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One-Lung Ventilation (OLV)

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Indications

Absolute Indications

Relative Indications

• Thoracic aortic aneurysm repair
• Pneumonectomy, lobectomy, esophageal surgery
• Upper lobe thoracoscopic procedures
• Thoracoscopy, pleurodesis

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Physiology of OLV

The Core Problem: Obligatory Shunt

• Only the dependent (lower) lung is ventilated
• The nondependent (upper, operative) lung is collapsed but continues to receive blood flow
• This creates a right-to-left intrapulmonary shunt of 20-30%

Two Main Contributors to Hypoxemia During OLV

1. Persistent blood flow through the nonventilated nondependent lung - obligatory shunt
2. Atelectasis in the dependent (ventilated) lung - caused by absorption atelectasis, compression, decreased FRC in lateral decubitus position

Hypoxic Pulmonary Vasoconstriction (HPV) - The Compensatory Mechanism

• HPV has a biphasic response: rapid onset (minutes) and a slower phase peaking over hours
• HPV is the most important determinant of blood flow diversion away from the nonventilated lung (more so than mechanical vascular kinking)

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Factors That Inhibit HPV (Worsen Hypoxemia)

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Techniques for Lung Isolation

1. Double-Lumen Endobronchial Tube (DLT) - Most Common

• Has one tracheal lumen and one bronchial lumen
• Allows independent ventilation, suctioning, and CPAP to each lung
• Left-sided DLT preferred in most cases - the left main bronchus is longer (5 cm vs. 2.5 cm on right), providing more margin of safety for cuff placement
• Right-sided DLT: more technically demanding; the bronchial cuff must align with the right upper lobe (RUL) ventilation slot, with only 1-8 mm margin - so right-sided DLTs are used when left bronchial intubation is contraindicated (e.g., left pneumonectomy)
• Position confirmation is mandatory with fiberoptic bronchoscopy

1. Tube advanced too far into right or left bronchus - absent breath sounds contralaterally
2. Tube not inserted far enough - bronchial cuff above carina
3. Right upper lobe orifice occlusion with right-sided DLT
4. Left upper lobe occlusion with deep left-sided DLT
5. Bronchial cuff herniation over carina

2. Single-Lumen Tube + Bronchial Blocker

• A blocker (e.g., Arndt, Cohen, EZ-Blocker) is placed through or alongside a regular ETT
• Useful in difficult airways, tracheostomized patients, patients who need postoperative ventilation
• Allows conversion from two-lung to one-lung ventilation without tube exchange

3. Endobronchial Intubation with Single-Lumen Tube

• A standard ETT intentionally advanced into a mainstem bronchus
• Simplest but offers no ability to selectively manage the opposite lung

4. Tubeless / VATS Techniques

• Used for minimally invasive thoracoscopic procedures
• High-frequency ventilation or apneic oxygenation may be employed

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Ventilation Strategy During OLV

Pre-OLV Preparation

• De-nitrogenate the operative (nondependent) lung by ventilating with 100% O2 before lung collapse - nitrogen's low blood solubility delays absorption atelectasis and slows collapse; pre-oxygenation speeds collapse
• Perform a recruitment maneuver of the dependent lung immediately after starting OLV (hold at ~20 cmH2O for 15-20 seconds) to reduce dependent atelectasis

Tidal Volume

• Use low tidal volumes: 4-6 mL/kg predicted body weight (not 10 mL/kg as used historically)
• Tidal volumes <3 mL/kg may cause derecruitment and worsen atelectasis

PEEP

• Apply sufficient PEEP to the ventilated dependent lung to prevent atelectasis
• Optimal PEEP is titrated to best compliance - excessive PEEP can redirect blood to the nonventilated lung and worsen oxygenation

Ventilatory Mode

• Pressure-controlled ventilation (PCV) may reduce barotrauma risk by limiting peak/plateau pressures and provides a more homogeneous tidal volume distribution
• Plateau pressure: <25 cmH2O; peak airway pressure: <35 cmH2O

FiO2

• Use FiO2 0.5-1.0; some evidence favors avoiding 100% O2 due to oxygen toxicity and absorption atelectasis concerns
• Use FiO2 of 1.0 during episodes of hypoxemia

