In epidural anaesthesia, a test dose containing 3 mL of 1.5% lidocaine with 1:200,000 epinephrine is administered. What constitutes a positive intravascular test dose response?

• A Immediate total spinal from subarachnoid injection
• B Hypotension within 2 minutes from systemic lidocaine effect
• C Paraesthesiae in the lower limbs from subdural injection
• D Heart rate increase >20 bpm within 45 seconds, consistent with intravenous epinephrine absorption ✓

Explanation

The standard epidural test dose (3 mL of 1.5% lidocaine + 15 µg epinephrine) serves a dual purpose: the epinephrine (15 µg) detects intravascular injection by causing a heart rate increase >20 bpm within 45 seconds if injected IV; the lidocaine 45 mg detects subarachnoid injection by causing rapid dense motor block within 3-5 minutes. In obstetric patients, tachycardia may be less reliable (beta-blockade, sympathetic activation of labour), so both components must be interpreted together. A positive test mandates catheter repositioning before proceeding.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.