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Anaesthesia for Morbid Obesity and Bariatric Surgery

MD Anaesthesia PG Exam - Detailed Answer

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SECTION 1: BODY MASS INDEX (BMI)

Definition

WHO Classification of Obesity

Related Weight Concepts (Critical for Drug Dosing)

• Ideal Body Weight (IBW) - Broca Index:
  • Males: IBW (kg) = Height (cm) - 100
  • Females: IBW (kg) = Height (cm) - 105
• Predicted Body Weight (PBW):
  • Males: PBW = 50 + 0.91 × (Height[cm] - 152.4)
  • Females: PBW = 45.5 + 0.91 × (Height[cm] - 152.4)
• Lean Body Weight (LBW):
  • Males: 1.10 × TBW - 0.0128 × BMI × TBW
  • Females: 1.07 × TBW - 0.0148 × BMI × TBW
  • Approximation: IBW + 20-30% in morbidly obese
• Total Body Weight (TBW): Actual measured weight

Metabolic Syndrome (Syndrome X)

1. Central obesity: waist > 102 cm (M), > 88 cm (F)
2. Triglycerides ≥ 150 mg/dL
3. HDL < 40 mg/dL (M), < 50 mg/dL (F)
4. BP ≥ 130/85 mmHg
5. Fasting glucose ≥ 100 mg/dL

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SECTION 2: TYPES OF BARIATRIC SURGERIES

A. Restrictive Procedures

• Inflatable silicone band placed laparoscopically around the gastric cardia
• Creates a small pouch (15-30 mL); band tightness adjustable via subcutaneous port
• Advantages: reversible, adjustable, lowest mortality
• Disadvantages: least weight loss, high revision rate, band slippage/erosion, port complications
• Weight loss: ~20% excess body weight (EBW)

• Laparoscopic resection of ~80% of stomach along the greater curvature
• Creates a tubular "sleeve" (60-100 mL capacity)
• Irreversible; reduces ghrelin production (hunger hormone)
• Advantages: no anastomosis, no foreign body, good weight loss
• Disadvantages: GERD may worsen, irreversible, gastric leak risk
• Weight loss: ~60-70% EBW
• Now the most commonly performed bariatric procedure worldwide

B. Restrictive + Malabsorptive Procedures

• Gold standard bariatric procedure
• Small gastric pouch (~30 mL) connected to a Roux limb of jejunum (~75-150 cm)
• Biliopancreatic limb and alimentary limb reunite at jejunojejunostomy
• Mechanism: restriction + malabsorption + gut hormone changes (GLP-1, PYY ↑)
• Advantages: best long-term weight loss, excellent T2DM resolution, durable
• Disadvantages: anastomotic leak, dumping syndrome, nutritional deficiencies (B12, iron, folate, Ca), internal hernias
• Weight loss: ~70-80% EBW

• Most malabsorptive procedure
• Distal gastrectomy + long Roux limb (250 cm alimentary limb); common channel only 50-100 cm
• With duodenal switch: sleeve gastrectomy + duodenal-ileal anastomosis
• Advantages: maximum weight loss (>80% EBW), excellent metabolic outcomes
• Disadvantages: highest complication rate, severe nutritional deficiencies (fat-soluble vitamins, protein), highest mortality, complex surgery
• Reserved for BMI > 50 or super-obese patients

C. Other/Newer Procedures

• Intragastric balloon: Endoscopic, temporary (6 months), adjunct for pre-op weight loss
• Gastric plication: Endoscopic; experimental
• One anastomosis gastric bypass (OAGB/MGB): Single anastomosis, gaining popularity

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SECTION 3: PATHOPHYSIOLOGY OF MORBID OBESITY - ANAESTHETIC RELEVANCE

Respiratory System

• Reduced FRC: Fat deposition on chest wall and diaphragm compresses the lung - FRC may fall below closing capacity in supine position, causing V/Q mismatch and hypoxemia
• Increased oxygen consumption and CO₂ production (proportional to body mass)
• Rapid desaturation on apnea - reduced apnea tolerance (FRC acts as O₂ reservoir)
• Obesity Hypoventilation Syndrome (OHS/Pickwickian): BMI > 30 + PaCO₂ > 45 mmHg without other cause
• Obstructive Sleep Apnea (OSA): High prevalence; screen with STOP-BANG questionnaire
• Pulmonary hypertension in chronic hypoxemia
• Restrictive pattern on spirometry; reduced TLC, RV, ERV

