One-Lung Ventilation (OLV)

MD Anaesthesia PG Examination Answer (30 Marks)

1. INTRODUCTION (2 marks)

2. INDICATIONS FOR OLV (4 marks)

Absolute Indications (Lung Isolation - to protect the other lung)

Relative Indications (Lung Separation - to improve surgical exposure)

• Thoracic aortic aneurysm repair
• Pneumonectomy, lobectomy, segmentectomy
• Video-assisted thoracoscopic surgery (VATS), robotic thoracic surgery
• Oesophageal surgery
• Anterior thoracic spine surgery
• Minimally invasive cardiac surgery

3. PHYSIOLOGICAL CHANGES DURING OLV (6 marks)

3a. Lateral Decubitus Position Effects (Two-Lung Ventilation)

• Ventilation preferentially goes to the dependent (lower) lung due to gravity and diaphragmatic mechanics
• Perfusion also preferentially goes to the dependent lung due to gravity (greater hydrostatic pressure)
• In the closed-chest lateral position, V/Q matching is relatively preserved

3b. Transition to OLV - The Shunt Problem

• During two-lung ventilation: Shunt fraction (Qs/Qt) ≈ 10% (5% per lung)
• During OLV: Shunt fraction rises to approximately 20-30%
• The non-dependent lung receives approximately 35-40% of total pulmonary blood flow
• This mixing of deoxygenated blood from the collapsed lung with oxygenated blood from the dependent lung widens the alveolar-arterial (A-a) O₂ gradient and causes hypoxaemia

Two-lung vs OLV: The essential difference is that the non-ventilated lung has obligatory shunt flow (Barash 9e)

3c. Hypoxic Pulmonary Vasoconstriction (HPV) - The Protective Mechanism

• HPV reduces blood flow to the non-ventilated lung by approximately 50% of its baseline perfusion
• This reduces the effective shunt from ~40% (if no HPV) to ~20-25%
• HPV develops over minutes (biphasic: early rapid phase within minutes, sustained phase over hours)

• Volatile anaesthetic agents (dose-dependent; halothane > enflurane > isoflurane ≈ sevoflurane ≈ desflurane)
• Pulmonary hypertension
• Hypocapnia / alkalosis
• High cardiac output / increased mixed venous PO₂
• Vasodilators: nitroglycerin, nitroprusside, nitric oxide, phosphodiesterase inhibitors (milrinone), calcium channel blockers
• β-adrenergic agonists
• Hypothermia

• Almitrine (pulmonary vasoconstrictor - augments HPV)
• TIVA (propofol-based)
• Normocarbia / mild hypocarbia
• Appropriate cardiac output

• High mean airway pressure (excessive PEEP, high tidal volumes, intrinsic PEEP)
• Low FiO₂ to ventilated lung (causing HPV there too)
• Vasoconstrictors acting more on normoxic than hypoxic vessels (Morgan & Mikhail 7e, Miller's 10e)

3d. CO₂ Elimination During OLV

4. TECHNIQUES FOR OLV (5 marks)

4a. Double-Lumen Endobronchial Tube (DLT) - Most Commonly Used

• Two lumens: a longer endobronchial lumen (enters main bronchus) + a shorter endotracheal lumen (terminates in lower trachea)
• Two cuffs: endobronchial cuff + endotracheal cuff
• Designed specifically for left or right bronchus
• Available sizes: 35F, 37F, 39F, 41F (French); 28F and 32F for smaller patients

• Left-sided DLT is preferred in most cases because the left main bronchus is longer (4-5 cm) and there is less risk of occluding the left upper lobe bronchus
• Right-sided DLT is used when left bronchial intubation is contraindicated (e.g., left pneumonectomy, left mainstem bronchus tumour, left main bronchus stenosis, thoracic aortic surgery compressing left main bronchus). It has a modified cuff with a ventilation slot for the right upper lobe

• Tracheal width on CXR or CT correlates with appropriate DLT size
• Generally: males ≥170 cm → 41F; males <170 cm → 39F; females ≥160 cm → 37F; females <160 cm → 35F

• Easy placement
• Can ventilate one or both lungs
• Can suction either lung
• Rapid switching between one- and two-lung ventilation
• Ability to apply CPAP/PEEP selectively

4b. Bronchial Blockers

• Univent tube: Single-lumen ETT with a movable endobronchial blocker in a side channel; blocker is advanced under fiberoptic guidance
• Arndt wire-guided endobronchial blocker (Cook Critical Care): Guide wire loop snares FOB, directs blocker into desired bronchus
• Cohen Flexitip bronchial blocker: Flexion at tip directed using a wheel mechanism
• EZ-Blocker: Y-shaped, self-positioning at the carina

• Single-lumen tube remains in place (no tube change needed post-operatively)
• Useful in patients with difficult airways, tracheostomy
• Can be directed to specific lobar bronchi for selective lobar collapse

• More difficult to position (FOB required)
• Risk of displacement during surgery
• Limited suctioning capacity
• Not ideal for lung isolation (low-volume high-pressure cuffs less effective at preventing contamination)

4c. Endobronchial Intubation with Single-Lumen Tube

4d. Tubeless Techniques

5. CONFIRMATION OF CORRECT DLT PLACEMENT (3 marks)

Clinical Method (3-step auscultation protocol for left-sided DLT):

Fiberoptic Bronchoscopy (Gold Standard)

