Epidural anesthesia and the test dose
Why examiners ask this
The epidural test dose is a small ritual with a large rationale, and examiners use it to see whether you understand what it detects, what it misses, and what you do afterwards. The broader topic tests application of knowledge about the epidural space, segmental spread and volume-based dosing, and judgment about the catheter that is intravascular, intrathecal, subdural, or simply not working. It also tests organization in a safety sequence that should be automatic.
The 60-second answer
I identify the epidural space by loss of resistance to saline, thread the catheter 4–6 cm into the space, and aspirate for blood and CSF. I then give a test dose of 3 mL of lidocaine 1.5% with epinephrine 1:200,000, which is 45 mg of lidocaine and 15 mcg of epinephrine. An intrathecal catheter produces a rapid motor and sensory block within 3–5 minutes; an intravascular catheter produces a heart-rate rise of 20 bpm or more, or a systolic rise of 15 mmHg, within about a minute. A negative test is reassuring but not proof, so every subsequent dose is incremental, 5 mL at a time with aspiration, because each increment is itself a test. I dose by volume: roughly 1–2 mL per segment I need to cover.
Decision points and the "why"
- Epinephrine as an intravascular marker — 15 mcg IV raises the heart rate in an unblocked, non-beta-blocked patient. It is unreliable in labor (contractions change the heart rate), with beta-blockers, under general anesthesia, and in the elderly, where I look for a 15 mmHg systolic rise or T-wave changes instead.
- Fractionated dosing beats any test — 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 and dizziness; I use it when I want to avoid epinephrine, accepting that it needs a cooperative patient.
- Catheter depth — 4–6 cm in the space balances migration out against a too-deep catheter that exits a foramen and gives a unilateral block.
- Spread — epidural spread is dose- and volume-dependent and fills toward the site of injection; position matters less than for a spinal. I place the catheter at the dermatomal center of the incision.
- Onset and choice of drug — chloroprocaine 3% for speed, lidocaine 2% with epinephrine and bicarbonate for intermediate, bupivacaine or ropivacaine for duration and differential block.
The complication they will inject
After a negative test dose, I give 10 mL of 0.5% bupivacaine as a single bolus, and the patient complains of tinnitus and a metallic taste, then seizes. I stop the injection, call for help, and say "local anesthetic systemic toxicity." I manage the airway with 100% oxygen and avoid hypoventilation, give a benzodiazepine for the seizure, and start lipid emulsion 20% at 1.5 mL/kg over 2–3 minutes at the first sign of cardiovascular instability. If the patient arrests, I give small epinephrine doses (1 mcg/kg or less) and avoid vasopressin, beta-blockers, calcium-channel blockers and local anesthetic antiarrhythmics. I prepare for prolonged resuscitation and bypass. Afterwards I disclose, and I name the error: a single large bolus despite the negative test.
Pitfalls that cost points
- Saying a negative test dose means the catheter is correctly placed.
- Forgetting that heart-rate criteria fail in labor and with beta-blockers.
- Giving the whole calculated dose at once.
- Reciting the test-dose composition without the reason for each component.
- Not mentioning subdural placement when the block is slow, patchy and high.
Rapid-why
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 tap 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.
Numbers to know
| Item | Value |
|---|---|
| Test dose | Lidocaine 1.5% 3 mL + epinephrine 1:200,000 (45 mg + 15 mcg) |
| Positive intravascular | HR +20 bpm or SBP +15 mmHg within 1 min |
| Positive intrathecal | Motor/sensory block within 3–5 min |
| Incremental dosing | 5 mL with aspiration |
| Catheter depth in space | 4–6 cm |
| Lumbar dosing | 1–2 mL per segment |
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 (coming)
- ASRA Practice Advisory on local anesthetic systemic toxicity; ASA Practice Guidelines for Obstetric Anesthesia
Sources
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