Obstetrics · 9 min
OB anesthesia — last-night quick guide
Pregnancy physiology, labor analgesia, c-section, OB emergencies.
Mnemonic — Up · Down · Anatomic
Pregnancy physiology shifts
Pregnancy physiology shifts
- Up↑ Cardiac output 30-50%, ↑ blood volume 30-40%, ↑ minute ventilation 50%, ↑ FRC eaten by gravid uterus, ↑ aspiration risk, ↑ thyroid binding globulin, ↑ factors VII/VIII/X (hypercoag)
- Down↓ FRC 20% supine, ↓ residual volume, ↓ albumin (dilutional), ↓ pseudocholinesterase 25% (succ effects mostly unchanged), ↓ MAC 25-40% (volatile)
- AnatomicDifficult airway 8x more common (engorged mucosa, larger breasts, edema, weight gain)
- AortocavalAfter 20 wk — LEFT lateral tilt 15° to prevent supine hypotension; aortocaval compression by gravid uterus
These shifts mean: full-stomach precautions always, RSI default for general anesthesia, ↓ induction doses, anticipate difficult airway, lateral tilt always, expect hypotension with neuraxial.
Labor analgesia options
| Technique | Onset | Notes |
|---|---|---|
| Epidural | 10-20 min | Catheter-based, titratable, can extend for c-section |
| CSE (combined spinal-epidural) | 5 min | Fast onset spinal + extendable epidural |
| DPE (dural puncture epidural) | 15-20 min | Pierce dura without injecting → faster sacral spread |
| IV remifentanil PCA | Immediate | When neuraxial contraindicated; close monitoring (apnea risk) |
| Pudendal block | 5 min | Stage 2 only — perineal anesthesia |
Rule
C-section spinal — bread + butter
Bupivacaine 0.5% hyperbaric 9-15 mg + fentanyl 10-25 mcg + morphine 0.1-0.2 mg (intrathecal Duramorph; 18-24h post-op analgesia). Target T4 level. Phenylephrine FIRST-LINE for hypotension (better fetal acid-base than ephedrine). Co-load 250-500 mL crystalloid. Aortocaval tilt 15° left.
Rule
C-section GA when
Cat 1 emergency without functional epidural; contraindication to neuraxial (severe coag, refusal, severe ICH); failed neuraxial. RSI: propofol 2 mg/kg or thio 4-5; succ 1.5 mg/kg or roc 1.2; volatile + N2O AFTER delivery (avoid uterine atony); oxytocin 3 IU IV slow + infusion after delivery. Difficult airway 8x more common in pregnancy.
Watch out
Oxytocin Rule of 3s
AVOID IV bolus >3 IU (hypotension, arrhythmia). 3 IU slow IV over 30 sec → reassess in 3 min → repeat × max 3 → infusion 3 IU/hr. Cardiac patients: even slower + lower.
Refractory atony — 2nd line
| Drug | Dose | Avoid in |
|---|---|---|
| Methylergonovine (methergine) | 0.2 mg IM | HTN, preeclampsia |
| Carboprost (hemabate) | 0.25 mg IM q15 min | Asthma |
| Misoprostol | 800-1000 mcg PR | (few cont) |
Watch out
Preeclampsia / HELLP
Treat severe HTN (>160/110): labetalol 20 mg IV → 40 → 80, hydralazine 5-10 mg IV, nicardipine infusion. Mg sulfate 4-6 g load + 2 g/hr (seizure prophylaxis). HELLP — platelets often <100; check before neuraxial (>70 OK most). Mg + NMBs: mg potentiates non-depolarizers; reduce roc dose 30-50%.