Obstetric Anesthesia and Analgesia Flashcards

A parturient at 32 weeks gestation with severe preeclampsia requires urgent cesarean delivery. She has a platelet count of 68,000/μL and received her last dose of low-molecular-weight heparin (enoxaparin 40 mg) 8 hours ago. Which anesthetic approach is most appropriate?

Answer: General anesthesia with rapid sequence induction is preferred

This patient has two contraindications to neuraxial anesthesia: thrombocytopenia (platelets <80,000/μL is a relative contraindication, and <70,000/μL is generally considered a contraindication by most guidelines) and recent LMWH administration within 12 hours of a prophylactic dose (or 24 hours of a therapeutic dose). General anesthesia with RSI — using modified cricoid pressure and anticipating difficult airway due to airway edema in preeclampsia — is the correct choice. Spinal anesthesia carries risk of spinal hematoma. Epidurals have a larger needle and catheter, increasing the risk even further. Delaying is not appropriate given the urgency.

During establishment of epidural labor analgesia, an unintentional intravascular catheter placement is suspected. Which test dose finding most specifically indicates intravascular rather than intrathecal placement in a labouring patient receiving oxytocin?

Answer: Heart rate increase of ≥20 bpm within 60 seconds of epinephrine 15 mcg

The standard epidural test dose contains epinephrine 15 mcg (typically as 3 mL of 1.5% lidocaine with 1:200,000 epinephrine). An intravascular injection produces a heart rate increase of ≥20 bpm within 45–60 seconds due to epinephrine's beta-1 effect — this is the most specific marker for intravascular placement. Hypotension can occur with either intravascular or intrathecal injection. Motor block (option C) is characteristic of intrathecal placement. Tinnitus and metallic taste (option D) suggest intravascular local anesthetic toxicity but are unreliable in a labouring patient with background contraction pain and are not part of the standard test dose protocol.

A patient with known Eisenmenger syndrome due to a large ventricular septal defect presents at 28 weeks gestation requesting pain relief in early labour. Which analgesic strategy carries the lowest risk of precipitating acute right-to-left shunt reversal?

Answer: Systemic remifentanil patient-controlled analgesia

In Eisenmenger syndrome, the right-to-left shunt is driven by elevated pulmonary vascular resistance (PVR) exceeding systemic vascular resistance (SVR). Any reduction in SVR — such as from neuraxial sympathectomy — risks worsening the shunt and causing severe hypoxemia or cardiovascular collapse. Systemic remifentanil PCA avoids any sympatholytic effect on SVR and is the safest labour analgesic in this context. CSE with intrathecal opioid alone (option A) is theoretically possible but introduces epidural catheter risk and incremental bolus risk. Incremental epidural bupivacaine (option C) causes sympathetic block and SVR reduction. Single-shot spinal with bupivacaine (option D) causes rapid, profound sympathectomy — the most dangerous option in Eisenmenger syndrome.

A primiparous patient at 39 weeks receives a combined spinal-epidural (CSE) for labour analgesia. Thirty minutes after intrathecal sufentanil 10 mcg + bupivacaine 2.5 mg, she develops severe fetal bradycardia to 60 bpm. Her blood pressure is 118/74 mmHg, SpO2 is 99%, and she has no uterine hyperstimulation on tocometry. Which mechanism BEST explains this complication?

Answer: Uterine hypertonus secondary to rapid fall in circulating catecholamines

Fetal bradycardia following CSE without maternal hypotension is a well-documented phenomenon, occurring in approximately 8–14% of cases. The most accepted mechanism is uterine hypertonus caused by a sudden decrease in circulating maternal epinephrine (which has tocolytic beta-2 effects) following rapid opioid-induced pain relief. With pain abolished, catecholamine levels fall precipitously, removing their tocolytic effect and allowing oxytocin-driven uterine tone to increase, reducing intervillous blood flow. Maternal hypotension (option C) is excluded by the normal blood pressure. Systemic opioid absorption (option A) is minor and not the dominant pathway. Direct fetal myocardial depression (option D) does not occur at these intrathecal doses with intact placental metabolism.

A woman with a history of lumbar laminectomy at L4–L5 and L5–S1 presents for elective cesarean section. She specifically requests spinal anesthesia. After two failed attempts at L3–L4, you successfully place the needle at L2–L3 and inject hyperbaric bupivacaine 12 mg with fentanyl 25 mcg. Ten minutes later, the block level is T10 bilaterally with no sacral spread. The most likely explanation is:

Answer: Extensive arachnoid adhesions from prior surgery compartmentalizing CSF flow

This presentation — a block reaching T10 but with absent or patchy sacral (S2–S5) coverage despite injection at L2–L3 — is the hallmark of arachnoid adhesions from prior lumbar surgery. Post-laminectomy changes can create adhesive arachnoiditis that compartmentalizes the subarachnoid space, producing unpredictable, patchy, or directionally limited block spread. Importantly, the adhesions may preferentially allow cephalad CSF spread while limiting caudal flow to the sacral roots. A subdural injection (option A) typically produces a high, extensive, but unexpectedly dense block. Inadequate dose (option C) is incorrect — 12 mg hyperbaric bupivacaine easily achieves T10 in a cesarean-section patient. Epidural scarring (option B) is an extrathecal phenomenon and does not directly limit intrathecal drug distribution.

In the management of refractory hypotension following spinal anesthesia for cesarean section, which vasopressor strategy has the strongest evidence for preserving fetal acid-base status while maintaining maternal blood pressure?

Answer: Phenylephrine infusion (25–50 mcg/min) with rescue ephedrine for bradycardia

Multiple RCTs and meta-analyses (Ngan Kee et al., Heesen et al.) demonstrate that phenylephrine (an alpha-1 agonist) is superior to ephedrine for maintaining uteroplacental blood flow and fetal acid-base status during spinal anesthesia for cesarean delivery. Ephedrine crosses the placenta and stimulates fetal beta-adrenoceptors, increasing fetal metabolic rate and producing lower umbilical arterial pH and higher lactate — even when maternal blood pressure is equally maintained. The current standard is a phenylephrine infusion with rescue ephedrine only if bradycardia occurs (since phenylephrine can cause reflex bradycardia). Norepinephrine (option D) is emerging as an alternative with similar fetal outcomes to phenylephrine, but current guidelines still support phenylephrine as first-line. Dopamine (option C) has no established role in obstetric spinal hypotension management.