Cardiovascular System

• Increased blood volume and cardiac output (extra 100 mL blood per kg fat)
• Left ventricular hypertrophy (concentric - from HTN; eccentric - from volume overload)
• High incidence of HTN, CAD, cardiomyopathy
• Difficult ECG interpretation - low voltage from fat insulation, false LVH criteria
• Increased risk of arrhythmias

Gastrointestinal System

• Gastroparesis and GERD common
• Increased gastric volume, increased intra-abdominal pressure
• Full stomach risk: Hiatus hernia, increased aspiration risk
• Fatty liver / NAFLD in up to 90%

Endocrine/Metabolic

• High prevalence of Type 2 DM - insulin resistance; risk ↑ by 25% for every 1 kg/m² BMI above 22
• Subclinical hypothyroidism in ~25% of morbidly obese
• Dyslipidemia
• Hypercoagulable state

Pharmacokinetic Changes

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SECTION 4: PRE-OPERATIVE EVALUATION

Indications for Bariatric Surgery

• BMI ≥ 40 kg/m² OR
• BMI ≥ 35 with significant obesity-related comorbidities (T2DM, OSA, HTN, joint disease)
• Failure of ≥ 6 months conservative management
• Age 18-65 years (relative limits)
• Psychologically stable; committed to lifestyle changes

Contraindications

• Unstable CAD
• Uncontrolled severe OSA
• Uncontrolled psychiatric disorder (absolute)
• Intellectual disability (IQ < 60)
• Active malignancy with poor 5-year prognosis
• Continued drug/alcohol abuse
• Cirrhotic liver disease with portal hypertension
• Inability to comply with postoperative restrictions

Pre-operative Assessment Framework

• Duration and degree of obesity
• Previous attempts at weight loss
• Comorbidities: HTN, CAD, T2DM, OSA, GERD, hypothyroidism
• Drug history: antihypertensives, oral hypoglycemics, antidepressants
• Functional capacity (METs): stair climbing, exercise tolerance
• Previous anesthetic difficulties
• STOP-BANG score for OSA (≥ 3 = high risk)

• Snoring
• Tiredness daytime
• Observed apnea
• Pressure (high BP)
• BMI > 35
• Age > 50
• Neck circumference > 40 cm
• Gender = Male

• Mallampati class III/IV (independent risk factor for difficult intubation)
• Neck circumference > 40 cm
• Thyromental distance
• Mouth opening
• Neck extension
• Pretracheal soft tissue thickness on ultrasound
• History of OSA (pharyngeal tissue redundancy)
• Ramped positioning improves laryngoscopic view significantly

• ACC/AHA guidelines; Class III obesity (BMI ≥ 40) warrants ECG + CXR regardless of age if even one CAD risk factor or poor functional capacity
• Echocardiography if LVH, cardiomyopathy, pulmonary HTN suspected
• Stress testing if > 4 METs threshold not met

• SpO₂ at rest and on exertion
• ABG if OHS suspected (PaCO₂ > 45 mmHg)
• PFTs if COPD/asthma; restrictive pattern expected
• Polysomnography for OSA
• Chest X-ray

• CBC, blood group and crossmatch
• Blood glucose, HbA1c
• LFTs (NAFLD very common)
• Renal function tests (creatinine, BUN)
• TFTs (hypothyroidism screen)
• Serum electrolytes
• Coagulation profile (PT, aPTT)
• Lipid profile
• 12-lead ECG
• Helicobacter pylori testing (eradicate if positive)

• Initiate/optimize CPAP therapy if OSA diagnosed - continue perioperatively
• Optimize blood glucose (HbA1c < 8%)
• Control BP
• Pre-operative weight reduction (VLCD - very low calorie diet for 2-4 weeks pre-op reduces liver size, improves surgical access)
• Thromboprophylaxis planning
• Stop metformin 24-48 hours pre-op
• Pre-operative CPAP reduces incidence of post-op pulmonary complications

