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Review Note

Last Update: 03/27/2025 10:27 PM

Current Deck: ACG Part 2::Obstetrics

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SS_OB 1.36 Discuss the pathophysiology and anaesthetic management of co-existing maternal conditions as described in the Perioperative medicine Clinical Fundamental, in particular:

∙ Morbid obesity
Obesity
  • BMI > 40 = morbid obesity.
  • Associated with reduced life expectancy and increased obstetric and anesthetic-related morbidity.
  • Normal weight gain in pregnancy from baseline: 10-16kg.
Pathophysiology of Morbid Obesity Affecting Pregnancy:
  • A combination of physiological changes associated with pregnancy and obesity results in organ systems nearing their physiological limits.
Respiratory:
  • Reduced FRC, RV, and ERV from fat deposition in the chest wall and diaphragm and cephalad movement of the diaphragm associated with pregnancy.
  • FRC may fall below closing capacity, resulting in shunting.
  • Reduced compliance due to increased pulmonary blood volume, reduced FRC, and reduced chest wall compliance. This may be exacerbated by supine, lithotomy, and Trendelenberg positions.
  • Worsening V/Q matching as lung apices are preferentially ventilated, resulting in lower PaO2 (80-85mmHg vs 104-108mmHg in non-obese).
  • Increased work of breathing (WOB) from reduced compliance and chest wall weight.
  • Rapid shallow breathing with relative restrictive respiratory defect.
  • Linear increase in oxygen consumption (VO2) and CO2 production (VCO2).
  • Increased risk of sleep apnoea.
  • Airway: Soft tissue adiposity, capillary engorgement, and mucosal edema.
Cardiovascular:
  • Increase in CO proportional to obesity (30-50ml/100g adipose tissue/min).
  • Increase in blood volume, though the ratio of blood volume to mass is reduced.
  • Hypertension due to increased plasma leptin, insulin, and inflammatory markers (IL-6, CRP, TNFa), leading to LVH or dilation in severe cases, and eventually systolic dysfunction.
  • Diastolic dysfunction from reduced diastolic time (from increased HR), LVH, and fatty infiltration of tissue.
  • Increased risk of arrhythmias from hypertrophy, dilation, fat deposition in the conducting system, IHD, sleep apnea, and increased catecholamines.
  • Cardiomyopathy (obesity is a risk factor), with difficulty diagnosing it via TTE.
Gastrointestinal:
  • Increased reflux risk due to increased IAP, relaxation of LOS, reduced GIT motility, increased acidity, and volume.
Metabolic:
  • Poor glycaemic control from increased HPL, human chorionic gonadotropin, and steroid hormones, which increase resistance of target tissues to insulin.
  • Hyperlipidemia due to insulin secretion.
Renal:
  • Increased ACE and renin levels.
  • Reduced renal blood flow (RBF) from increased IAP.
  • Increased sodium reabsorption from increased leptin.
Endocrine:
  • Increased SNS drive leading to hypertension, sodium and water reabsorption from increased leptin.
Coagulation:
  • Increased risk of DVT and thromboembolic events.
Pregnancy-related Morbidity – Increased Risk of:
  • Fetal congenital malformations (e.g., neural tube defects).
  • Miscarriage and premature delivery.
  • Pre-eclampsia.
  • GDM.
  • LSCS.
  • Wound infection and breakdown.
  • Endometritis.
  • Induction and augmentation of labor.
  • Poor contractility of uterus pre and post delivery due to fatty infiltration.
  • Obstructed labor due to intra-abdominal fat.
  • Shoulder dystocia.
Pharmacokinetics:
  • Increased fat and lean body mass.
  • Reduced total body water (TBW).
Implications for Obstetric Anaesthetic Care:
Labor:
  • LSCS:
    • Pre-op: Early regional analgesia in labor is recommended as:
      • More likely to require emergency operative intervention, and general anesthesia (GA) confers greater risks.
      • Optimal positioning with less pain early in labor.
    • Difficult placement due to loss of landmarks and limited back flexion.
    • Epidural is associated with:
      • Higher failure rates.
      • More than 1 attempt.
      • Migration of catheter.
      • Inconsistent spread of epidural solution, with potential for greater cephalad spread due to more epidural fat and venous distension.
      • Higher risk of accidental dural puncture (4% vs 2.5%).
      • Lower risk of PDPH due to increased IAP and epidural space pressure.
    • Antacid prophylaxis to reduce risk of reflux and aspiration.
Intra-op:
  • Neuraxial anesthesia is preferred.
  • CSE is recommended as:
    • Spinal dose and height of block are difficult to predict.
    • Surgery may be prolonged due to operative difficulty.
    • Long spinal and epidural needles need to be available.
  • Airway:
    • Higher risk of failure to intubate (1:250).
    • Difficult BMV due to reduced chest wall compliance and increased IAP.
    • Rapid desaturation from reduced FRC and increased O2 consumption.
    • Adequate pre-oxygenation is essential.
    • Appropriate positioning with head up or ramped, allowing breasts to fall toward axillae to laryngoscopy and reduce pressure on chest wall.
    • Range of difficult intubation equipment should be available.
  • Breathing:
    • Ventilation may be difficult, leading to barotrauma and volutrauma.
    • Head up positioning may improve respiratory compliance.
    • PEEP can aid oxygenation.
Drug Dosage Adjustment:
  • Thiopentone and propofol doses – based on LBM/IBW.
  • NDMR – based on IBW to avoid unwanted prolonged blockade and residual neuromuscular blockade.
  • Suxamethonium – use TBW for dosing (offset is quick due to increased activity of pseudocholinesterase in obesity).
Other (Positioning/Equipment):
  • Adequate and trained theater assistants and staff to facilitate transfer and prevent injury to both staff and patient.
  • Bariatric hover mattress.
  • Wide BP cuffs to provide accurate readings (small cuffs may result in falsely high readings).
  • Large compression stockings and calf compression.
  • Adequate left tilt to avoid aorto-caval compression may be tricky.
Post-op:
  • Airway: Preferable to extubate awake in sitting position to optimize ventilation.
  • Analgesia: Adequate, regular, and multi-modal analgesia and encourage mobilization.
  • Destination: May need transfer to HDU for appropriate monitoring and CPAP.
  • Reduce risk of pulmonary complications (e.g., hypoxemia, atelectasis, pneumonia, pulmonary edema).
  • Reduce risk of thromboembolic events with thrombophylaxis, TEDS, and SCUDS.

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