Spinal and Epidural Technique
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Direct answer
Spinal anaesthesia injects a small hyperbaric dose — typically 3 mL of 0.5% bupivacaine in 8% dextrose (15 mg) — through a 25-27G pencil-point needle inserted at the L3-L4 or L4-L5 interspace (identified by Tuffier's line joining the iliac crests) into the cerebrospinal fluid, producing surgical anaesthesia within 5-10 minutes. Epidural anaesthesia advances a 16-18G Tuohy needle to the epidural space, identified by loss of resistance to saline, then threads a catheter for incremental top-ups such as 3-5 mL of 0.5% bupivacaine or a 0.125-0.5% infusion. The tissue sequence — skin, subcutaneous fat, supraspinous and interspinous ligaments, the tough ligamentum flavum, then the epidural space with its negative pressure, and finally dura — is the anatomical script for both techniques, and hypotension from sympathetic block, treated with fluids, left tilt and phenylephrine 50-100 microgram boluses or ephedrine 5-10 mg, is the complication to anticipate every single time.
What you must remember
- Landmark safety: Tuffier's line across the highest iliac crests crosses roughly L4 or the L3-L4 interspace; since the adult spinal cord ends at L1 (L3 in neonates), injection stays at or below L3-L4 to avoid the conus.
- Tissue planes in order: skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum (the sudden loss of resistance after it), epidural space (negative pressure, may aspirate vein), dura and arachnoid — a "click" classically marks dural puncture in spinals.
- Standard doses: hyperbaric bupivacaine 0.5% 12.5-15 mg for caesarean section (aiming for a T4 sensory level), 10-15 mg for lower-limb surgery; lignocaine 5% hyperbaric 50-75 mg is the shorter alternative; epidural test dose is 3 mL of 2% lignocaine with 1:200,000 adrenaline.
- Hypotension protocol: crystalloid coload 500 mL, left lateral tilt or wedge from 20 weeks of pregnancy, phenylephrine 50-100 microgram boluses (current international consensus first line in obstetrics) or ephedrine 5-10 mg; hypotension complicates roughly a third of spinals.
- Complications map: hypotension and bradycardia from a high block, post-dural-puncture headache, total spinal (apnoea with hypotension — airway, ventilation, fluids, vasopressors), urinary retention, and rare epidural haematoma or abscess (new severe back pain with weakness — an emergency MRI).
- Post-dural-puncture headache management: fluids, analgesics, caffeine, and an epidural blood patch (15-20 mL of the patient's blood) if it persists beyond 48 hours; pencil-point 25-27G needles cut the incidence steeply.
Walking through a caesarean spinal
A 26-year-old at term for an emergency lower-segment caesarean arrives with a pulse of 92. Monitoring goes on first — ECG, NIBP, oximeter — and a 500 mL crystalloid coload runs while the technician opens the spinal tray: 27G Whitacre, introducer, and 3 mL of heavy bupivacaine drawn into a 5 mL syringe. Sitting the patient up or lying her lateral with knees drawn, the anaesthesiologist identifies Tuffier's line, infiltrates 2-3 mL of 1% lignocaine, then walks the needle through the ligaments until a distinct give signals the subarachnoid space; free flow of cerebrospinal fluid confirms it before the slow injection.
What follows is pure timing and positioning. She is laid supine with a 15-degree left wedge to keep the gravid uterus off the inferior vena cava, and the block is tested with alcohol swab or pinprick every two minutes until T4 is reached. The blood pressure is checked every minute for the first fifteen: when it falls more than 20%, phenylephrine 100 microgram goes in, and the technician is the one who has it ready in a labelled syringe precisely because there is no time to draw it during a drop. Nausea, oddly, is often the first symptom of hypotension rather than a drug effect — a viva favourite.
How the exam frames it
University papers ask the sequence of structures the needle traverses verbatim, and the order of ligaments is where a mark is lost: supraspinous, interspinous, then flavum, with the epidural space's negative pressure explained by the needle hanging-drop or saline-loss methods. The two classic discriminating questions are phenylephrine versus ephedrine in obstetrics — phenylephrine is now preferred because ephedrine crosses the placenta and causes fetal acidosis when used repeatedly — and total spinal management, where the pass-fail sequence is airway, 100% oxygen with controlled ventilation, fluids, vasopressor, and then, only then, calling for help on paper. Indian examiners additionally expect the L3-L4/L4-L5 interspace and Tuffier's line named explicitly, because surface-anatomy questions anchor this whole topic.
Frequently asked questions
At which interspace is a spinal performed and why?
L3-L4 or L4-L5, located via Tuffier's line joining the iliac crests, because below L1-L2 in adults there is only the cauda equina, so the conus medullaris cannot be injured by the needle.
How is the epidural space identified?
By loss of resistance: the Tuohy needle is advanced with its syringe of saline pressing continuously until the ligamentum flavum's resistance gives way abruptly, or by the hanging-drop method using the space's negative pressure.
What is the standard spinal dose for caesarean section?
12.5-15 mg of 0.5% hyperbaric bupivacaine (2.5-3 mL), aiming for a T4 sensory block; adding fentanyl 15-25 microgram improves the quality without much motor block.
How is spinal hypotension managed?
Coload crystalloids, left uterine displacement in pregnancy, phenylephrine 50-100 microgram boluses or an infusion as first line, with ephedrine 5-10 mg as an alternative, plus oxygen and leg elevation if severe.
What is a total spinal and its immediate management?
An excessively high block producing apnoea, hypotension and loss of consciousness: secure the airway and ventilate with 100% oxygen, give fluids and vasopressors, and support the circulation until the block regresses.