Can 20 mL of 0.5 % bupivacaine and 20 mL of 1 % lidocaine with epinephrine plus 10 mL of normal saline be administered to an 80‑kg male with bilateral forearm fracture for bilateral axillary nerve blocks?

Safety Assessment of Proposed Local Anesthetic Dosing for Bilateral Axillary Blocks

The proposed regimen of 20 mL of 0.5% bupivacaine plus 20 mL of 1% lidocaine with epinephrine (total 40 mL) for bilateral axillary blocks in an 80-kg male is NOT safe and should not be administered as described.

Critical Dose Calculations

For unilateral axillary block, the proposed mixture would deliver:

• Bupivacaine: 100 mg (20 mL × 0.5%) = 1.25 mg/kg in an 80-kg patient
• Lidocaine: 200 mg (20 mL × 1%) = 2.5 mg/kg in an 80-kg patient 1

For bilateral blocks (which the question implies), these doses would double:

• Bupivacaine: 200 mg total = 2.5 mg/kg
• Lidocaine: 400 mg total = 5.0 mg/kg

Maximum Safe Dose Thresholds

The established maximum safe doses are:

• Bupivacaine with epinephrine: 3 mg/kg (240 mg for 80 kg) 2
• Lidocaine with epinephrine: 7 mg/kg (560 mg for 80 kg) 3, 2

Why This Regimen Exceeds Safety Limits

The bilateral administration approaches the maximum safe dose of bupivacaine (2.5 mg/kg vs. 3 mg/kg ceiling) with minimal safety margin. 2 This leaves no room for:

• Individual pharmacokinetic variability
• Inadvertent intravascular injection
• Cumulative toxicity from the lidocaine component 3

Bupivacaine carries significantly higher cardiotoxicity risk than lidocaine, with greater affinity and longer binding duration to cardiac sodium channels, making dose precision critical. 2 Potential adverse events include hypotension, cardiac arrhythmias, and cardiac arrest. 1

Volume Considerations for Bilateral Blocks

The proposed 40 mL total volume (20 mL per side) for bilateral axillary blocks is at the upper limit of safety. 1 Guidelines recommend not exceeding 30 mL of injectate per axillary side when performing bilateral blocks, as higher volumes increase systemic absorption and toxicity risk. 1

Recommended Safe Alternative

For bilateral forearm fractures in an 80-kg male, use a reduced-volume approach:

• Per side: 15 mL of mixture (7.5 mL of 0.5% bupivacaine + 7.5 mL of 1% lidocaine with epinephrine 1:200,000)
• Total bilateral dose:
  • Bupivacaine: 75 mg (0.94 mg/kg)
  • Lidocaine: 150 mg (1.88 mg/kg)
• Total volume: 30 mL (15 mL per side)

This provides adequate anesthesia while maintaining a substantial safety margin below toxic thresholds. 1, 4

Evidence Supporting Lidocaine-Bupivacaine Mixtures

Mixing lidocaine and bupivacaine in equal volumes is considered safe and effective for peripheral nerve blocks. 1 A randomized study of 82 patients receiving femoral-sciatic blocks demonstrated that bupivacaine-lidocaine mixtures (100 mg bupivacaine + 400 mg lidocaine with epinephrine 1:200,000) provided faster onset (16±9 min vs. 28±12 min for bupivacaine alone) without increased adverse events. 4

The mixture approach offers pharmacodynamic advantages: lidocaine provides rapid onset while bupivacaine extends duration. 4, 5 A 2024 randomized trial confirmed that equal-volume mixtures of 2% lidocaine with epinephrine and 0.5% bupivacaine provided significantly faster complete conduction blockade (95% vs. 48% at 40 minutes) compared to bupivacaine alone during supraclavicular blocks. 5

Critical Safety Precautions

Calculate maximum allowable dose before any procedure to prevent cumulative toxicity, accounting for additive toxic effects when multiple local anesthetics are used concurrently. 3

Aspirate before each injection to avoid inadvertent intravascular administration, which markedly raises toxicity risk. 3

Monitor vigilantly for at least 50 minutes after injection (approximate time to peak plasma concentrations) to detect early signs of local anesthetic systemic toxicity: circumoral numbness, metallic taste, tinnitus, CNS changes, or cardiovascular instability. 3, 1

Have 20% lipid emulsion immediately available when administering bupivacaine in high-risk scenarios; it is the recommended rescue therapy for local anesthetic systemic toxicity. 3, 2

Common Pitfalls to Avoid

Do not buffer bupivacaine with sodium bicarbonate, as precipitation can reduce drug efficacy and compromise block quality. 1

Do not assume that adding epinephrine eliminates toxicity risk—while epinephrine reduces peak plasma concentrations of both lidocaine (by 23%) and bupivacaine (by 28%), it does not eliminate the need for dose discipline. 6

Avoid the misconception that "more volume equals better block"—ultrasound guidance allows effective blocks with lower volumes (15-20 mL per side), reducing systemic absorption and toxicity risk while maintaining efficacy. 5, 7