17.1 Obstetric Analgesia and Anaesthesia: Labour, Caesarean Delivery, and High-Risk Conditions

17.1 Obstetric Analgesia and Anaesthesia: Labour, Caesarean Delivery, and High-Risk Conditions

Obstetric anaesthesia requires an acute understanding of unique maternal-fetal physiology, dynamic alterations in pain pathways during parturition, and rapid resuscitation algorithms for life-threatening obstetric crises. Anaesthetic care must simultaneously safeguard two interdependent patients: the mother and the fetus.

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1. Neuroanatomy of Labour Pain and Neuraxial Analgesia Pathways

Labour pain evolves through two distinct anatomical stages governed by separate neurological mechanisms and spinal cord segmental distributions.

Segmental Pathways of Labour Pain

1. First Stage of Labour (Cervical Dilation & Uterine Contractions):

   • Mechanism: Mechanical stretching of the lower uterine segment, progressive dilation and effacement of the cervix, and myometrial ischaemia during contractions stimulate mechanoreceptors and chemosensitive nociceptive free nerve endings.
   • Afferent Pathway: Visceral nociceptive afferents (unmyelinated C-fibers) travel retrogradely along sympathetic nerve pathways. They traverse the paracervical tissue, pelvic plexus, inferior hypogastric plexus, hypogastric nerve, superior hypogastric plexus, and the lumbar sympathetic chain to enter the spinal cord at T10, T11, T12, and L1 via dorsal nerve roots.
   • Pain Quality: Dull, aching, poorly localized visceral pain referred to the lower abdominal wall, lumbosacral region, and anterior thighs.

2. Second Stage of Labour (Fetal Descent & Pelvic Expulsion):

   • Mechanism: Direct mechanical compression, distension, and tearing of pelvic fascia, vaginal vault, levator ani muscles, and perineal skin by the presenting fetal part.
   • Afferent Pathway: Somatic nociceptive afferents (predominantly myelinated $A\delta$ and unmyelinated C-fibers) travel via the pudendal nerve (formed by the ventral rami of S2, S3, and S4). The pudendal nerve traverses the greater sciatic foramen, crosses behind the ischial spine and sacrospinous ligament, enters the ischiorectal fossa through the lesser sciatic foramen via Alcock's canal, and innervates the perineum, vulva, and distal vagina.
   • Pain Quality: Sharp, intense, well-localized somatic pain focused on the perineum, anus, and vulva.

                             [ PAIN PATHWAYS OF LABOUR ]
                                          |
             +----------------------------+----------------------------+
             |                                                         |
     [ First Stage (T10-L1) ]                                  [ Second Stage (S2-S4) ]
     - Origin: Cervix & lower uterine segment                 - Origin: Perineum, vagina & pelvic floor
     - Type: Visceral C-fibers via sympathetics                - Type: Somatic A-delta & C-fibers
     - Pathway: Paracervical -> Hypogastric                   - Pathway: Pudendal nerve (S2, S3, S4)
       plexus -> Lumbar sympathetic chain                       via Alcock's canal in ischiorectal fossa
     - Sensation: Dull, poorly localized                      - Sensation: Sharp, intense, well-localized

Modern Epidural Analgesia Regimens

Modern obstetric epidural analgesia emphasizes motor-sparing regimens utilizing ultra-low concentrations of local anaesthetics combined with lipophilic opioids to provide reliable sensory analgesia while preserving pelvic floor tone and maternal mobility ("walking epidural").

