Physiological Changes in Laparoscopic Surgery

MD Anaesthesia Exam Notes

1. OVERVIEW

1. Mechanical effects of raised intra-abdominal pressure (IAP)
2. CO₂ absorption and resultant hypercapnic acidosis
3. Patient positioning effects

2. CARDIOVASCULAR EFFECTS

Cardiac Output

• ↓ Cardiac output ~30% at institution of pneumoperitoneum
• Due to: ↓ venous return (IVC compression), ↑ afterload, direct myocardial depression from hypercapnic acidosis
• Typically returns toward normal within 10 minutes of establishing pneumoperitoneum as compensatory mechanisms engage
• IAP >20 mmHg causes more severe reduction; keep IAP <12–15 mmHg

Systemic Vascular Resistance (SVR) & BP

• ↑ SVR — from mechanical compression of splanchnic vessels + catecholamine/renin-angiotensin activation
• MAP may increase or be unchanged (up to 16% ↑) despite ↓ cardiac output, because ↑ SVR compensates
• ↑ Myocardial oxygen consumption — clinically significant in CAD patients

Venous Return

• Initially ↑ (leg veins compressed → auto-transfusion effect) then ↓ (IVC compression dominates at higher IAP)
• Trendelenburg (head-down) position partially offsets ↓ venous return by aiding cephalad venous drainage
• Reverse Trendelenburg (head-up) worsens venous return reduction

Arrhythmias

• Hypercapnia → catecholamine release → sensitises myocardium to arrhythmias (especially with volatile anaesthetics)
• Bradycardia and nodal rhythms possible from peritoneal stretch (vagal reflex) at CO₂ insufflation

Regional Blood Flow

• ↓ Renal blood flow — IAP >15 mmHg associated with postoperative AKI; oliguria can occur even with haemodynamic stability
• ↓ Portal and splanchnic flow
• ↓ Hepatic blood flow — relevant in prolonged surgery

Summary Table (Barash Table 50-5)

3. RESPIRATORY EFFECTS

Mechanics

• ↓ Functional Residual Capacity (FRC) — cephalad diaphragm displacement
• ↓ Vital capacity (VC)
• ↓ Pulmonary compliance — stiff chest in Trendelenburg; up to 50% compliance reduction in obese patients
• ↑ Peak airway pressure — monitor continuously; risk of barotrauma
• Atelectasis — particularly dependent zones; worsened in obese and Trendelenburg position

Gas Exchange — Key Paradox (Miller's)

• ↑ Ventilation–perfusion mismatch would predict ↑ shunt and ↓ PaO₂
• But: CO₂ potentiates hypoxic pulmonary vasoconstriction (HPV), redistributing blood away from collapsed regions
• Net result: shunt is reduced and arterial oxygenation is often maintained or improved
• This CO₂-specific HPV potentiation explains why air insufflation produces far worse shunting than CO₂ insufflation

CO₂ Absorption & Acid-Base

• CO₂ is highly soluble — absorbed rapidly from peritoneum into bloodstream
• Results in ↑ PaCO₂, respiratory acidosis
• EtCO₂ typically underestimates PaCO₂ by 2–5 mmHg in healthy patients; gradient widens in sick patients
• COPD patients may fail to compensate by hyperventilation — monitor with ABGs
• CO₂ stored in body compartments (viscera, bone, muscle) — delayed hypercapnia possible post-extubation, especially after prolonged cases
• Management: increase minute ventilation (↑ RR preferred over ↑ TV to avoid barotrauma)

4. CENTRAL NERVOUS SYSTEM EFFECTS

• ↑ Intracranial pressure (ICP) — from hypercapnia (cerebral vasodilation) + Trendelenburg position (↓ venous drainage from head)
  • Immediate rise at institution of pneumoperitoneum
  • Critical concern in patients with raised ICP or intracranial pathology
• ↑ Cerebral blood flow (CBF) — CO₂ is a potent cerebral vasodilator
• ↑ Intraocular pressure (IOP) — Trendelenburg + hypercapnia; concern in glaucoma
• Catecholamine release — sympathetic activation from CO₂ and surgical stress

5. ENDOCRINE & NEUROHUMORAL EFFECTS

• Renin-angiotensin-aldosterone system (RAAS) activation — from ↓ renal perfusion pressure
• ↑ Vasopressin (ADH) — contributes to oliguria
• Cortisol and catecholamine surge — similar to major open surgery, though usually attenuated
• These persist beyond deflation of pneumoperitoneum

6. RENAL EFFECTS

• ↓ Urine output — even with haemodynamic stability and liberal fluids
• Mechanism: ↓ renal perfusion (IAP compresses renal parenchyma + renal veins), ↑ RAAS/ADH, ↓ cardiac output
• IAP >15 mmHg associated with postoperative AKI
• Recommended strategy: keep IAP <12 mmHg, ensure adequate hydration
• Diuretics (furosemide, mannitol), "renal-dose" dopamine, fenoldopam — no proven renal protective benefit

7. GASTROINTESTINAL & HEPATIC EFFECTS

• ↑ Risk of gastro-oesophageal regurgitation — raised IAP + positioning; consider RSI or modified RSI
• ↓ Hepatic blood flow and portal perfusion — relevant in liver disease and prolonged cases
• Bowel distension risk if N₂O used (avoid in laparoscopy — supports combustion and enters bowel)

