What is the recommended technique and dosage for a digital block of the thumb using lidocaine (local anesthetic)?

Digital Block of the Thumb: Technique and Dosage

For a digital block of the thumb, inject 2-3 mL of 1% lidocaine (10 mg/mL) without epinephrine using a single-injection transthecal approach into the flexor tendon sheath at the mid-proximal phalanx, staying well below the maximum safe dose of 4.5 mg/kg (300 mg total) for adults. 1, 2, 3

Recommended Technique

Transthecal (Single-Injection) Approach

• Inject into the flexor tendon sheath at the midpoint between the proximal digital crease and the proximal interphalangeal joint crease on the volar (palmar) surface of the thumb 2, 3
• This single-injection technique achieves anesthesia of all four digital nerves through centrifugal diffusion from the tendon sheath 3
• The transthecal approach has a 94% success rate overall and 83% success rate specifically for thumbs in pediatric patients, with similar efficacy expected in adults 2
• Use a 25-gauge needle with a 3 mL syringe for injection 3

Alternative Tumescent Technique

• If the transthecal approach is unsuccessful, use a subcutaneous single-injection tumescent technique by injecting into the volar subcutaneous space at the proximal digit until achieving a firm, turgid feel to the tissue 4
• This tumescent technique has demonstrated 100% success rate in 123 digits including thumbs 4

Dosage Recommendations

Standard Lidocaine Dosing

• Use 2-3 mL of 1% lidocaine (10 mg/mL) for thumb blocks 2, 3
• This provides 20-30 mg of lidocaine, well below toxic thresholds 3
• Maximum safe dose without epinephrine: 4.5 mg/kg body weight, not exceeding 300 mg total 5, 1
• Maximum safe dose with epinephrine: 7.0 mg/kg body weight, not exceeding 500 mg total 5, 1

Concentration Conversions

• 1% lidocaine = 10 mg/mL 5
• 2% lidocaine = 20 mg/mL 5

Pediatric Dosing Adjustments

• For children, volume should be based on age: typically 1.5-2.5 mL depending on digit size 2
• Reduce amide local anesthetic doses by 30% in infants younger than 6 months 5
• Maximum dose for children: 4.4 mg/kg without epinephrine 5

Critical Safety Considerations

Avoiding Cumulative Toxicity

• Calculate the total dose of ALL local anesthetics administered from any route (infiltration, nerve blocks, topical) to avoid exceeding maximum safe doses 6, 7
• Do not perform digital blocks within 4 hours of any other local anesthetic intervention 5, 6, 7
• Remove any lidocaine patches before administering injectable lidocaine 7, 8

Monitoring for Toxicity

• Early warning signs (plasma concentration 5-10 μg/mL): circumoral numbness, facial tingling, tongue tingling, tinnitus, metallic taste, light-headedness, slurred speech 6, 7, 8
• Severe toxicity signs (>10 μg/mL): muscle twitching, loss of consciousness, seizures, respiratory arrest, cardiac arrhythmias, myocardial depression, ventricular arrest 6, 7, 8
• If any signs of toxicity appear, immediately discontinue lidocaine, provide airway support, manage seizures with benzodiazepines, and use lipid emulsion therapy for cardiovascular collapse 7, 8

Equipment and Preparation

• Have suction equipment, adequate oxygen supply, and properly stocked rescue cart with age-appropriate resuscitation equipment available 5
• Keep 20% lipid emulsion readily available for treatment of local anesthetic systemic toxicity 5

Onset and Duration

Lidocaine Characteristics

• Onset of anesthesia: 1-3 minutes 9
• Duration of anesthesia: 90-200 minutes (1.5-3.3 hours) 5, 9
• Procedure time averages 113 seconds for transthecal technique 2

Alternative Long-Acting Agent

• Ropivacaine 0.75% (2.5 mL) provides onset in 4.5 minutes but extends duration to 21.5 hours with reduced postoperative analgesic requirements 9
• However, lidocaine remains the standard first-line agent for digital blocks 1

Common Pitfalls to Avoid

• Never use epinephrine-containing solutions in digital blocks due to risk of digital ischemia from end-artery vasoconstriction (though not explicitly stated in provided evidence, this is critical general medicine knowledge for digital blocks)
• Avoid injecting into highly vascular areas where lower doses should be used due to increased systemic absorption 5
• Do not exceed recommended volumes, as excessive injection pressure can compromise digital perfusion 4
• Ensure proper needle placement in the flexor tendon sheath by feeling for loss of resistance as the needle enters the sheath 3