CO2 Elimination

• Usually maintained adequately with unchanged minute ventilation
• End-tidal CO2 monitoring is useful as a trend but may underestimate PaCO2 (increased dead space)
• Permissive hypercapnia is acceptable when oxygenation is adequate

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Management of Hypoxemia During OLV

• Almitrine (IV pulmonary vasoconstrictor): potentiates HPV, reduces shunt - effective when combined with inhaled NO
• Inhaled NO: vasodilator to ventilated regions, increases Va/Q - alone has minimal effect but improves oxygenation when combined with almitrine

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Choice of Anesthetic Agent

• At ≤1 MAC, modern volatile agents (isoflurane, sevoflurane, desflurane) are equipotent and weak HPV inhibitors - responsible for only ~4% net increase in shunt; acceptable for routine OLV
• TIVA (total IV anesthesia) may be preferred in special situations (e.g., bleomycin-treated patients requiring FiO2 limitation)
• Dexmedetomidine added to volatile anesthesia improves oxygenation during OLV
• Nitrous oxide should be avoided: raises pulmonary artery pressures, inhibits HPV, associated with increased postoperative atelectasis (51% vs. 24% with air/O2)

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Lung Injury During OLV

• Dependent (ventilated) lung: hyperperfusion + ventilator-induced trauma (volutrauma, barotrauma) from large tidal volumes
• Nondependent (collapsed) lung: surgical trauma + ischemia-reperfusion injury on re-expansion

• Incidence: 2.5% overall; 7.9% after pneumonectomy
• Mortality/major morbidity: ~40% when it occurs
• Lung-protective ventilation (low VT, low FiO2, low pressures) reduces ALI risk

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Lung Isolation in Special Situations

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Postoperative Considerations

• Extubate early when possible to reduce barotrauma risk (especially risk of bronchial suture "blowout")
• When post-op ventilation is needed, replace the DLT with a standard single-lumen tube using a tube exchanger
• Common post-op complications: atelectasis, hypoxemia from splinting, reexpansion pulmonary edema of the collapsed lung, supraventricular arrhythmias (~common), hemorrhage (3% of thoracotomies)
• Adequate analgesia is essential - pain-related splinting leads to atelectasis and hypoxemia; thoracic epidural analgesia is the gold standard for open thoracotomy

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Oxygen-Hemoglobin Dissociation Curve (ODC)

DNB Anesthesiology Exam Notes

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1. The Curve - Basic Description

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2. Key Reference Points (Must Memorize for Exams)

• Normal P50 = 26.5 mmHg
• Increased P50 = right shift = decreased affinity
• Decreased P50 = left shift = increased affinity

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3. Why the Sigmoid Shape is Physiologically Perfect

Upper Flat Portion (PaO2 60-100 mmHg) - "Loading Zone" (Lungs)

• Saturation stays high (~89-97%) despite large PaO2 swings
• Protective: even with lung disease causing PaO2 drop to 60 mmHg, Hb is still ~89% saturated
• O2 loading in the lungs is efficient and buffered

Steep Portion (PaO2 20-60 mmHg) - "Unloading Zone" (Tissues)

• Small drop in PO2 → large release of O2
• At rest PO2 falls from 95 → 40 mmHg in tissues, releasing ~25% of O2 (5 mL/100 mL blood)
• During exercise, active muscle can extract much more O2 with only modest PO2 reduction
• This is impossible with a hyperbolic curve (e.g., myoglobin has high affinity throughout this range and cannot unload O2 efficiently)

Exam pearl: Myoglobin has a hyperbolic curve and acts as an O2 store in muscle, not a transporter. It only releases O2 when intracellular PO2 falls very low.

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4. Shifts of the ODC - The Most Exam-Tested Topic

Causes of RIGHT SHIFT (CADET mnemonic):

Causes of LEFT SHIFT (opposite of above):

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5. The Bohr Effect - Exam Favorite

1. Active tissues produce CO2
2. Carbonic anhydrase: CO2 + H2O → H2CO3 → HCO3⁻ + H⁺
3. H⁺ binds to specific histidine residues on Hb (β-chain His146)
4. Salt bridges form → stabilizes T (deoxy) state
5. Hb dumps O2

• Low PCO2, high pH in alveolar capillaries → destabilizes T state → shifts Hb to R state → O2 loading enhanced

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6. 2,3-Bisphosphoglycerate (2,3-BPG / DPG)