• Drug Formulations: Bupivacaine $0.0625\% - 0.1\%$ or ropivacaine $0.1\%$ combined with fentanyl $2\text{ }\mu\text{g/mL}$ or sufentanil $0.5\text{ }\mu\text{g/mL}$. Ropivacaine produces slightly less motor block and possesses a lower cardiotoxic liability than racemic bupivacaine.
• Delivery Modalities:
  • Programmed Intermittent Epidural Bolus (PIEB): High-yield clinical trials demonstrate that PIEB (administering fixed boluses of $8 - 10\text{ mL}$ every $40 - 45\text{ minutes}$) is markedly superior to traditional Continuous Epidural Infusion (CEI).
  • Biophysical Mechanism: Intermittent high-flow boluses generate significant peak opening pressures inside the epidural space, ejecting solution uniformly through all lateral orifices of multi-orifice epidural catheters. This produces broad, circumferential radial spread. Conversely, low-rate continuous infusions ($5 - 10\text{ mL/h}$) produce negligible ejection pressure; fluid trickles exclusively through the single proximal eyelet and tracks along path-of-least-resistance tissue planes, predisposing to patchy, unilateral blocks.
  • Clinical Outcomes: PIEB significantly reduces total hourly local anaesthetic consumption, decreases maternal motor block, reduces instrumental vaginal delivery rates, and enhances maternal satisfaction compared to CEI.
  • Patient-Controlled Epidural Analgesia (PCEA): Used in conjunction with PIEB, allowing maternal breakthrough boluses ($5\text{ mL}$ with a $10 - 15\text{ minute}$ lockout period).

Combined Spinal-Epidural (CSE) and Dural Puncture Epidural (DPE)

• Combined Spinal-Epidural (CSE): Utilizes a needle-through-needle technique. Following Tuohy needle placement in the epidural space, a long 25G or 27G pencil-point spinal needle punctures the dura mater. Intrathecal injection of lipophilic opioid (fentanyl $15 - 25\text{ }\mu\text{g}$) with low-dose bupivacaine ($1.25 - 2.5\text{ mg}$) provides rapid analgesia within $2 - 5\text{ minutes}$ with zero motor block. The spinal needle is withdrawn, and an epidural catheter is inserted. CSE is particularly valuable in advanced or rapidly progressing labour. Drawbacks include transient fetal bradycardia (induced by sudden reduction in circulating maternal catecholamines precipitating uterine hypertonus) and pruritus.
• Dural Puncture Epidural (DPE): A technique where the dura is intentionally punctured with a 25G or 27G pencil-point needle through the Tuohy needle, but no medication is injected intrathecally. An epidural catheter is then placed conventionally. The micro-dural puncture allows subarachnoid translocation of epidural medications along hydrostatic pressure gradients. DPE yields faster onset, denser sacral block for second-stage pain, and fewer asymmetric blocks than standard epidural analgesia, without increasing the incidence of post-dural puncture headache (PDPH).

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2. Anaesthesia for Caesarean Delivery: Regional vs General

Regional anaesthesia is the gold standard for Caesarean delivery, avoiding the morbidity of general anaesthesia in the obstetric patient (difficult intubation, gastric aspiration, fetal drug depression).

Spinal Anaesthesia Regimens and Dermatomal Targets

• Standard Intrathecal Drug Regimen:
  • Hyperbaric bupivacaine $0.5\%$: typically $10 - 12.5\text{ mg}$ ($2.0 - 2.5\text{ mL}$); some centres use $8 - 10\text{ mg}$ with opioid adjuncts.
  • Fentanyl: $15 - 20\text{ }\mu\text{g}$ (rapid visceral analgesia mitigating peritoneal traction, exteriorization pain, and bladder flap manipulation).
  • Preservative-free morphine: $100\text{ }\mu\text{g}$ ($0.1\text{ mg}$, provides profound postoperative analgesia for $18 - 24\text{ hours}$ via spinal $\mu$-opioid receptor activation).
• Target Sensory Level: Bilateral T4 dermatome (nipple line) to cold or light touch.
  • Clinical Rationale: Surgical incision involves T10-L1; however, uterine exteriorization, manipulation of pelvic viscera, and traction on the peritoneum stimulate peritoneal and diaphragmatic afferents carried by high thoracic sympathetic fibers (T5-T9) and the vagus nerve, causing severe visceral pain, retching, and autonomic distress unless a T4 sensory level is established.

