Mechanical ventilation strategies

12 min read Last updated April 8, 2026

Surgical approach

  • Not an operation: no surgical approach.
  • Context where strategies are commonly required
    • Major abdominal/thoracic surgery (reduced FRC, atelectasis, one-lung ventilation).
    • ICU respiratory failure (ARDS, pneumonia, sepsis, trauma).
    • Neurosurgery (PaCO2 targets, ICP considerations).

Anaesthetic management

  • Type of anaesthesia
    • Usually GA when invasive ventilation required; regional/awake techniques may avoid intubation in selected cases.
  • Airway device
    • ETT for controlled ventilation, high pressures/PEEP, aspiration risk, long cases, laparoscopy, prone, thoracic surgery.
    • SGA may be acceptable for short, low-risk cases with modest pressures; avoid if high airway pressures/PEEP required.
  • Duration
    • Highly variable: minutes (short cases) to days/weeks (ICU).
  • Analgesia / “How painful?”
    • Ventilation strategy itself is not painful; discomfort relates to ETT, underlying disease, and surgical insult.
    • Ensure adequate analgesia and sedation; consider neuromuscular blockade for severe ventilator dyssynchrony/ARDS early phase.
  • Immediate ventilator starting point (adult, normal lungs)
    • Mode: volume-controlled or pressure-controlled with targets.
    • VT 6–8 mL/kg predicted body weight; RR to maintain pH; PEEP 5 cmH2O; FiO2 to SpO2 92–96% (or 88–92% in CO2 retainers).
    • Plateau pressure aim < 30 cmH2O; driving pressure (Pplat − PEEP) as low as feasible (often < 15).

1) Core physiology and definitions

  • Goals of mechanical ventilation
    • Maintain oxygenation and ventilation (CO2/pH).
    • Reduce work of breathing; treat/avoid fatigue.
    • Prevent ventilator-induced lung injury (VILI): volutrauma, barotrauma, atelectrauma, biotrauma.
    • Facilitate surgery/anaesthesia and airway protection.
  • Key pressures/volumes
    • Peak inspiratory pressure (PIP): includes resistive + elastic components; rises with bronchospasm/secretions/kinked tube.
    • Plateau pressure (Pplat): alveolar pressure at end-inspiration during inspiratory hold; reflects compliance; target < 30 cmH2O in ARDS.
    • PEEP: prevents end-expiratory alveolar collapse; improves FRC and V/Q matching but may impair venous return and increase PVR.
    • Driving pressure: Pplat − PEEP; surrogate of cyclic lung stress; lower is better (often < 15).
    • Compliance (C) = ΔV/ΔP; Resistance (R) = ΔP/flow.
  • Predicted body weight (PBW) for VT
    • Male: PBW(kg) = 50 + 0.91 × (height(cm) − 152.4).
    • Female: PBW(kg) = 45.5 + 0.91 × (height(cm) − 152.4).

2) Modes of ventilation (what changes, what you set, pros/cons)

  • Controlled vs assisted
    • Controlled: ventilator delivers breaths regardless of patient effort (often with sedation/NMB).
    • Assisted: patient triggers some/all breaths; ventilator supports with set parameters.
  • Volume-controlled ventilation (VCV)
    • Set: VT, RR, inspiratory flow pattern, PEEP, FiO2.
    • Pressure varies with compliance/resistance; guarantees minute ventilation (if no major leak).
    • Risks: high pressures if compliance worsens; monitor Pplat and driving pressure.
  • Pressure-controlled ventilation (PCV)
    • Set: inspiratory pressure (Pinsp above PEEP), RR, I:E (Tinsp), PEEP, FiO2.
    • VT varies with compliance/resistance; decelerating flow may improve distribution and reduce PIP.
    • Useful with leaks (e.g., SGA), laparoscopy, and to limit peak pressures; must watch delivered VT/minute ventilation.
  • Pressure support ventilation (PSV)
    • Patient-triggered, pressure-targeted; set PS level, PEEP, FiO2; patient controls RR and Tinsp.
    • Used for weaning and spontaneous breathing; avoid in apnoea unless backup mode present.
  • SIMV / PSV + backup
    • Mandatory breaths at set rate plus spontaneous breaths with/without pressure support; less commonly used now than PSV/assist-control strategies.
  • CPAP
    • Spontaneous breathing with continuous positive pressure; improves oxygenation by recruiting alveoli; no mandatory breaths.
  • Advanced strategies (ICU)
    • APRV/BiLevel: prolonged high pressure with brief releases; aims to improve recruitment while allowing spontaneous breathing; evidence mixed; requires expertise.
    • PRVC/volume-targeted pressure modes: pressure-controlled breath adjusted to achieve target VT; combines features of PCV and VCV.