• ASA classification: Morbid obesity with comorbidities typically ASA III-IV
• Obesity Surgery Mortality Risk Score (OS-MRS): five risk factors
  • BMI ≥ 50
  • Male sex
  • HTN
  • Pulmonary embolism risk (OHS, right heart failure, previous VTE)
  • Age ≥ 45 years
  • Score 0-1: Class A (0.2% mortality); 2-3: Class B (1.1%); 4-5: Class C (2.4%)

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SECTION 5: INTRAOPERATIVE MANAGEMENT

Premedication

• Avoid sedative premedication (benzodiazepines, opioids) - risk of respiratory depression/airway obstruction
• H₂ blockers (ranitidine) + proton pump inhibitors (omeprazole) - reduce gastric volume and acidity
• Metoclopramide 10 mg IV - prokinetic, reduces gastric volume, increases LOS tone
• If CPAP-dependent, apply pre-operatively

Monitoring (Standard + Additional)

• Standard: SpO₂, ETCO₂, ECG, NIBP, temperature
• Arterial line (invasive BP): Often needed - NIBP unreliable with large arm circumference; use correct large cuff (bladder covering ≥ 80% of arm circumference)
• Central venous access if peripheral access difficult
• Foley catheter (urine output monitoring during prolonged procedures)
• Nerve stimulator/TOF monitor - mandatory, neuromuscular blockade harder to assess clinically
• BIS monitor (anesthetic depth - avoid awareness and overdose)
• Oesophageal Doppler / PICCO for haemodynamic monitoring in high-risk patients

Patient Positioning

• Ramped position for intubation: elevate upper body and head until ear aligns with sternum horizontally - significantly improves laryngoscopic view compared to sniffing position alone
• Can achieve with commercially available ramp devices or blanket ramp
• Reverse Trendelenburg (20-30°) improves FRC and reduces regurgitation risk
• Supine position → rhabdomyolysis from gluteal muscle pressure reported
• Careful padding of all pressure points (gel pads)
• Operating table must have adequate weight capacity (bariatric tables)

Airway Management

• 3-5 minutes of 100% O₂ via tight-fitting mask
• CPAP/BiPAP at 10 cmH₂O during pre-oxygenation dramatically extends safe apnea time by recruiting alveoli and increasing FRC
• Target SpO₂ > 98% before induction
• Reverse Trendelenburg position during pre-oxygenation

• Rapid Sequence Induction (RSI) standard approach - full stomach risk
• Cricoid pressure (Sellick's maneuver) - controversial but widely used
• Drugs: Propofol (2 mg/kg LBW) + Succinylcholine (1.5 mg/kg TBW) or Rocuronium (1.2 mg/kg IBW) for RSI
• Have difficult airway equipment immediately available (videolaryngoscope, airway exchange catheter, emergency surgical airway kit)
• Videolaryngoscopy (e.g. McGrath, GlideScope, C-MAC) preferred or immediately available - improves first-pass success

• Oral endotracheal intubation preferred
• Confirm with waveform capnography (not just SpO₂)
• Lubricated tube stylet, bougie available
• Backup: LMA Supreme as bridge to surgical airway

• Awake fibreoptic intubation (AFOI) - gold standard for anticipated difficult airway
• Nasal route preferred for AFOI
• Adequate topical anaesthesia: lignocaine 4% spray; transtracheal block

Induction Agents

• Propofol: 1-2 mg/kg LBW (induction); avoid overdose
• Thiopentone: LBW dosing
• Ketamine: useful in haemodynamically compromised; 1-2 mg/kg IBW

Maintenance of Anaesthesia

• Total Intravenous Anaesthesia (TIVA): Preferred for bariatric surgery
  • Propofol infusion: TBW-based dosing for maintenance
  • Remifentanil: LBW (context-insensitive - ideal for obese)
  • Reduces PONV vs inhalational agents
• Inhalational agents: Desflurane preferred (lowest fat solubility, fastest emergence) over isoflurane and sevoflurane; however avoid in PONV-high risk
• Nitrous oxide: Avoid - increases bowel distension (laparoscopic surgery), increases PONV