Pathophysiology and Prevention of Spinal-Induced Maternal Hypotension

Intrathecal local anaesthetics produce rapid preganglionic sympathetic blockade ($B\text{ fibers}$), resulting in extensive arteriolar and venous vasodilation. Massive venous pooling in the lower extremities and splanchnic vascular bed reduces venous return ($VR$), stroke volume, and cardiac output. This is drastically exacerbated in the supine position by aortocaval compression by the gravid uterus.

        [ Intrathecal Hyperbaric Bupivacaine ]
                         |
         [ Extensive Sympathetic Denervation ]
                         |
      +------------------+------------------+
      |                                     |
[ Venodilation -> Massive ]             [ Arteriolar Dilation -> ]
[ Venous Pooling in Legs  ]             [ Precipitous Drop in    ]
[ & Splanchnic Bed        ]             [ Systemic Vascular Res. ]
      |                                     |
      +------------------+------------------+
                         |
           [ Reduced Venous Return (VR) ]
           [ Exacerbated by Aortocaval  ]
           [ Compression of Gravid Uterus]
                         |
         [ Severe Maternal Hypotension & ]
         [ Reduced Uteroplacental Flow   ]

• Preventative and Therapeutic Strategy:
  1. Left Uterine Displacement: Mandatory $15^\circ$ left lateral table tilt or pelvic wedge to relieve inferior vena cava and aortic compression.
  2. Fluid Co-loading: Rapid infusion of $1000 - 1500\text{ mL}$ balanced crystalloid administered concurrently with intrathecal injection. Co-loading is substantially superior to pre-loading; pre-loaded crystalloid rapidly redistributes into the extracellular interstitial space before sympathetic block establishes, failing to prevent hypotension and promoting tissue oedema.
  3. First-Line Vasopressor: Phenylephrine Infusion:
     • Phenylephrine (pure selective $\alpha_1$-adrenergic agonist) is the gold-standard first-line agent, administered as a prophylactic infusion ($25 - 50\text{ }\mu\text{g/min}$) titrated to maintain maternal systolic blood pressure at $\ge 90 - 100\%$ of baseline.
     • Phenylephrine vs Ephedrine: Ephedrine is a mixed $\alpha$- and $\beta$-adrenergic agonist that readily crosses the placenta. In the fetus, ephedrine stimulates $\beta$-adrenoceptors, markedly increasing fetal myocardial work, oxygen consumption, and glucose metabolism, producing lactic acidosis and lower umbilical arterial blood pH. In contrast, phenylephrine preserves fetal acid-base equilibrium. If phenylephrine induces maternal reflex bradycardia (mediated by baroreceptor reflexes), low-dose ephedrine boluses ($3 - 6\text{ mg}$) or norepinephrine infusion ($2 - 4\text{ }\mu\text{g/min}$) can be substituted.

General Anaesthesia for Caesarean Section

Historically, general anaesthesia for Caesarean delivery carried substantially higher maternal mortality than neuraxial techniques; the gap has narrowed with better airway management, but airway catastrophe and pulmonary aspiration remain the main risks.