3) Lung-protective ventilation (perioperative and ICU)

  • Principles
    • Use low VT based on PBW; avoid high Pplat and high driving pressure.
    • Use adequate PEEP to prevent atelectasis; avoid excessive FiO2 (absorption atelectasis, oxygen toxicity).
    • Treat the cause (sepsis control, diuresis, bronchodilation, suction, recruitment where appropriate).
  • Typical intraoperative “protective” settings (adult, non-ARDS)
    • VT 6–8 mL/kg PBW; PEEP 5–8 cmH2O (higher in obesity/laparoscopy); avoid Pplat > 30.
    • FiO2 titrated to SpO2; consider periodic recruitment manoeuvres selectively (after disconnections, desaturation, induction) rather than routine aggressive manoeuvres.
  • Recruitment manoeuvres (RM)
    • Aim: open collapsed alveoli; must be followed by sufficient PEEP to maintain recruitment.
    • Risks: hypotension (reduced venous return), barotrauma, overdistension; avoid/modify in haemodynamic instability, bullous disease, raised ICP (relative).

4) ARDS ventilation strategy (adult)

  • Diagnosis (Berlin definition summary)
    • Timing: within 1 week of known insult/new or worsening symptoms.
    • Imaging: bilateral opacities not fully explained by effusions/collapse/nodules.
    • Origin: respiratory failure not fully explained by cardiac failure/fluid overload.
    • Oxygenation severity by PaO2/FiO2 with PEEP/CPAP ≥ 5 cmH2O: mild 200–300, moderate 100–200, severe < 100 (mmHg).
  • Ventilation targets (ARDSNet-style)
    • VT ~ 6 mL/kg PBW (consider 4–6 if severe) and permissive hypercapnia if needed.
    • Pplat ≤ 30 cmH2O; driving pressure as low as possible.
    • PEEP: use moderate/high PEEP strategies in moderate–severe ARDS; individualise using oxygenation, compliance, haemodynamics; consider transpulmonary pressure if oesophageal manometry available.
    • Oxygenation: accept SpO2 ~88–95% / PaO2 ~7.3–10.7 kPa (55–80 mmHg) to reduce FiO2 and ventilator stress.
  • Adjuncts in moderate–severe ARDS
    • Prone positioning (often 12–16 h/day) improves oxygenation and mortality in severe ARDS when applied early with expertise.
    • Neuromuscular blockade: short course may help severe dyssynchrony/high pressures early; balance against ICU weakness risk.
    • Inhaled pulmonary vasodilators (e.g., iNO) as rescue for refractory hypoxaemia/right heart strain; no clear mortality benefit.
    • ECMO referral when refractory hypoxaemia/hypercapnia despite optimal lung-protective ventilation and proning (centre criteria vary).

5) Obstructive lung disease (asthma/COPD): avoid dynamic hyperinflation

  • Problem
    • High airway resistance → prolonged expiration → air trapping → intrinsic PEEP (auto-PEEP) → hypotension, barotrauma, difficult triggering.
  • Ventilation strategy
    • Reduce minute ventilation demand where possible (treat fever, pain, agitation).
    • Allow long expiratory time: low RR, high inspiratory flow, I:E 1:3–1:5 (or longer).
    • Use modest VT (6–8 mL/kg PBW) and accept permissive hypercapnia if pH acceptable.
    • Monitor for auto-PEEP: expiratory flow not returning to zero; rising end-expiratory volume; hypotension.
    • External PEEP: may help triggering in COPD by offsetting auto-PEEP (typically set to ~70–85% of measured auto-PEEP); avoid excessive PEEP which worsens hyperinflation.
    • Treat bronchospasm: inhaled β2-agonist, ipratropium, IV magnesium, steroids; consider ketamine/sevoflurane in theatre; suction secretions; check tube patency.
  • Emergency decompensation on ventilator (suspected severe air trapping)
    • Disconnect circuit briefly to allow full exhalation; then re-ventilate with longer expiratory time and lower RR.