8. EFFECTS OF CO₂ AS THE INSUFFLANT

Properties of CO₂ (why it's preferred)

• Colourless, non-combustible, very soluble in blood, inexpensive
• Rapid absorption → reduced risk of massive gas embolism compared to insoluble gases
• Quickly eliminated post-operatively in healthy patients

Downsides

• Hypercapnia and respiratory acidosis
• Catecholamine release → arrhythmias
• ↑ ICP and IOP
• Prolonged post-op elimination in COPD/obese patients

Alternative Insufflants

9. POSITIONAL EFFECTS

Trendelenburg (head-down) — gynaecological, colorectal, robotic pelvic surgery

• ↑ Venous return (partially offsets pneumoperitoneum effect)
• ↑ ICP, ↑ IOP
• Cephalad diaphragm shift → ↓ FRC, ↑ atelectasis, risk of endobronchial intubation (tube migrates down)
• Facial and laryngeal oedema in prolonged steep Trendelenburg
• Brachial plexus neuropraxia from shoulder braces — avoid shoulder braces; use non-slip mattress
• Raised ICP — contraindicated or use with extreme caution in neurosurgical patients

Reverse Trendelenburg (head-up) — cholecystectomy, upper GI surgery

• ↓ Venous return → ↓ CO more pronounced
• Reduces diaphragm compression → better respiratory mechanics
• Risk of air embolism (surgical field above heart level)

10. COMPLICATIONS SPECIFIC TO LAPAROSCOPY

Gas Embolism (most dangerous)

• CO₂ embolism — rare but life-threatening
• Causes: direct vascular needle/trocar placement, hepatic laceration
• Presentation: sudden ↓ EtCO₂ (obstructed pulmonary circulation), ↓ SpO₂, ↑ CVP, "mill-wheel" murmur, cardiovascular collapse
• Management: immediately deflate pneumoperitoneum, left lateral decubitus + head-down (Durant's manoeuvre), 100% O₂, aspirate gas via CVP catheter, CPR if arrest

Trocar Injuries

• ~0.5% incidence — vascular or bowel injury at trocar insertion
• Veress needle blind insertion — risk at umbilicus (aorta, IVC)

Venous Thromboembolism

• ↑ DVT risk from: venous stasis (IAP compresses IVC + leg veins), Trendelenburg, prolonged surgery
• Use: TED stockings, LMWH, pneumatic compression devices

Surgical Emphysema

• CO₂ tracking subcutaneously → hypercapnia
• Detected clinically (crepitus) + ↑ EtCO₂ out of proportion
• Usually self-limiting; may require increased ventilation

Pneumothorax/Pneumomediastinum

• CO₂ tracking via diaphragmatic defect or pleural tear
• Monitor: ↑ airway pressures, ↓ SpO₂, asymmetric breath sounds

11. ANAESTHETIC MANAGEMENT IMPLICATIONS

Monitoring

• Continuous EtCO₂ (mandatory), SpO₂, invasive BP for high-risk patients
• ABG in COPD, obese, prolonged cases — EtCO₂ may underestimate PaCO₂ significantly

Airway

• IPPV with cuffed ETT — essential (positive pressure ventilation needed to offset reduced compliance)
• LMA generally avoid (aspiration risk, difficulty maintaining adequate ventilation with ↑ airway pressures)
• Check ETT position after Trendelenburg positioning — re-check with auscultation

Ventilation

• ↑ Minute ventilation to compensate for CO₂ absorption
• Lung-protective strategy: low TV (6–8 mL/kg IBW) + PEEP (5–8 cmH₂O) + moderate RR increase
• PEEP helps maintain FRC and reduce atelectasis

Haemodynamics

• Preoperative fluid loading — colloid bolus before pneumoperitoneum institution → higher stroke volume
• Keep IAP <12 mmHg where possible
• SVR-increasing agents (phenylephrine, noradrenaline) for ↓ MAP — avoid if ↓ CO is primary issue; vasopressors increase afterload further

Neuromuscular Blockade

• Deep NMB allows lower insufflation pressures for equivalent surgical exposure → reduces all IAP-related effects
• Use of sugammadex allows reversal of deep block at end

Specific Considerations

• CAD patients: monitor for ischaemia (ST changes) — ↑ myocardial O₂ demand + ↓ coronary perfusion
• COPD: consider helium insufflation; ABG monitoring; plan for delayed post-op hypercapnia
• Obese: ↓ FRC at baseline, larger atelectasis, higher IAP for equivalent exposure
• Pregnancy: lateral tilt, ↑ risk of aspiration, concern for uteroplacental blood flow
• ↑ ICP: avoid or use with extreme caution — steep Trendelenburg contraindicated

12. QUICK-RECALL SUMMARY TABLE

• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e — Chapter 50, Table 50-5 (Physiology of CO₂ Pneumoperitoneum)
• Miller's Anesthesia, 10e — Chapter 12 (Pneumoperitoneum & Respiratory Physiology)
• Campbell-Walsh-Wein Urology — Chapter 14 (Laparoscopic Physiology & Insufflants)