During labour epidural catheter placement, a test dose of 3 mL of 1.5% lidocaine with 1:200,000 adrenaline is injected. Which statement best explains why the traditional heart rate criterion for detecting intravascular injection is LESS reliable in a labouring parturient?

• A Pregnancy blunts the beta-adrenergic response to adrenaline
• B The gravid uterus compresses the inferior vena cava, abolishing the chronotropic response
• C Progesterone accelerates lidocaine uptake into the epidural fat
• D Uterine contraction pain produces spontaneous heart rate surges that mimic the test response ✓

Explanation

The classic intravascular test dose sign is a rise in heart rate of at least 10 beats per minute within 30 to 45 seconds from 15 micrograms of adrenaline. In labour, each uterine contraction causes pain-driven catecholamine release and heart rate spikes indistinguishable from a positive test, producing false positives. Therefore subjective symptoms such as tinnitus, circumoral tingling and dizziness are considered more reliable markers in parturients, a point emphasised in Chestnut's text.

Reference: Chestnut's Obstetric Anesthesia, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.