What is the proper composition and ratio for a test dose to verify epidural catheter placement, including lidocaine concentration and epinephrine concentration?

Epidural Test Dose Composition and Administration

For standard epidural catheter placement verification, use a test dose containing lidocaine 45 mg (3 mL of 1.5% lidocaine) combined with epinephrine 15 mcg (1:200,000 concentration), which provides 100% sensitivity for detecting intravascular placement in awake laboring patients. 1

Test Dose Composition by Clinical Context

For Detecting Intravascular Placement (Standard Epidural)

Lidocaine 1.5% with epinephrine 1:200,000 (15 mcg in 3 mL total volume) is the validated standard test dose for detecting accidental intravascular catheter placement. 2, 1

• This combination achieves 100% sensitivity, 96% specificity, and 100% negative predictive value for identifying IV catheters in laboring patients 1
• A positive test manifests as sudden heart rate increase ≥10 bpm above resting rate within one minute, with rapid acceleratory phase >1 bpm 1
• Systolic blood pressure increase ≥15 mm Hg also indicates intravascular placement with 80-100% sensitivity and 80-100% positive predictive value 3

Alternative epinephrine dosing:

• In patients premedicated with clonidine 5 mcg/kg, reduce epinephrine to 7.5 mcg (half the standard dose) as clonidine enhances cardiovascular responses 4
• In children, use 0.5 mcg/kg epinephrine with lidocaine 0.1 mL/kg of 1% solution 5

For Detecting Intrathecal Placement (Preventing High Spinal)

The test dose must not exceed 10 mg bupivacaine equivalent (or approximately 30-45 mg lidocaine) to minimize risk of total spinal anesthesia while producing clinically evident effects if intrathecally placed. 2, 6, 7

• The Royal College of Anaesthetists specifically recommends ≤10 mg bupivacaine equivalent as the maximum safe test dose 2, 6
• This dose produces detectable sensory, motor, or autonomic changes within 10-15 minutes if intrathecal, while maintaining safety margins 2, 7
• An alternative validated dose is 8 mg bupivacaine, which demonstrated ≥80% sensitivity and positive predictive value 2, 7

Motor block assessment is the most reliable indicator:

• Wait 5 minutes after injection to assess motor block (inability to raise legs), which has 95-100% specificity for intrathecal placement 6, 8
• At 3 minutes, 30 mg lidocaine has 83% sensitivity while 45 mg has 100% sensitivity for detecting motor block 2
• Sensory block extending to T1-T2 dermatomes develops in 10-15 minutes with intrathecal placement 2, 7

Practical Administration Algorithm

Step 1: Prepare and Aspirate

• After catheter placement, aspirate for blood or CSF 1
• If aspiration positive, remove and replace catheter 1
• Critical caveat: Negative aspiration does NOT rule out misplacement—direct intrathecal injection after negative aspiration occurs in 1:1,750 to 1:126,000 cases 6

Step 2: Administer Combined Test Dose

• Inject 3 mL of lidocaine 1.5% with epinephrine 15 mcg (1:200,000) during uterine diastole with stable heart rate 1
• This single test dose simultaneously screens for both intravascular AND intrathecal placement 2, 1

Step 3: Monitor for Intravascular Signs (0-1 minute)

• Observe for heart rate increase ≥10 bpm within 60 seconds 1
• Monitor for systolic blood pressure increase ≥15 mm Hg 3
• If positive: Assume intravascular placement, remove catheter 1

Step 4: Monitor for Intrathecal Signs (3-5 minutes)

• At 3 minutes: Check for subjective leg warmth/heaviness (59-74% specificity—unreliable alone) 2, 6
• At 4-5 minutes: Assess motor block by leg raising (93-100% specificity—most reliable) 6, 8
• Check sensory level to cold/pinprick over lumbar/sacral dermatomes 8
• If positive: Assume intrathecal placement, decide whether to convert to intrathecal catheter or remove 2, 6

Step 5: Proceed if Negative

• If both tests negative at 5 minutes, proceed with therapeutic epidural dosing 1
• Administer initial epidural dose in fractionated increments, never as single bolus 9

Critical Safety Warnings

Never rely solely on subjective symptoms (warmth, heaviness) as these have only 59-74% specificity for intrathecal detection and will miss 26-41% of misplacements. 2, 6

Do not proceed with full epidural dosing if any uncertainty exists about catheter position—the consequences of unrecognized intrathecal injection include high or total spinal block requiring airway management. 6

Multi-orifice catheters produce more reliable aspiration than single-orifice catheters, but negative aspiration never completely excludes misplacement. 2

In children under sevoflurane anesthesia, use modified heart rate criterion of ≥10 bpm increase (not the conventional ≥20 bpm) as the test dose is unreliable with the higher threshold. 5

Invalidate and repeat the test if uterine contraction occurs within 1 minute of injection or if heart rate response is equivocal, as contractions cause physiologic tachycardia that confounds interpretation. 1