• Absolute & Urgent Indications:
  • Immediate Category 1 emergency Caesarean delivery with sustained profound fetal bradycardia ($<80\text{ bpm}$) or cord prolapse, where neuraxial blockade cannot be established in time.
  • Severe maternal haemorrhage with uncorrected hypovolemic shock.
  • Severe coagulopathy or profound thrombocytopenia ($<50 \times 10^9/\text{L}$).
  • Failed or patchy regional block converting during surgery.
• Technique: Strict Rapid Sequence Induction (RSI):
  • Positioning: $30^\circ$ head-up ramped position to optimize laryngeal alignment, counteract maternal breast tissue interference, and reduce gastro-oesophageal passive regurgitation.
  • Denitrogenation / Pre-oxygenation: $100\%\text{ O}_2$ for 3 minutes of tidal breathing or 8 vital capacity breaths. Obstetric patients have an elevated metabolic oxygen consumption ($\dot{V}\text{O}_2$ increased by $35 - 50\%$) and decreased functional residual capacity ($FRC$ reduced by $20 - 30\%$), leading to precipitously fast arterial desaturation upon induction.
  • Cricoid pressure (Sellick's maneuver): Applied with $10\text{ N}$ force awake, increasing to $30\text{ N}$ upon loss of consciousness. Released immediately if active vomiting occurs or if it impedes direct/videolaryngoscopic glottic view.
  • Induction agents: Propofol ($2.0 - 2.5\text{ mg/kg}$) or thiopental ($4 - 5\text{ mg/kg}$). Thiopental historically preserves uterine contractility slightly better, but propofol is widely accepted.
  • Neuromuscular blockade: Succinylcholine ($1.0 - 1.5\text{ mg/kg}$) remains the classic choice for ultra-rapid onset ($45 - 60\text{ s}$) and brief duration. Alternatively, high-dose rocuronium ($1.0 - 1.2\text{ mg/kg}$) is utilized provided sugammadex ($16\text{ mg/kg}$) is immediately available in the operating room for rescue reversal in a "Cannot Intubate, Cannot Oxygenate" (CICO) crisis.
  • Maintenance: Prior to delivery, administer $50\%\text{ O}_2$ in nitrous oxide or air with volatile anaesthetic at $0.5 - 0.75\text{ MAC}$ (e.g. sevoflurane $1 - 1.5\%$) to ensure maternal awareness does not occur while avoiding volatile-induced myometrial relaxation.
• Obstetric Difficult Airway Preparedness:
  • Maternal airway oedema (mucosal friability), increased capillary engorgement, enlarged breasts, and obesity increase the incidence of failed intubation (about 1 in 390 obstetric general anaesthetics, or 2.6 per 1,000, in a 2015 meta-analysis; several times the non-obstetric rate).
  • Preparedness checklist: Smaller endotracheal tubes ($6.0 - 6.5\text{ mm}$ ID), short-handled laryngoscope, primary videolaryngoscopy, and second-generation supraglottic airway devices (SAD, e.g. ProSeal or i-gel) immediately accessible.
  • DAS Obstetric Algorithm: If intubation fails after 2 attempts: insert second-generation SAD. If oxygenation is successful: in life-threatening fetal compromise, proceed with surgery using SAD maintaining cricoid pressure/gastric suctioning; in non-life-threatening situations, wake the mother up and perform awake fibreoptic intubation or regional anaesthesia.

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3. High-Risk Obstetric Conditions: Preeclampsia, Eclampsia, and HELLP

Hypertensive disorders complicate $5 - 10\%$ of pregnancies and represent a leading cause of maternal and perinatal mortality.

Preeclampsia: Definition and Diagnostic Criteria

• Core Definition: New-onset hypertension occurring after 20 weeks of gestation in a previously normotensive woman, accompanied by proteinuria or new-onset maternal organ dysfunction:
  • Blood pressure: Systolic $\ge 140\text{ mmHg}$ or diastolic $\ge 90\text{ mmHg}$ on two separate occasions at least 4 hours apart.
  • Proteinuria: $\ge 300\text{ mg}$ per 24-hour urine collection, or protein-to-creatinine ratio $\ge 30\text{ mg/mmol}$ ($0.3\text{ mg/mg}$). In the absence of proteinuria, preeclampsia is confirmed by new-onset organ dysfunction (thrombocytopenia, renal insufficiency, impaired liver function, pulmonary oedema, or neurological symptoms).
• Features of Severe Preeclampsia:
  • Severe hypertension: $\text{SBP} \ge 160\text{ mmHg}$ or $\text{DBP} \ge 110\text{ mmHg}$ confirmed within 15 minutes.
  • Haematological: Thrombocytopenia with platelet count $<100 \times 10^9/\text{L}$.
  • Hepatic: Serum transaminases (AST/ALT) elevated to $\ge 2 \times$ the upper limit of normal; severe persistent right upper quadrant or epigastric pain unresponsive to medication.
  • Renal: Serum creatinine $>97\text{ }\mu\text{mol/L}$ ($>1.1\text{ mg/dL}$) or doubling of serum creatinine in the absence of renal disease.
  • Respiratory: Acute pulmonary oedema.
  • Neurological: New-onset persistent cerebral symptoms (severe throbbing frontal/occipital headache, photopsia, scotomata, cortical blindness, hyperreflexia with sustained ankle clonus $\ge 3$ beats).

