A patient undergoes lumbar epidural anaesthesia. The test dose contains 3 mL of 1.5% lidocaine with 1:200,000 adrenaline (15 mcg). Three minutes after injection, the patient's heart rate increases from 75 to 100 bpm. This finding indicates:

• A Correct epidural placement; adrenaline absorbed from the epidural space
• B Inadvertent intrathecal placement producing high spinal symptoms
• C Allergic reaction to the adrenaline vehicle
• D Intravascular injection of the test dose into an epidural vein ✓

Explanation

The epidural test dose containing 15 mcg of adrenaline relies on the cardiovascular marker of intravascular injection: if the dose is injected intravascularly, systemic absorption of adrenaline causes a heart rate increase of ≥20 bpm within 30–60 seconds. This is the vascular marker. An intrathecal injection marker would manifest as rapid onset dense motor block and sensory block (onset within 5 minutes with lidocaine 1.5%). Slow absorption from a correctly placed epidural space would produce a much smaller cardiovascular effect after a longer latency. A heart rate increase of 25 bpm within 3 minutes is the definitive positive vascular marker.

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

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Written and medically reviewed by the StethoPrep medical team.