• Produced in RBCs via Rapoport-Luebering shunt (glycolysis bypass)
• Binds to the central cavity formed by the β-chains of deoxy-Hb (T state); cannot bind in R (oxy) state
• Stabilizes T state → reduces O2 affinity → right shift

• HbF has γ-chains replacing β-chains
• γ-chains bind 2,3-DPG less avidly than β-chains
• Result: HbF stays more in R state → higher O2 affinity → LEFT-shifted ODC
• This allows fetal blood to extract O2 from maternal blood across the placenta (fetal PO2 is low, ~30-40 mmHg)

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7. Special Hemoglobins and ODC

1. CO occupies heme sites (reducing O2-carrying capacity)
2. Remaining oxy-Hb subunits shift LEFT (won't release O2 to tissues) Result: SpO2 reads falsely normal on pulse oximeter (COHb absorbs light like OxyHb at 660 nm)

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8. Oxygen Content of Blood

Formula:

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9. Clinical Anesthesia Applications (High-Yield)

Hypothermia and ODC

• Hypothermia causes LEFT shift → Hb retains O2 → tissue hypoxia risk even if SpO2 appears normal
• Clinically relevant in cardiac bypass, deep hypothermic circulatory arrest
• At 20°C, P50 falls dramatically; blood gas analyzers measure PO2 at 37°C - must correct for patient temperature

Stored Blood

• 2,3-DPG depletes in 24-48h of storage → left shift
• However, regenerated within 24h after transfusion
• Clinical impact debated - large RCTs (INFORM trial) showed no survival difference with older vs. newer blood

One-Lung Ventilation (OLV) / Thoracic Anesthesia

• HPV depends partly on the ODC - hypoxia in the nonventilated lung promotes vasoconstriction (HPV)
• Alkalosis from hyperventilation → LEFT shift → impairs O2 delivery; also inhibits HPV

Carbon Monoxide Poisoning

• Standard pulse oximetry unreliable (falsely high SpO2 reading)
• Measure COHb directly by co-oximetry
• 100% O2 displaces CO; half-life of COHb falls from 5h (room air) → 90 min (100% O2) → 23 min (hyperbaric O2)

Neonates

• At birth HbF predominates (left-shifted curve); transitions to HbA over first 6 months
• Fetal "normal" PaO2 values are lower (25-35 mmHg) but Hb saturation still ~70% due to HbF

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10. Quick Summary Table - Shifts of ODC

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DNB Exam Buzzwords to Remember

• P50 = 26.5 mmHg (normal)
• Bohr effect = H⁺/CO2 → right shift
• Haldane effect = O2 binding → CO2 release (reciprocal)
• HbF left-shifted because γ-chains bind 2,3-DPG less
• Sigmoid shape = cooperative binding (not hyperbolic like myoglobin)
• "Cliff edge" at PaO2 60 mmHg = SpO2 90%; below this, further desaturation is rapid
• Hüfner's constant = 1.34 mL O2/g Hb
• Pulse oximetry blind to COHb and MetHb - use co-oximetry

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Recent Advances in One-Lung Ventilation

DNB Anesthesiology - Concise Update (2023-2026)

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1. Individualised Open-Lung Approach (iOLA) - iPROVE-OLV Trial (Lancet Respir Med, 2024)

• Design: Multicentre RCT, 1308 patients, 25 hospitals across 5 countries
• Intervention: iOLA = recruitment manoeuvre (RM) to 40 cmH2O end-inspiratory pressure + PEEP titrated to best respiratory compliance + postoperative high-flow nasal oxygen (HFNO)
• Control: Fixed PEEP 4 cmH2O + conventional O2 therapy
• Result: Severe postoperative pulmonary complications (PPCs) at 7 days:
  • iOLA group: 6% vs control: 15% (RR 0.39; 95% CI 0.28-0.56)
  • Absolute risk reduction: 9.2%
• Key message: Individualising PEEP to best compliance (rather than a fixed value) and adding postoperative HFNO significantly reduces PPCs after lung resection.