HELLP Syndrome

A severe variant of preeclampsia characterized by:

• H (Hemolysis): Microangiopathic hemolytic anemia, fragmented red blood cells (schistocytes, burr cells) on peripheral smear, total serum bilirubin $\ge 20.5\text{ }\mu\text{mol/L}$ ($1.2\text{ mg/dL}$), and elevated lactate dehydrogenase ($\text{LDH} > 600\text{ U/L}$).
• EL (Elevated Liver enzymes): Serum AST or ALT $\ge 70\text{ U/L}$.
• LP (Low Platelets): Platelet count $<100 \times 10^9/\text{L}$. Hepatic capsular distension can cause subcapsular haematoma formation; sudden hypotension and epigastric pain indicate spontaneous hepatic rupture, requiring emergent surgical laparotomy.

Medical Management: Antihypertensives and Magnesium Sulfate

• Antihypertensive Pharmacotherapy: Severe hypertension ($\ge 160/110\text{ mmHg}$) needs urgent treatment; NICE (2019) targets $\le 135/85\text{ mmHg}$ and ISSHP (2021) suggests a diastolic target of about $85\text{ mmHg}$, preventing haemorrhagic stroke without compromising uteroplacental perfusion:
  • Labetalol: Combined $\alpha_1$- and non-selective $\beta$-blocker ($1:7$ IV ratio). Initial dose $20\text{ mg}$ IV over 2 minutes, repeated at $40 - 80\text{ mg}$ every 10 minutes (maximum total $300\text{ mg}$) or continuous infusion ($1 - 2\text{ mg/min}$). Avoid in severe asthma or bradycardia.
  • Hydralazine: Direct arteriolar smooth muscle vasodilator. Administer $5 - 10\text{ mg}$ IV slowly every 20 minutes (maximum $20\text{ mg}$). Potential adverse effects include reflex tachycardia, maternal hypotension, and headache.
  • Nifedipine: Oral immediate-release or modified-release tablets ($10 - 20\text{ mg}$ orally). Critical Trap: Sublingual nifedipine is strictly contraindicated due to uncontrolled, precipitous maternal hypotension, leading to catastrophic placental hypoperfusion and fetal demise.
• Seizure Prophylaxis and Treatment ($MgSO_4$):
  • Magnesium sulfate is the definitive agent for preventing and treating eclamptic seizures (superior to diazepam, phenytoin, or lytic cocktails in the Magpie trial).
  • Dosing Regimen: Intravenous loading dose of $4\text{ g}$ IV over $15 - 20\text{ minutes}$, followed by a continuous maintenance infusion of $1\text{ g/h}$ for 24 hours postpartum or post-last seizure.
  • Therapeutic Serum Level: $2.0 - 3.5\text{ mmol/L}$ ($4.0 - 7.0\text{ mEq/L}$). Physiological normal serum magnesium is $0.7 - 1.0\text{ mmol/L}$.
  • Magnesium Toxicity Monitoring & Management:
    • Patellar Reflexes: Loss of deep tendon (patellar) reflexes occurs at $4.0 - 5.0\text{ mmol/L}$ ($8 - 10\text{ mEq/L}$) and serves as the earliest, most reliable clinical indicator of toxicity.
    • Respiratory Depression: Occurs at $5.0 - 6.5\text{ mmol/L}$ ($10 - 13\text{ mEq/L}$).
    • Cardiac Conduction Impairment & Arrest: PR prolongation, QRS widening, and asystole occur at $>7.0\text{ mmol/L}$ ($>15\text{ mEq/L}$).
    • Antidote: Stop the magnesium infusion immediately. Administer $10\%\text{ calcium gluconate } 10\text{ mL}$ ($1\text{ g}$) IV slowly over 10 minutes.

