Newer Ventilatory Strategies for ARDS

MD Anaesthesiology — 10 Marks Exam Notes

Definition & Pathophysiologic Basis

• Dependent zones: consolidated/atelectatic but recruitable
• Non-dependent zones: relatively normal but small
• Middle zones: poorly aerated, recruitable with PEEP

Mechanisms of VILI (Background for Exam)

I. Low Tidal Volume (LTV) Ventilation — ARDSNet Protocol (Core Strategy)

• Mode: Volume-controlled Assist/Control (A/C mode) — the only ventilator intervention proven to improve long-term survival in ARDS
• Tidal volume: 6 mL/kg PBW (reduce in steps of 1 mL/kg every ≤2 hours from initial 8 mL/kg)
• Plateau pressure (Pplat): ≤30 cmH₂O (if >30, reduce TV to minimum 4 mL/kg PBW)
• Respiratory rate: Up to 35 breaths/min to maintain pH
• PBW formula: Males = 50 + 2.3 × [height (inches) − 60]; Females = 45.5 + 2.3 × [height (inches) − 60]

Important: Tidal volume is based on predicted (ideal) body weight, not actual weight — lung volume correlates with height, not weight.

II. PEEP Optimisation

• Higher PEEP (~12–13 cmH₂O) is beneficial in moderate-severe ARDS (PaO₂/FiO₂ <200 mmHg)
• PEEP titration methods: FiO₂-PEEP table; oesophageal pressure-guided PEEP; recruitment-to-inflation ratio (R/I ratio)
• Very high PEEP with aggressive recruitment maneuvers can be harmful — not routinely recommended (PHARLAP, ART trials)

III. Driving Pressure Limitation

• Driving pressure (ΔP) = Pplat − PEEP (reflects dynamic strain on the lung)
• Target: ΔP ≤15 cmH₂O
• Driving pressure may better reflect risk of injury than plateau pressure alone; lower driving pressure independently associated with improved survival
• Larger tidal volumes may be used safely in patients with higher compliance to avoid detrimental effects of overly low tidal volumes (acidosis, atelectasis, dyssynchrony)

IV. Prone Positioning

• Improves oxygenation by:
  • More uniform distribution of pleural pressure
  • Redistribution of ventilation away from dependent (collapsed) regions
  • Reducing VILI through more homogeneous stress distribution
• PROSEVA trial (2013): >16 hours/day prone positioning reduced 28-day mortality from 32.8% → 16% in severe ARDS (PaO₂/FiO₂ <150 mmHg)
• Current recommendation: Prone positioning for ≥16 hours/day in moderate-severe ARDS (P/F <150) — indicated irrespective of its effect on oxygenation in any given patient
• Contraindications: spinal instability, open chest/abdomen, unstable haemodynamics

V. Neuromuscular Blockade (NMB)

• ACURASYS trial: Early 48-hour cisatracurium infusion improved outcomes in severe ARDS
• ROSE trial (2019): Did not confirm survival benefit when light sedation was used in the control arm
• Current recommendation: Routine NMB not recommended in moderate-severe ARDS; however, judicious use is appropriate in:
  • Patient-ventilator asynchrony with breath stacking
  • Very large spontaneous inspiratory efforts (P-SILI risk)
  • Severe refractory hypoxaemia

VI. Permissive Hypercapnia

• Deliberate acceptance of elevated PaCO₂ to avoid injurious ventilator settings
• pH Goal: 7.30–7.45
• If pH 7.15–7.30: increase RR (max 35/min) → consider NaHCO₃
• If pH <7.15: increase TV even if Pplat exceeds 30 cmH₂O (life-threatening acidosis takes precedence)

• Raised intracranial pressure
• Acute cerebrovascular events / stroke
• Myocardial ischaemia
• Severe pulmonary hypertension / right ventricular failure
• Uncorrected severe metabolic acidosis
• Pregnancy