6) Restrictive physiology/poor compliance (obesity, laparoscopy, pulmonary oedema, fibrosis)

  • Features
    • Low compliance → higher pressures for given VT; atelectasis common; oxygenation improves with PEEP and recruitment.
  • Strategy
    • VT 6–8 mL/kg PBW; consider higher PEEP (8–12+) especially in obesity/laparoscopy; monitor haemodynamics.
    • Consider PCV/volume-targeted pressure modes to control pressures; ensure adequate minute ventilation.

7) One-lung ventilation (OLV) essentials

  • Goals
    • Maintain oxygenation while minimising lung injury to ventilated lung and avoiding excessive pressures.
  • Typical OLV settings
    • VT 4–6 mL/kg PBW; PEEP 5 cmH2O to dependent lung (individualise); keep Pplat < 25–30.
    • FiO2 initially high then titrate; avoid unnecessary hyperoxia if stable.
    • Manage hypoxaemia: check tube position (FOB), optimise CO and Hb, recruitment/PEEP to dependent lung, CPAP to non-dependent lung if feasible, intermittent two-lung ventilation.

8) Permissive hypercapnia and controlled hypocapnia

  • Permissive hypercapnia
    • Used to avoid injurious ventilation (low VT/low pressures), especially ARDS/asthma.
    • Contraindications/relative cautions: raised ICP, severe pulmonary hypertension/RV failure, severe metabolic acidosis, significant arrhythmias/ischaemia (individualise).
  • Hypocapnia (e.g., neuroanaesthesia)
    • Brief hyperventilation may reduce ICP via cerebral vasoconstriction; avoid prolonged profound hypocapnia (cerebral ischaemia risk).

9) Troubleshooting: high pressure, low volume, hypoxaemia, hypercapnia

  • High PIP approach (differentiate resistance vs compliance)
    • Check Pplat: if PIP↑ but Pplat normal → resistance problem (kink, secretions, bronchospasm, biting, small tube).
    • If PIP↑ and Pplat↑ → compliance problem (atelectasis, pneumothorax, pulmonary oedema, ARDS, abdominal insufflation, chest wall rigidity).
  • Hypoxaemia on ventilator (structured approach)
    • Immediate: increase FiO2; check patient, circuit, oxygen supply; confirm ETT position and bilateral air entry; suction.
    • Then: assess for atelectasis (recruit/PEEP), bronchospasm, pneumothorax, pulmonary oedema, embolism, low CO, shunt (OLV).
  • Hypercapnia
    • Check ventilation: minute ventilation, leaks, dead space, rebreathing (valves/absorber), increased CO2 production (sepsis, MH).

10) Weaning and liberation from ventilation (ICU principles)

  • Readiness criteria (typical)
    • Cause improving; adequate oxygenation (e.g., FiO2 ≤ 0.4–0.5, PEEP ≤ 5–8), haemodynamic stability, manageable secretions, adequate consciousness/airway protection.
  • Spontaneous breathing trial (SBT)
    • PSV low support or T-piece for 30–120 min; monitor RR, VT, SpO2, HR/BP, work of breathing, agitation.
  • Extubation considerations
    • Cuff leak if concern for laryngeal oedema; plan post-extubation support (HFNO/NIV) in high-risk patients.
Explain the differences between volume-controlled and pressure-controlled ventilation. What are the advantages and disadvantages of each?

Structure: what you set, what varies, clinical use, hazards, monitoring.