Neuromuscular Blockade

• Succinylcholine: 1.5 mg/kg TBW (plasma cholinesterase activity increases with TBW)
• Rocuronium/Vecuronium: Dose on IBW to avoid prolonged blockade
• Mandatory TOF monitoring throughout
• Sugammadex: Dose on TBW (16 mg/kg TBW for immediate reversal; 4 mg/kg for T2 recovery); preferred reversal agent - faster, more reliable than neostigmine

Ventilation Strategy

• Lung-protective ventilation (LPV):
  • Tidal volume (VT): 6-8 mL/kg PBW (NOT TBW - prevents volutrauma)
  • PEEP: 10-15 cmH₂O (reduces atelectasis, improves oxygenation)
  • Recruitment maneuvers: Apply at induction and during surgery (sustained inflation 40 cmH₂O × 30 sec or incremental PEEP)
  • Respiratory rate: titrate to normocapnia (PaCO₂ 35-45)
  • FiO₂: lowest to maintain SpO₂ ≥ 95%
• Reverse Trendelenburg position maintains FRC better than supine
• ETCO₂ may underestimate PaCO₂ by 5-10 mmHg in obese patients - ABG correlation recommended

Laparoscopic Considerations

• Pneumoperitoneum (CO₂ insufflation to 12-15 mmHg):
  • Further reduces FRC
  • Increases airway pressures
  • CO₂ absorption increases ETCO₂ - increase RR accordingly
  • Head-down (Trendelenburg) → even worse FRC; head-up preferred for bariatric procedures (reverse Trendelenburg or beach chair)
  • Vasovagal response on peritoneal insufflation
  • Risk of pneumothorax, surgical emphysema, pneumomediastinum

Fluid Management

• Goal-directed fluid therapy recommended
• Avoid excess crystalloid (increased tissue oedema, worse oxygenation post-op)
• Estimated blood volume (EBV) ~50 mL/kg TBW
• Urine output target ≥ 0.5 mL/kg/hr (based on IBW, not TBW)

Analgesia (Multimodal)

• Opioid-sparing strategy preferred (reduces respiratory depression, PONV):
  • Paracetamol (acetaminophen) - dose on LBW (max 2g/dose)
  • NSAIDs (ketorolac, diclofenac) - unless contraindicated
  • Dexmedetomidine infusion: reduces opioid requirements, anxiolytic, no respiratory depression
  • Ketamine (sub-anaesthetic doses 0.1-0.5 mg/kg): adjunct analgesia
  • Lignocaine infusion: perioperative systemic local anaesthetic reduces opioid requirements
  • Regional techniques: TAP block (Transversus Abdominis Plane), wound infiltration, epidural (for open procedures)
• If opioids required: use short-acting agents (remifentanil intraop; fentanyl PCA carefully titrated)

VTE Prophylaxis

• Mechanical: Pneumatic compression devices (PCDs) from induction; TED stockings
• Pharmacological: Low-molecular-weight heparin (LMWH - enoxaparin) - dose on TBW (higher doses needed: enoxaparin 0.5 mg/kg TBW BD or 40 mg BD); start 12 hrs post-op
• Prophylactic IVC filter in very high-risk patients (prior VTE, OHS, pulmonary hypertension)
• Maintain early mobilisation post-operatively

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SECTION 6: POST-OPERATIVE MANAGEMENT

Post-Anaesthesia Care Unit (PACU)

• Semi-sitting / head-up position (30-45°) at all times - prevents atelectasis and aspiration
• Never nurse flat or supine
• SpO₂ monitoring continuously
• Supplemental O₂ initially; target SpO₂ ≥ 92-95%
• Resume CPAP/BiPAP immediately in OSA patients - apply in PACU before fully awake if tolerated
• Criteria for safe extubation:
  • Awake, following commands
  • TOF ratio ≥ 0.9 (neuromuscular blockade fully reversed)
  • SpO₂ ≥ 95% on room air or minimal O₂
  • Adequate minute ventilation (RR 10-20)
  • Normothermic

Extubation Strategy

• Extubate awake (not deep extubation) - aspiration risk
• Extubate in reverse Trendelenburg or semi-sitting
• Have re-intubation equipment immediately ready
• Consider laryngeal mask (LMA) as bridge if oedema anticipated

Respiratory Management Post-Op

• CPAP/BiPAP for OSA patients (continue home settings)
• Incentive spirometry
• Deep breathing exercises, early physiotherapy
• High-Flow Nasal Cannula (HFNC) if standard O₂ insufficient
• Monitor for pulmonary oedema, atelectasis, pneumonia, respiratory failure

