Epidural anesthesia and the test dose
Stem
A 58-year-old man, 84 kg, on metoprolol for hypertension, is scheduled for an open left hemicolectomy. He agrees to a lumbar-thoracic epidural for intraoperative anesthesia and postoperative analgesia, to be placed awake before induction. Loss of resistance to saline is found at 5 cm, the catheter threads easily, and aspiration returns neither blood nor CSF.
Question
How will you test this catheter, and how will you dose it safely?
Ideal answer
HeadlineExclude an intravascular or intrathecal catheter before any large dose: a 3 mL test dose of lidocaine 1.5% with epinephrine 1:200,000, then only incremental doses.1
- About 5–6 cm in the space, gentle aspiration, then the test: 45 mg of lidocaine and 15 mcg of epinephrine.1
- Intrathecal: a motor and sensory block within 3–5 minutes; unable to lift his legs at 4 minutes is a clear positive.2
- Intravascular: heart rate up 10 bpm or more, or systolic up 15 mmHg or more. On metoprolol the heart rate is blunted, so I watch systolic pressure and T wave, which are preserved.2
- Negative is not proof: every later dose incremental, 5 mL with aspiration, each itself a test, about 1–2 mL per segment.3
Watch for: tinnitus, perioral numbness, a rising block level or hypotension, talking to him throughout.
Breaking down the answer
- Epinephrine as a marker — 15 mcg IV raises heart rate within a minute in an unblocked, non-beta-blocked patient; unreliable in labor (contractions), beta-blockade, general anesthesia, the elderly: a 15 mmHg systolic rise or T-wave fall of 25% or more instead.2
- Why fractionate — a catheter can migrate into a vein after a negative test; aspiration plus incremental doses with observation catches it.
- Fentanyl as an alternative marker in labor — 100 mcg IV causes sedation, drowsiness or dizziness within 5 minutes; it avoids epinephrine but needs a cooperative patient.2
- Catheter depth — 5–6 cm balances migration out against a deep catheter that exits a foramen and gives a unilateral block; there is no consensus on exact length.1
- Spread — dose- and volume-dependent, toward the injection site; position matters less than for a spinal, so the catheter goes at the dermatomal center of the incision.
- Drug choice — chloroprocaine 3% for speed, lidocaine 2% with epinephrine and bicarbonate for intermediate, bupivacaine or ropivacaine for duration and differential block.
Follow-up question
After a negative test dose, a colleague gives 10 mL of 0.5% bupivacaine as a single bolus. The patient complains of tinnitus and a metallic taste, then seizes. What do you do?
Follow-up answer
HeadlineLocal anesthetic systemic toxicity; I stop the injection, call for help and the lipid kit, and say it out loud.
Airway with 100% oxygen, no hypoventilation, a benzodiazepine for the seizure.4 Lipid emulsion 20% early: over 70 kg, about 100 mL over 2–3 minutes, then an infusion of about 250 mL over 15–20 minutes, repeating the bolus if unstable.4 In arrest: epinephrine under 1 mcg/kg; no vasopressin, beta-blockers, calcium-channel blockers or local anesthetic antiarrhythmics; bypass team alerted.4 Then a monitored bed, disclosure, and the named error: a single large bolus despite the negative test.
Pitfalls that cost points
- Reciting the test-dose composition without the reason for each component.
- Not mentioning subdural placement when the block is slow, patchy and high.
Rapid-fire
Q: Why lidocaine 45 mg in the test dose? A: Intrathecal, it produces a recognizable sensory and motor block within minutes without causing a total spinal.
Q: Why does a catheter give a unilateral block? A: It has exited the space through a foramen or lies too far laterally; I withdraw 1–2 cm and bolus with the bad side down.
Q: Why is the epidural space wider at L2 than at C7? A: It is about 5–6 mm in the lumbar region and narrows to 1.5–2 mm in the cervical region, which is why a cervical dural puncture is more likely.
Q: What is the differential for a block that fails? A: Catheter not in the space, too little volume, a septated or scarred space, or migration out.
Q: How do you manage a suspected subdural catheter? A: Supportive care and removal of the catheter; it is rare and hard to diagnose.3
Numbers to know
| Item | Value |
|---|---|
| Test dose | Lidocaine 1.5% 3 mL + epinephrine 1:200,000 (45 mg + 15 mcg)1 |
| Positive intravascular | HR +10 bpm (many use 20), SBP +15 mmHg, or T-wave amplitude −25%2 |
| Positive intrathecal | Motor/sensory block within 3–5 min |
| Incremental dosing | 5 mL with aspiration |
| Catheter length in space | About 5–6 cm1 |
| Dosing by segment | 1–2 mL per segment3 |
| Lipid bolus | ~1.5 mL/kg over 2–3 min (~100 mL if >70 kg)4 |
Links
- Spinal anesthesia: dosing, spread, hypotension, High or total spinal, Local anesthetic systemic toxicity, Neuraxial labor analgesia and its complications, Thoracic epidural analgesia, Epidural hematoma and abscess
- ASRA Checklist for Treatment of Local Anesthetic Systemic Toxicity (2020); ASA Practice Guidelines for Obstetric Anesthesia
Sources
- 1.Epidural Anesthesia · StatPearls · ncbi.nlm.nih.gov
- 2.The Epidural Test Dose: A Review · Anesthesia & Analgesia (Guay J), 2006 · faculty.washington.eduAnesth Analg 2006;102:921-9. Positive intravascular criteria HR +10 bpm, SBP +15 mmHg, or T-wave amplitude fall of 25%; StatPearls quotes HR +20-30 bpm.
- 3.Epidural and Combined Spinal-Epidural: Pharmacology, Technique, and Side Effects · OpenAnesthesia · openanesthesia.org
- 4.Checklist for Treatment of Local Anesthetic Systemic Toxicity · ASRA Pain Medicine, 2020 · asra.com
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.