  • Volume-controlled ventilation (VCV)
    • Set VT, RR, flow, PEEP, FiO2; pressure varies with compliance/resistance.
    • Pros: guaranteed VT/minute ventilation (if no leak); easy CO2 control.
    • Cons: if compliance worsens → high PIP/Pplat; risk of baro/volutrauma if not monitored.
  • Pressure-controlled ventilation (PCV)
    • Set Pinsp, RR, Tinsp/I:E, PEEP, FiO2; VT varies with compliance/resistance.
    • Pros: limits peak pressure; decelerating flow may improve distribution; useful with leaks (SGA).
    • Cons: VT can fall with worsening compliance/bronchospasm → hypercapnia; requires close monitoring of VT and minute ventilation.
  • Monitoring common to both
    • Pplat (in VCV via inspiratory hold; in PCV may be close to set pressure if no flow at end-inspiration), driving pressure, ETCO2/ABG, waveforms, haemodynamics.
A ventilated patient develops a sudden rise in peak airway pressure. How would you assess and manage this?

Aim: rapid safety actions + differentiate resistance vs compliance using plateau pressure and clinical assessment.

  • Immediate actions
    • Assess patient (SpO2, BP, chest movement), increase FiO2, call for help if unstable.
    • Check circuit: kinks, water, filter obstruction, ventilator malfunction; consider switching to manual ventilation to feel compliance.
  • Differentiate resistance vs compliance
    • Measure Pplat (inspiratory hold) if VCV: PIP↑ with normal Pplat → increased resistance (bronchospasm, secretions, kink/biting, ETT obstruction).
    • PIP↑ with Pplat↑ → reduced compliance (atelectasis, pneumothorax, pulmonary oedema/ARDS, abdominal insufflation, mainstem intubation).
  • Targeted management examples
    • Bronchospasm: deepen anaesthesia, β2-agonist, ipratropium, steroids, magnesium; check for anaphylaxis.
    • Secretions/ETT obstruction: suction, pass catheter, consider tube change.
    • Pneumothorax: suspect with hypotension/unilateral breath sounds; decompress if tension suspected.
    • Atelectasis: recruitment manoeuvre + increase PEEP if appropriate; ensure adequate analgesia and positioning.
Describe lung-protective ventilation. What parameters would you target in ARDS?

Key marks: PBW-based VT, plateau and driving pressure, PEEP strategy, oxygenation targets, permissive hypercapnia, adjuncts.

  • Ventilator targets
    • VT ~6 mL/kg PBW (consider 4–6 in severe ARDS).
    • Pplat ≤30 cmH2O; minimise driving pressure (Pplat−PEEP).
    • PEEP: moderate–high, individualised; avoid derecruitment (avoid frequent disconnections).
    • Oxygenation: accept lower PaO2/SpO2 to reduce FiO2 and ventilator stress (e.g., SpO2 88–95%).
    • Permissive hypercapnia if needed (treat severe acidaemia; consider contraindications such as raised ICP).
  • Adjuncts for moderate–severe ARDS
    • Early prone positioning; consider short course NMB for dyssynchrony; ECMO referral if refractory.
How do you set tidal volume in an obese patient? Explain predicted body weight and why it matters.

Exam focus: VT relates to lung size (height/sex), not actual weight; obesity increases chest wall load not lung volume.

  • Use PBW not actual body weight
    • Male PBW = 50 + 0.91 × (height(cm) − 152.4). Female PBW = 45.5 + 0.91 × (height(cm) − 152.4).
    • Set VT typically 6–8 mL/kg PBW (lower if lung injury).
  • Why PBW matters
    • Lung size correlates with height/sex; using actual weight in obesity risks excessive VT → volutrauma and high driving pressures.
    • Obesity often needs higher PEEP and recruitment to counter atelectasis; monitor haemodynamics.
A ventilated COPD patient becomes hypotensive with rising airway pressures. Explain auto-PEEP and how you would manage it.

Core: dynamic hyperinflation → increased intrathoracic pressure → reduced venous return and barotrauma risk.

  • Mechanism and recognition
    • Expiratory time insufficient → incomplete exhalation → air trapping → intrinsic PEEP; expiratory flow does not return to zero.
    • Consequences: hypotension, difficult triggering, hypercapnia, barotrauma.
  • Immediate management
    • Disconnect circuit briefly to allow exhalation if severe; treat hypotension with fluids/vasopressors as appropriate.
  • Ventilator adjustments
    • Reduce RR; increase inspiratory flow to shorten Tinsp; set I:E 1:3–1:5+; avoid excessive VT.
    • Accept permissive hypercapnia if pH acceptable; reduce CO2 production (analgesia, sedation, treat sepsis/fever).
    • Consider external PEEP to aid triggering in COPD (careful titration).
  • Treat cause of obstruction
    • Bronchodilators, steroids, magnesium; suction; check ETT position/patency; exclude pneumothorax.
How would you manage refractory hypoxaemia during one-lung ventilation?

