Correct Sequence for Combined Spinal-Epidural Using Separate Needles
When using separate spinal and epidural needles without a CSE kit, first identify the epidural space with the 18G Tuohy needle and thread the catheter, then perform the spinal injection with the 25G needle at the same or adjacent interspace, followed by an epidural test dose through the catheter. 1, 2
Step-by-Step Technical Sequence
1. Epidural Placement First
- Insert the 18G Tuohy needle at L2-3 or L3-4 interspace using loss-of-resistance technique 2, 3
- Thread the epidural catheter 3-5 cm into the epidural space 2
- Do NOT inject any drugs through the epidural catheter yet 2, 3
- Secure the catheter in place 2
2. Spinal Injection Second
- Insert the 25G spinal needle at the same interspace or one space below (e.g., L3-4 if epidural was at L2-3) 2, 3
- Confirm free flow of cerebrospinal fluid (CSF) - this is critical as it validates both your spinal placement AND confirms the epidural needle was correctly positioned in the midline 4, 1
- Inject heavy bupivacaine (hyperbaric 0.5% bupivacaine 2 mL) with or without opioid immediately after confirming CSF flow 3, 1
- Remove the spinal needle 2, 3
3. Epidural Test Dose Last
- After completing the spinal injection and removing the spinal needle, administer the epidural test dose 3
- Use 3-4 mL of 2% lidocaine with epinephrine 1:200,000 as the test dose 3
- Wait 3-5 minutes and assess for signs of intravascular injection (tachycardia, hypertension) or intrathecal placement (rapid onset of dense block) 3
Timing of Heavy Bupivacaine Administration
Inject the heavy (hyperbaric) bupivacaine immediately after confirming free CSF flow through the spinal needle, before removing it. 3, 1 This ensures:
- The spinal component provides rapid, reliable anesthesia 4
- You don't lose CSF flow or needle position 2
- The patient can be positioned appropriately for the surgical site while the block develops 3
Timing of Epidural Test Dose
The epidural test dose must be given AFTER the spinal injection is complete and the spinal needle is removed. 3 This sequence is critical because:
- Testing the epidural before the spinal makes it impossible to distinguish between spinal and epidural effects 5
- If you test the epidural first and it's accidentally intrathecal, you've wasted time and may have an inadequate block 5
- The spinal provides immediate surgical anesthesia while you verify epidural catheter function 3, 5
Critical Advantages of This Separate-Needle Sequence
- Success rate of 99.5-100% when epidural is placed first, followed by spinal 2, 3
- Free CSF flow through the spinal needle confirms correct midline epidural placement, reducing epidural failure risk 4, 1
- Allows immediate repositioning of epidural catheter if problems occur (blood, paresthesia) before committing to spinal injection 2
- Avoids the 37% failure rate seen with some needle-through-needle techniques 3
Common Pitfalls and How to Avoid Them
- Never inject the epidural test dose before the spinal - you won't be able to interpret the results and may mask spinal failure 5
- Always confirm free CSF flow before injecting spinal medication - this single step validates both needle placements 4, 1
- If you encounter blood or paresthesia during epidural catheter threading, reposition the catheter immediately before proceeding to spinal injection 2
- Use pencil-point (atraumatic) 25G spinal needles to minimize post-dural puncture headache risk to <1% 4, 1