07 / Anaesthesia · Regional
Block the nerve, not the brain
5 local anaesthetics with their ceilings, 7 operations with the block height each one needs, the needle’s nine layers, and the 10 peripheral blocks a student meets on the list.
Photograph: DocP, CC BY-SA 2.0 de, via Wikimedia Commons.
Before every block
The maximum dose, in millilitres of the bottle in your hand
Ceiling
3 mg/kg
Capped at 200 mg however heavy the patient.
For this patient
200 mg
3 times 70 kg, against the ceiling.
Of the 1% solution
20 mL
200 mg over 10 mg/mL. One percent is ten milligrams a millilitre; every sum starts there.
Lidocaine: onset fast: 2 to 5 minutes, lasts 1 to 2 hours plain, 2 to 4 with adrenaline. The workhorse for infiltration, the epidural top-up and intravenous use. Toxicity announces itself with tingling and tinnitus before the heart is involved.
The map
Dermatomes, and how high the block must be
Point at a level
T10: The umbilicus
How high must the block be?
Caesarean section
The landmarks to know cold
- The top of the shoulders and the clavicles
- The thumb
- The middle finger
- The little finger
- The inner arm
- The nipples
- The xiphisternum
- The costal margin
- The umbilicus
- The inguinal ligament and the iliac crest
- The groin crease
- The front of the thigh
- The knee
- The medial malleolus
- The dorsum of the foot and the great toe
- The lateral foot, the little toe and the heel
- The perineum, the saddle
Midline, L3/4
Walk the needle in, layer by layer
- 1…
- 2…
- 3…
- 4…
- 5…
- 6…
- 7…
- 8…
- 9…
Reference
The tables the ward round asks about
| Feature | Spinal | Epidural |
|---|---|---|
| Where the drug goes | Into cerebrospinal fluid below the conus | Into the epidural space at any level, through a catheter |
| Dose | Tiny: 2 to 3 mL of 0.5 percent bupivacaine, 10 to 15 mg | Large: 10 to 20 mL for surgery, or an infusion of 8 to 15 mL/h of a dilute mixture |
| Onset | Minutes; dense | 15 to 30 minutes; segmental and titratable |
| Duration | Finite: 1.5 to 3 hours, cannot be extended | As long as the catheter runs, for days |
| Motor block | Profound | Adjustable by concentration; a labour epidural spares the legs |
| Hypotension | Fast and often marked | Slower, more gradual |
| Headache risk | About one percent with a 25 gauge pencil-point needle | Only after an accidental dural puncture, then over half |
| The catastrophic error | A high block from too large a dose, or from a dose meant for the epidural space | An epidural dose given intrathecally through a migrated catheter: total spinal. Every top-up is preceded by an aspiration and a test dose. |