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2. Individualised PEEP Titration by Compliance - Meta-Analysis (Crit Care, 2025)

• Design: Meta-analysis of 10 RCTs, 3426 patients
• Finding: Individualised PEEP by lung compliance vs. fixed PEEP:
  • PPCs: RR 0.55 (95% CI 0.38-0.78) - 45% relative reduction
  • Pneumonia: RR 0.71; Atelectasis: RR 0.63
  • Better PaO2/FiO2 and lower driving pressure
  • No difference in ARDS, mortality, or haemodynamics
• Best method: Titration by dynamic compliance using a stepwise decremental strategy outperformed incremental or driving-pressure based titration
• Key message: PEEP should be titrated to optimal compliance for each patient during OLV - a fixed 5 cmH2O is insufficient.

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3. Driving Pressure (DP) as the New Ventilatory Target

What is Driving Pressure?

• Emerged from ARDS literature (Amato, NEJM 2015) - applied now to OLV
• During OLV, only one lung is ventilated - DP captures the actual strain on available lung units better than tidal volume alone

2026 Systematic Review (Front Med, 2026 - PMID 42245953)

• Pooled 4 RCTs (654 patients): DP-guided ventilation trended toward reduced PPCs (RR 0.60) but not statistically significant (p = 0.14); evidence graded as low certainty (GRADE)
• Conclusion: DP-guided OLV is hypothesis-generating - larger multicentre trials needed

Large Observational Study (J Clin Anesth, 2025 - PMID 40876391)

• Retrospective cohort, ~4951 patients with OLV surgery
• DP independently associated with PPCs (OR 1.04 per cmH2O increase)
• Proposed thresholds: DP (plateau-based) ≤15 cmH2O; DP (peak-based) ≤18 cmH2O
• Mechanical power was NOT independently associated with PPCs in this cohort

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4. Volatile vs. TIVA - Anti-inflammatory Advantage of Volatiles

Systematic Review + Meta-Analysis (Crit Care Explor, 2025 - PMID 40637441)

• 28 studies, 4175 patients undergoing lung resection
• Inhaled anaesthesia (vs TIVA) → lower alveolar TNF-α and IL-6 at 1-2 hours post-resection
• Suggests volatile agents (sevoflurane, desflurane) reduce the inflammatory cascade triggered by OLV ischaemia-reperfusion injury
• Limitation: Clinical outcome differences (mortality, ICU stay) not consistently reported
• Practical implication: Volatile anaesthesia may be preferred over TIVA specifically to attenuate OLV-associated lung inflammation, particularly in pneumonectomy or high-risk cases

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5. Bronchial Blockers (BB) vs. Double-Lumen Tubes (DLT) - Updated Evidence

Meta-Analysis (BMC Anesthesiol, 2025 - PMID 40450255)

• DLT and BB have comparable efficacy and safety for OLV
• BB advantages: preferred in tracheostomy, difficult airway, patients requiring postoperative ventilation
• DLT advantages: faster lung collapse, easier suctioning, better CPAP delivery to operative lung

New Placement Technique (J Clin Anesth, 2025 - PMID 40367865)

• Multicentre RCT: Placing the BB in the lateral decubitus position (rather than supine before positioning) improved first-attempt success rate and reduced displacement
• 2026 RCT (BMC Anesthesiol): Right lateral decubitus position improved left BB placement success

Video-DLT / Video-Imaging Bronchial Blockers

• Emerging: video-imaging DLTs and video-guided blockers allow real-time visualisation of cuff position without fibreoptic bronchoscope
• 2025 RCT showed comparable outcomes between video-DLT and video-blocker + single-lumen tube for left lung isolation in minimally invasive cardiac surgery

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6. Permissive Hypercarbia During OLV (EJA RCT, 2023 - PMID 37455644)

• Prospective RCT, thoracic surgery patients
• Permissive hypercapnia (allowing PaCO2 rise) during OLV:
  • Improved PaO2 during OLV (vasoconstricts pulmonary vessels → potentiates HPV)
  • Reduced postoperative pulmonary complications
• Mechanism: Hypercapnia enhances HPV in the collapsed lung, reducing shunt
• Corroborates allowing moderate CO2 rise rather than aggressive ventilation to normalise PaCO2

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7. Cerebral Oximetry Monitoring During OLV (J Int Med Res, 2024 - PMID 39275973)

• RCT in elderly patients undergoing OLV thoracoscopic surgery
• Optimising cerebral O2 saturation (rSO2) by targeted interventions during OLV reduced postoperative delirium incidence
• Adds neurocognitive monitoring as a new dimension of OLV management in the elderly

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Summary: What Has Changed in OLV Practice

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