Mechanical ventilation strategies

12 min read Last updated April 9, 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 &lt, 30 cmH2O, driving pressure (Pplat − PEEP) as low as feasible (often &lt, 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 &lt, 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 &lt, 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 &gt, 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 &lt, 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 &lt, 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.

Test yourself…

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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