Neuraxial Anaesthesia in Preeclampsia

Epidural analgesia provides substantial therapeutic benefit in preeclampsia by blunting excessive sympathetic hyperreactivity and improving uteroplacental blood flow. Neuraxial placement and catheter removal are considered safe when the platelet count is $\ge 70 - 80 \times 10^9/\text{L}$, provided platelet count is stable, platelet function is not inhibited, and INR/aPTT are normal. In HELLP syndrome, platelet counts can plummet within hours; an immediate pre-procedure platelet count is mandatory.

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4. Obstetric Haemorrhage and Postpartum Resuscitation

Obstetric haemorrhage accounts for over $25\%$ of maternal deaths worldwide. Definitions of postpartum haemorrhage (PPH) differ: ACOG uses cumulative blood loss $\ge 1000\text{ mL}$ within 24 hours of delivery, or blood loss accompanied by signs or symptoms of hypovolemia, whereas WHO and RCOG use $\ge 500\text{ mL}$ after birth (major PPH $> 1000\text{ mL}$).

Etiological Classification: The 4 Ts

1. Tone (~70-80%): Uterine atony (overdistension from twins, polyhydramnios, macrosomia; prolonged labour; chorioamnionitis; tocolytic agents).
2. Tissue (~10%): Retained placenta or membranes; Placenta Accreta Spectrum (PAS: accreta attaching directly to myometrium, increta invading into myometrium, percreta penetrating through uterine serosa into bladder or surrounding pelvic structures).
3. Trauma (~15%): Cervical, vaginal, or perineal lacerations; uterine inversion; uterine rupture (previous Caesarean scar).
4. Thrombin (<1%): Pre-existing coagulopathies, placental abruption, amniotic fluid embolism (AFE), consumption coagulopathy.

Pharmacological Uterotonics: Mechanism, Dosing, and Contraindications

Antifibrinolytic Therapy: The WOMAN Trial

• Tranexamic Acid (TXA): Synthetic lysine analogue that competitively inhibits plasminogen activation.
• The landmark WOMAN trial (20,000+ women) established that early administration of IV tranexamic acid ($1\text{ g}$ IV over 10 minutes) administered within 3 hours of delivery reduces maternal mortality due to bleeding by nearly one-third without increasing thrombotic complications.
• If bleeding continues after 30 minutes, or restarts within 24 hours, a second $1\text{ g}$ dose is given.
• Critical Window: In WOMAN, the benefit was seen when TXA was given within 3 hours of birth; no benefit was shown with later treatment.

Massive Transfusion Protocol (MTP) and Fibrinogen Dynamics

• Coagulopathy in Obstetrics: In normal term pregnancy, procoagulant factors rise significantly, with plasma fibrinogen physiologically elevating to $4.0 - 6.0\text{ g/L}$ (compared to $2.0 - 4.0\text{ g/L}$ in non-pregnant adults).
• Critical Fibrinogen Threshold: A plasma fibrinogen level $<2.0\text{ g/L}$ during active PPH is an independent, highly sensitive predictor of severe haemorrhage, progression to massive transfusion, and maternal death. Fibrinogen must be aggressively repleted early.
• Transfusion Protocol:
  • Begin early blood product resuscitation; while results are awaited, many protocols give Packed Red Blood Cells (PRBCs) and Fresh Frozen Plasma (FFP) in a fixed ratio (for example 1:1, RCOG), then switch to replacement guided by laboratory or viscoelastic results, with platelets given if the count falls below about $75 \times 10^9/\text{L}$.
  • Replete fibrinogen using cryoprecipitate ($2\text{ pools}$, supplying $\sim 3 - 4\text{ g}$ of fibrinogen) or fibrinogen concentrate ($30 - 60\text{ mg/kg}$ IV, typically $2 - 4\text{ g}$), targeting a circulating fibrinogen concentration $>2.0\text{ g/L}$.
  • Utilize point-of-care viscoelastic testing (rotational thromboelastometry [ROTEM] or thromboelastography [TEG]). An abnormal FIBTEM (e.g. A10 $<10\text{ mm}$) specifically confirms hypofibrinogenemia and directs immediate targeted fibrinogen replacement before standard laboratory coagulopathy tests return.