.76 L/min) 4
• Heart rate typically increases at unconsciousness 5

High-Risk Scenarios

In patients with severe hypovolemia, trauma, or nerve agent intoxication, thiopental's cardiovascular effects may be dangerously pronounced. 3

• Barbiturate-induced myocardial depression and vasodilation are exacerbated in hypovolemic states 3
• Consider etomidate as a safer alternative in traumatized or hemodynamically unstable patients due to minimal cardiovascular effects 3, 1
• Ketamine may be preferable in patients with asthma or COPD due to bronchodilation effects 3, 1

Specific Clinical Contexts

Pompe Disease

Thiopental should be used with extreme caution and reserved only for patients with less myocardial hypertrophy. 3

• Marked myocardial hypertrophy in Pompe disease creates physiological challenges with preload and afterload 3
• Ketamine or etomidate are safer alternatives, maintaining systemic vascular resistance and contractility 3
• Propofol may predispose to myocardial ischemia through afterload reduction in these patients 3

Respiratory Considerations

• May cause severe bronchoconstriction in patients with preexisting asthma, especially in presence of cholinergic stimulation 3
• Temporarily depresses respiration; gentle bag-mask ventilation (pressure <20 cmH₂O) recommended after induction 1

Pharmacokinetic Properties

• Hepatic extraction ratio: 0.29 with hepatic clearance of approximately 0.21 L/min 4
• The liver is the sole organ responsible for thiopental elimination 4
• Hepatic blood flow remains 26-28% of cardiac output and does not significantly change with induction 4
• Pharmacokinetics are linear at standard anesthetic doses; nonlinear kinetics occur with prolonged high-dose administration 6

Common Pitfalls and Safety Precautions

• Underdosing increases awareness risk: ensure adequate initial dosing rather than conservative approaches 1
• Have vasopressors immediately available to counteract potential severe hypotension 7
• Be prepared to reduce or remove cricoid pressure if intubation or mask ventilation proves difficult 1
• Thiopental provides sedation but NOT analgesia—ensure adequate analgesic coverage for painful procedures 8

Contemporary Context

Propofol has largely replaced thiopental in many clinical settings due to superior suppression of airway reflexes, easier preparation, fewer drug errors, and greater clinician familiarity 1. However, thiopental remains a valid option when propofol is contraindicated or unavailable, particularly given its established safety profile in breastfeeding women 3 and predictable pharmacokinetics 6.