Pain Management Post-Op

• Multimodal analgesia continues:
  • Regular paracetamol ± NSAIDs
  • Opioid PCA - careful monitoring, lowest effective dose
  • TAP block / epidural for open procedures
  • Avoid IM injections (unreliable absorption, difficult access)
• Monitor sedation score and respiratory rate with opioids

DVT/PE Prophylaxis

• Continue LMWH for 28 days post-op (extended prophylaxis)
• Early ambulation (within 24 hours)
• Graduated compression stockings

Other Post-Operative Concerns

• Very high risk (female, non-smoker, opioids, history of PONV)
• Triple prophylaxis: Ondansetron + Dexamethasone + Droperidol/Scopolamine
• TIVA preferred to reduce PONV

• From pressure on muscle groups (gluteal) intraoperatively
• Monitor CK levels, urine output, urine colour
• Aggressive IV hydration if suspected

• Obese patients have higher risk of wound infection, dehiscence
• Keep skin folds dry; regular inspection

• Blood glucose monitoring (insulin sliding scale)
• Early post-op nutritional support per bariatric team protocol
• Vitamin and mineral supplementation (B12, iron, folate, calcium - especially after RYGB)

• For day-case/short-stay: More stringent criteria in obese patients
• SOBA (Society for Obesity and Bariatric Anaesthesia) red flags for day surgery:
  • BMI ≥ 50
  • Uncontrolled OSA / not on CPAP
  • OHS
  • Uncontrolled comorbidities
  • History of difficult airway

• Anastomotic leak (most serious - presents with tachycardia, fever, peritonism; may need re-operation)
• Internal hernia (RYGB - intermittent abdominal pain)
• Dumping syndrome (RYGB - early or late hypoglycemia)
• Nutritional deficiencies (B12, iron, thiamine, folate, Ca, fat-soluble vitamins)
• GERD worsening (sleeve gastrectomy)
• DVT/PE - up to 3 months post-op
• Wound complications

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Summary Diagram

MORBID OBESITY (BMI ≥ 40) FOR BARIATRIC SURGERY
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PRE-OP ──────────────────────────────────────────
• Airway: Mallampati, neck circ, thyromental
• Cardiac: ECG, Echo, stress test (ACC/AHA)
• Pulmonary: SpO₂, ABG, PFTs, sleep study
• Labs: CBC, LFTs, RFTs, HbA1c, TFTs
• OS-MRS risk score
• Optimize: CPAP, glucose, BP, pre-op VLCD
         │
         ▼
INTRA-OP ────────────────────────────────────────
• RSI + Ramped position + Videolaryngoscope
• TIVA (Propofol + Remifentanil)
• Lung-protective ventilation (6-8 mL/kg PBW + PEEP 10-15)
• IBW dosing NMB, sugammadex reversal
• Multimodal analgesia (opioid-sparing)
• Mechanical + LMWH VTE prophylaxis
         │
         ▼
POST-OP ─────────────────────────────────────────
• Semi-sitting, SpO₂ monitoring
• Resume CPAP immediately
• Awake extubation, re-intubation ready
• Multimodal analgesia, PONV prophylaxis
• Early mobilisation, LMWH × 28 days
• Monitor for anastomotic leak, PE, rhabdo

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• Drug doses: Succinylcholine on TBW; NMB (rocuronium, vecuronium) on IBW; Sugammadex on TBW; Propofol induction on LBW, maintenance on TBW; Remifentanil on LBW
• Tidal volume always on PBW, never TBW
• Pre-oxygenation + CPAP extends safe apnea time significantly
• Ramped position (ear-sternum alignment) is optimal for intubation
• TIVA (propofol + remifentanil) preferred over inhalational agents for bariatric surgery
• Desflurane is preferred inhalational agent if used (lowest fat solubility)
• Never nurse post-op bariatric patient flat; always semi-sitting ≥ 30°
• PONV triple prophylaxis is standard
• Recent 2025 guidelines from the Society for Obesity and Bariatric Anaesthesia (SOBA) emphasize systematic airway management best practices in obese patients (PMID: 40468902)