• Through the tracheal (right) lumen: should see carina with blue bronchial cuff visible just below the carina in the left main bronchus (not herniated over)
• Through the bronchial (left) lumen: should see left upper and left lower lobar orifices

Malposition Problems (6 types - Barash 9e):

1. DLT directed to opposite main bronchus
2. DLT advanced too far into one bronchus
3. DLT not advanced far enough (bronchial lumen above carina)
4. Right upper lobe orifice occluded by right-sided DLT
5. Left upper lobe orifice obstructed by left-sided DLT
6. Herniation of bronchial cuff over carina

6. VENTILATORY MANAGEMENT DURING OLV (4 marks)

Lung-Protective Ventilation Strategy

• PEEP improves oxygenation when the dependent lung FRC is below optimal
• Excessive PEEP increases dependent lung pulmonary vascular resistance, redirecting blood to the non-ventilated lung and worsening shunt
• Optimal PEEP is titrated to lowest driving pressure (individualised)

7. MANAGEMENT OF HYPOXAEMIA DURING OLV (4 marks)

Step-wise Management (Miller's Box 49.12):

• Resume two-lung ventilation - reinflate non-ventilated lung, deflate bronchial cuff. This is the first priority and necessitates temporary interruption of surgery.

1. Confirm FiO₂ = 1.0 - Ensure oxygen delivery is maximal
2. Check DLT/blocker position - Fiberoptic bronchoscopy to rule out malposition (this is a very common cause)
3. Optimise cardiac output - Maintain adequate CO; neither too low (falls SvO₂) nor too high (inhibits HPV). Avoid inotropes (dopamine worsens PaO₂ at supranormal CO)
4. Reduce volatile agent to <1 MAC (or switch to TIVA with propofol) - minimises HPV inhibition
5. Apply CPAP to non-dependent/non-ventilated lung - 2-5 cmH₂O with 100% O₂; this is very effective, allows oxygen absorption through the non-ventilated lung without fully re-inflating it
6. Apply PEEP to ventilated lung - 5-10 cmH₂O if not already applied (only if FRC is low)
7. Recruitment manoeuvre to ventilated (dependent) lung - brief sustained inflation to recruit collapsed alveoli
8. Selective partial ventilation strategies:
   • Intermittent reinflation of non-ventilated lung (HPV more effective after repeated hypoxic exposure)
   • Intermittent positive airway pressure (IPAP) to non-ventilated lung via filter device
   • Selective lobar collapse only (blocker placed in ipsilateral lobe bronchus while rest of lung is ventilated)
9. Surgical ligation of pulmonary artery (pneumonectomy cases) - eliminates shunt entirely
10. Almitrine (pulmonary vasoconstrictor) - augments HPV; combination with inhaled NO improves oxygenation without systemic hypotension
11. ECMO - last resort in refractory hypoxaemia

8. CHOICE OF ANAESTHETIC AGENTS (2 marks)

Volatile Anaesthetics:

• At ≤1 MAC, modern volatile agents (isoflurane, sevoflurane, desflurane) cause only ~20% inhibition of HPV - equivalent to a net 4% increase in Qs/Qt
• This is clinically insignificant in most patients
• Volatile agents have anti-inflammatory effects, reducing post-thoracotomy cytokine release
• Avoid nitrous oxide (N₂O): increases risk of post-thoracotomy atelectasis (51% vs 24% with air/O₂), raises pulmonary artery pressure, inhibits HPV, and can expand bullae/blebs

TIVA (Total Intravenous Anaesthesia):

• Propofol-based TIVA does not inhibit HPV and is preferred when:
  • HPV preservation is critical (pre-existing pulmonary hypertension)
  • Patient has received bleomycin (risk of oxygen toxicity)
  • FiO₂ needs to be limited
• Addition of dexmedetomidine to volatile anaesthesia improves OLV oxygenation

Induction agents:

• Propofol or ketamine: both diminish bronchospasm (important as thoracic patients often have reactive airways)
• Avoid barbiturates, benzodiazepines, etomidate, or opioids alone for induction (no bronchodilatory effect)
• Sevoflurane: most potent bronchodilator among volatile agents

9. SPECIAL SITUATIONS (2 marks)

DLT or Bronchial Blocker?

• DLT is preferred when lung isolation is required (contamination risk): more reliable seal, better suctioning
• Bronchial blocker preferred for:
  • Difficult airway (avoids tube change post-operatively)
  • Tracheostomy patients
  • Patients requiring post-operative ventilation (avoid tube change from DLT to SLT)
  • Selective lobar collapse
  • Paediatric patients (<12 years) - appropriate small-size blockers

Patient with Pre-existing Pulmonary Disease:

• Emphysema patients paradoxically tolerate OLV well (auto-PEEP from obstructed airways maintains FRC in dependent lung)
• Patients with good pre-operative spirometry tend to have worse OLV oxygenation (more collapse in dependent lung)
• Trauma patients with dependent lung contusion are particularly high risk

10. SUMMARY TABLE: KEY POINTS AT A GLANCE

• Miller's Anesthesia, 10th Edition (Elsevier, 2020) - Chapter 49: Anesthesia for Thoracic Surgery
• Morgan & Mikhail's Clinical Anesthesiology, 7th Edition - Chapter 25: Thoracic Anesthesia
• Barash, Cullen & Stoelting's Clinical Anesthesia, 9th Edition - Chapter 38: Anesthesia for Thoracic Surgery