Spinal anesthesia: dosing, spread, hypotension
Stem
A 29-year-old woman, 74 kg at term, G2P1, is scheduled for an elective repeat cesarean delivery. She is healthy, her platelet count is 210,000 and she takes no anticoagulants. Her baseline blood pressure is 118/72 and heart rate 84. She requests to be awake for the birth.
Question
Walk me through your spinal anesthetic, including the dose you choose and how you will manage the blood pressure?
Ideal answer
HeadlineNo contraindication, so a reliable block to touch at T5 with hyperbaric bupivacaine 0.75%, 10.5–12 mg, and prophylactic phenylephrine from the start.
- Fentanyl 10–25 mcg, preservative-free morphine 100 mcg, for intraoperative quality and postoperative analgesia; 100 mcg is what most experts use.1
- Below L2, 25- or 27-gauge pencil-point, because the conus usually ends at L1 and can reach L3.2
- Sitting to inject, then supine with left uterine displacement, because baricity and position determine spread more than volume.3
- Phenylephrine infusion 25–50 mcg/min, fluid co-load, titrated to baseline pressure and heart rate.4
- Test before incision: light touch to T5 or higher, bilateral, with motor block, because I test the level only once motor block is present.5,6
Watch for: bradycardia, which can progress to asystole; breathing and sedation every 2 hours for 12 hours for the morphine.1
Breaking down the answer
- Baricity and position control spread — hyperbaric drug follows gravity and pools in the thoracic kyphosis at T4–T6 supine; isobaric drug stays near the injection and gives a more predictable, lower block.3 Dose matters more than volume or concentration.
- Why go low — the conus lies at about L1, ranging from T12 to L3;3 Tuffier's line crosses L4 or L4–L5 but is often a level higher than palpation suggests.
- Phenylephrine first — it restores afterload without tachycardia, and in obstetrics preserves fetal pH better than ephedrine;4 ephedrine when heart rate is low.
- Bradycardia warns of arrest — it can progress to asystole or heart block.3 The Bezold-Jarisch reflex and blocked cardiac accelerators cause it suddenly in young, vagal patients; early atropine or ephedrine, epinephrine if it progresses.
- Contraindications — refusal, site infection, coagulopathy or anticoagulant within ASRA windows, raised intracranial pressure; severe aortic or mitral stenosis and hypertrophic obstructive cardiomyopathy are relative,2 so a titrated epidural, continuous spinal catheter or general anesthesia, because a sudden afterload drop cannot be compensated.
- Morphine dose and monitoring — low-dose intrathecal morphine is 50–150 mcg for about a day of analgesia; delayed respiratory depression peaks about 6.5–7.5 hours after the dose, and none is reported in obstetric patients beyond 12 hours at clinical doses.1
Follow-up question
Ten minutes after the block, before incision, her heart rate falls to 38 bpm and then the monitor shows asystole. She is unresponsive.
What do you do?
Follow-up answer
HeadlineSpinal-associated cardiac arrest; I say so and call for help, obstetric and neonatal teams included.
CPR now with manual left uterine displacement, epinephrine 1 mg IV: an ACLS arrest, not a vagal episode to watch. Before full arrest the drugs are atropine 0.5–1 mg and epinephrine 10–100 mcg; delayed epinephrine is the common factor in poor outcomes. I secure oxygenation and ventilation, intubating if she does not respond at once. The obstetrician prepares to deliver immediately if circulation does not return, because emptying the uterus helps resuscitation. After return of circulation: block level for a high spinal, fluid status, team debrief.
Pitfalls that cost points
- Treating every hypotension with fluid alone.
- Missing that sedation plus a high block causes hypoventilation.
- Forgetting ASRA anticoagulant timing as a contraindication question.
Rapid-fire
Q: Why does the sympathetic block extend higher than the sensory block? A: Small preganglionic B fibers block at lower concentrations; the sympathetic level is two to six dermatomes above sensory,3 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: Low back and buttock pain with leg dysesthesia after spinal lidocaine, in about 14%, more with lithotomy;3 knee arthroscopy is the other classic association. It resolves within days without deficit.
Q: Why is CSF volume the strongest patient factor? A: Lumbosacral CSF volume varies widely and correlates inversely with peak block height; obesity and pregnancy reduce it.3
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 | Low dose 50–150 mcg; 100 mcg most common1 |
| Conus medullaris | About L1 (T12–L3) in adults; L3 at birth3 |
| Cesarean block check | Light touch to T5 or higher, bilateral, with motor block; cold runs higher and does not substitute5 |
| Phenylephrine infusion | 25–50 mcg/min, titrated4 |
| Monitoring after low-dose morphine | RR and sedation every 2 h for 12 h1 |
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 (2018); SOAP neuraxial morphine monitoring consensus (2019)
Sources
- 1.SOAP Consensus Statement: Monitoring Recommendations for Prevention and Detection of Respiratory Depression Associated With Administration of Neuraxial Morphine for Cesarean Delivery Analgesia · SOAP (Bauchat et al., Anesth Analg), 2019 · soap.org
- 2.Spinal Anesthesia · StatPearls · ncbi.nlm.nih.gov
- 3.Spinal Anesthesia · NYSORA · nysora.com
- 4.Study Comparing Phenylephrine Bolus and Infusion for Maternal Hypotension and Neonatal Outcome during Cesarean Section under Spinal Anesthesia · Anesthesia, Essays and Researches (Choudhary, Bajaj), 2018 · pmc.ncbi.nlm.nih.govCites the international consensus (Kinsella et al., Anaesthesia 2018) recommending a variable-rate prophylactic phenylephrine infusion starting at 25-50 mcg/min; the consensus itself could not be opened.
- 5.Prevention and management of intra-operative pain during caesarean section under neuraxial anaesthesia: a technical and interpersonal approach · Obstetric Anaesthetists' Association (Plaat, Stanford, Lucas et al., Anaesthesia), 2022 · europepmc.orgRecommendations: light touch as the primary modality, block to T5 or higher; straight leg raise for motor block; cold and pinprick levels run several dermatomes above touch and do not predict it.
- 6.Failed spinal anesthesia for cesarean delivery: prevention, identification and management · Current Opinion in Anaesthesiology (Girard, Savoldelli), 2024 · pmc.ncbi.nlm.nih.govStates loss of touch to T5 is required (citing OAA 2022) and that the upper level is tested once some motor block is present.
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.