Spinal anesthesia: dosing, spread, hypotension
Why examiners ask this
Spinal anesthesia looks simple, so examiners use it to separate candidates who know why a block goes where it goes from those who only know a recipe. The questions test application of knowledge about baricity, position and dose; judgment about patients in whom an abrupt sympathectomy is dangerous; and adaptability when the block is too high, too low, or causes bradycardia and arrest in a young healthy patient.
The 60-second answer
I confirm there is no contraindication: refusal, infection at the site, coagulopathy or anticoagulant within ASRA windows, raised intracranial pressure, or a fixed cardiac output lesion where an abrupt drop in afterload is dangerous. I choose a drug and dose for the level and duration I need, for example hyperbaric bupivacaine 0.75% 10.5–12 mg for a T4 cesarean level or isobaric bupivacaine 0.5% 10–15 mg for a hip. I place it below L2 to avoid the conus, using a 25- or 27-gauge pencil-point needle. I position the patient deliberately because baricity and position determine spread more than volume. I prevent and treat hypotension with a phenylephrine infusion and a co-load of fluid, and I watch heart rate closely because bradycardia can progress to asystole.
Decision points and the "why"
- Baricity and position control spread — hyperbaric drug follows gravity and pools in the thoracic kyphosis at T4–T6 when the patient lies supine; isobaric drug stays near the injection site and gives a more predictable, lower block. Dose matters more than volume or concentration.
- Below L2–L3 in adults — the conus ends at about L1–L2; Tuffier's line crosses L4 or L4–L5 but is often a level higher than palpation suggests, so I go low.
- Phenylephrine first for hypotension — it restores afterload without tachycardia; ephedrine when heart rate is low. In obstetrics a prophylactic phenylephrine infusion is standard because it preserves fetal pH better than ephedrine.
- Bradycardia warns of arrest — the Bezold-Jarisch reflex and blocked cardiac accelerators cause sudden bradycardia in young patients with high vagal tone. I treat early with atropine or ephedrine, and go to epinephrine if it progresses.
- Fixed cardiac output lesions — in severe aortic stenosis or hypertrophic obstructive cardiomyopathy I prefer a titrated epidural or continuous spinal catheter, or general anesthesia, because a sudden afterload drop cannot be compensated.
- Adjuncts — intrathecal morphine 100–150 mcg for cesarean gives 18–24 hours of analgesia, with delayed respiratory depression requiring monitoring; fentanyl 10–25 mcg improves intraoperative quality.
The complication they will inject
A healthy 32-year-old having knee arthroscopy under spinal becomes bradycardic at 38 bpm, then asystolic, ten minutes after the block. I call for help and start CPR immediately, give epinephrine 1 mg IV, and treat it as an ACLS arrest, not a vagal episode to watch. Before full arrest, the right drugs are atropine 0.5–1 mg and epinephrine 10–100 mcg; delayed epinephrine is the common factor in poor outcomes from spinal arrests. I ensure oxygenation and ventilation, because sedation contributes. After return of circulation, I check the block level, review sedation and fluid status, and debrief.
Pitfalls that cost points
- Saying a larger volume of the same dose gives a higher block; dose drives spread more than volume.
- Treating every hypotension with fluid alone.
- Treating bradycardia with observation in a sedated young patient after a spinal.
- Forgetting ASRA anticoagulant timing as a contraindication question.
- Missing that sedation plus a high block causes hypoventilation.
Rapid-why
Q: Why does the sympathetic block extend higher than the sensory block? A: Small preganglionic B fibers are blocked at lower concentrations; sympathetic level is about two segments above sensory, and motor about two below.
Q: Why do elderly patients get higher blocks? A: Reduced CSF volume, kyphosis and greater neural sensitivity.
Q: What is transient neurologic symptoms? A: Buttock and leg pain after spinal lidocaine, especially with lithotomy or knee arthroscopy positioning; it resolves within days and has no deficit.
Q: Why is CSF volume the strongest patient factor? A: Lumbosacral CSF volume varies widely and inversely predicts peak block height; it is reduced by obesity and pregnancy.
Numbers to know
| Item | Value |
|---|---|
| Hyperbaric bupivacaine 0.75% | 10.5–12 mg for cesarean |
| Isobaric bupivacaine 0.5% | 10–15 mg for lower-limb surgery |
| Intrathecal morphine | 100–150 mcg for cesarean |
| Conus medullaris | L1–L2 in adults; L3 in neonates |
| Cesarean target level | T4 to pinprick or cold |
| Phenylephrine infusion | 25–50 mcg/min, titrated |
Links
- High or total spinal, Epidural anesthesia and the test dose, Post-dural puncture headache, Neuraxial blocks and anticoagulants (ASRA), Intraoperative hypotension: differential and management, Aortic stenosis
- ASRA Practice Advisory on neurologic complications in regional anesthesia; international consensus statement on vasopressors during spinal for cesarean
Sources
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