Prioritise: confirm tube position, optimise V/Q, recruit dependent lung, reduce shunt from non-dependent lung.

  • Immediate checks
    • Increase FiO2; confirm DLT/bronchial blocker position with fibreoptic bronchoscopy; suction both lumens.
  • Optimise ventilated (dependent) lung
    • Recruitment manoeuvre then appropriate PEEP (often 5 cmH2O; individualise to compliance/oxygenation).
    • Use VT 4–6 mL/kg PBW; avoid high pressures; ensure adequate CO and Hb.
  • Reduce shunt from non-dependent lung
    • Apply CPAP (e.g., 2–5 cmH2O) with oxygen to non-dependent lung if surgical field allows.
    • Intermittent two-lung ventilation if necessary; coordinate with surgeon.
Explain plateau pressure and driving pressure. How do you measure plateau pressure and why is it important?

This is a common viva: define, measure, interpret, and link to compliance and VILI.

  • Definitions
    • Plateau pressure (Pplat): alveolar pressure at end-inspiration when flow is zero; reflects elastic load (lung + chest wall).
    • Driving pressure: Pplat − PEEP; reflects cyclic distending pressure and correlates with outcomes in ARDS.
  • Measurement
    • In VCV: perform an inspiratory hold (0.5–2 s) to stop flow; read Pplat. In PCV, end-inspiratory pressure approximates Pplat if no flow at end-inspiration.
  • Clinical importance
    • High Pplat suggests low compliance and risk of overdistension; target ≤30 cmH2O in ARDS.
    • If PIP high but Pplat normal → resistance problem; if both high → compliance problem.
What are the haemodynamic effects of positive pressure ventilation and PEEP?

Link intrathoracic pressure to venous return, RV afterload, LV afterload, and clinical contexts.

  • Venous return and cardiac output
    • Increased intrathoracic pressure reduces venous return (↓ preload) → may reduce CO, especially if hypovolaemic.
  • Right ventricle and pulmonary circulation
    • High lung volumes/PEEP can increase PVR (alveolar vessel compression) → ↑ RV afterload; problematic in pulmonary hypertension/RV failure.
  • Left ventricle
    • May reduce LV afterload (reduced transmural pressure), sometimes improving CO in LV failure/pulmonary oedema.
  • Clinical implications
    • Titrate PEEP with haemodynamics; consider fluids/vasopressors; avoid excessive PEEP in RV failure unless clear oxygenation benefit.
Describe a strategy for ventilating a patient with severe metabolic acidosis (e.g., DKA) who requires intubation.

Key risk: loss of compensatory hyperventilation → rapid severe acidaemia. Ventilation strategy must match pre-intubation minute ventilation initially.

  • Pre-intubation planning
    • Recognise high minute ventilation requirement; preoxygenate; consider awake/intubation strategy if appropriate; minimise apnoea time.
  • Post-intubation ventilation
    • Set high minute ventilation initially (higher RR with appropriate VT) to approximate pre-intubation PaCO2; monitor ABG early and frequently.
    • Avoid excessive VT/pressures; consider increasing RR rather than VT; ensure no added dead space.
  • Treat the cause
    • DKA: insulin, fluids, electrolytes; sepsis: antibiotics/source control; consider bicarbonate only in selected severe cases per local guidance.
How would you set ventilation during laparoscopic surgery and why?

Pneumoperitoneum reduces compliance and increases atelectasis and CO2 load.

  • Expected changes
    • ↓ compliance, ↑ airway pressures, cephalad diaphragm, ↑ V/Q mismatch; ↑ CO2 absorption → ↑ ETCO2.
  • Ventilation strategy
    • VT 6–8 mL/kg PBW; increase RR to maintain ETCO2/pH; consider PCV to limit peak pressures while ensuring adequate VT.
    • Use PEEP (often 5–10; higher in obesity); consider recruitment after insufflation and before emergence.
    • Monitor haemodynamics (PEEP + pneumoperitoneum can reduce venous return).

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