VII. Rescue / Salvage Strategies (Refractory Hypoxaemia)

VIII. Fluid Management

• Conservative fluid strategy (FACTT trial): reduces duration of mechanical ventilation and ICU stay without worsening organ function
• Target: CVP 4–6 mmHg or PCWP 8–12 mmHg in euvolaemic state
• Avoid fluid overload — worsens pulmonary oedema and lung compliance

IX. Liberation from Mechanical Ventilation

• Daily spontaneous breathing trials (SBT) with pressure support
• Criteria: FiO₂ ≤0.4, PEEP ≤8, haemodynamic stability, adequate respiratory effort
• Avoid prolonged ventilation — associated with ICU-acquired weakness and VAP

Summary Table — Key Targets

Key Trials to Quote

Newer Ventilatory Strategies for ARDS — Updated with 2023 Global & ATS 2024 Guidelines

MD Anaesthesiology — 10 Marks Exam Notes

Definition & Pathophysiologic Basis

■ BERLIN DEFINITION (2012) — Still Current

■ 2023 GLOBAL DEFINITION OF ARDS (NEW)

1. Non-intubated ARDS: Includes patients on high-flow nasal oxygen (HFNO) ≥30 L/min or non-invasive ventilation (NIV/CPAP ≥5 cmH₂O) with PaO₂/FiO₂ ≤300 or SpO₂/FiO₂ ≤315
2. Resource-limited settings: Allows diagnosis without PEEP requirement and permits SpO₂/FiO₂ ratio in place of PaO₂/FiO₂ when ABG unavailable (SpO₂/FiO₂ ≤315 corresponds to P/F ≤300)
3. Ultrasound-confirmed bilateral infiltrates accepted in addition to CXR/CT
4. Timing: Onset within 1 week of known clinical insult or new/worsening respiratory symptoms
5. Non-cardiogenic origin: Respiratory failure not fully explained by cardiac failure or fluid overload
6. Removes SpO₂ cap at 97% — SpO₂/FiO₂ ratio valid only when SpO₂ ≤97%

I. LOW TIDAL VOLUME VENTILATION — Core Strategy (ARDSNet; confirmed by all guidelines)

• Mode: Volume-controlled Assist/Control (A/C)
• TV: 6 mL/kg predicted body weight (PBW) — reduce stepwise 1 mL/kg q2h from 8 mL/kg
• PBW: Males = 50 + 2.3 × [Ht(inches) − 60]; Females = 45.5 + 2.3 × [Ht(inches) − 60]
• Plateau pressure (Pplat): ≤30 cmH₂O; if >30 → reduce TV to minimum 4 mL/kg
• RR: Up to 35 breaths/min to maintain pH 7.30–7.45
• ARMA trial (2000): 31% vs 40% mortality — landmark evidence, endorsed by all guidelines

II. DRIVING PRESSURE LIMITATION — Emerging Core Target

• ΔP reflects the actual distending stress delivered to the available functional lung
• Reanalysis of randomised trials: ΔP was the strongest mechanical predictor of 60-day mortality in ARDS — stronger than TV or Pplat alone
• Target: ΔP ≤14–15 cmH₂O
• ATS 2024 Guideline recommendation: Titrate ventilator settings to minimise driving pressure; conditional recommendation to use driving pressure as a primary ventilator target in addition to TV and Pplat
• Transpulmonary driving pressure (TPDP) = (Pplat − PEEP) − (oesophageal plateau − end-expiratory oesophageal pressure): corrects for chest wall elastance; particularly important in obese patients or those with chest wall oedema

Murray & Nadel's Respiratory Medicine: "Limiting DP to 14 cm H₂O or less may reduce lung injury… DP was the pulmonary mechanical variable most predictive of 60-day mortality."

III. PEEP OPTIMISATION

• ARDSNet FiO₂/PEEP table: Standard initial approach; target SpO₂ 88–95% / PaO₂ 55–80 mmHg

• Higher PEEP benefits moderate-severe ARDS (P/F <200) but not mild ARDS
• ATS 2024: Suggests PEEP titration guided by driving pressure response — increase PEEP only if ΔP decreases (indicates recruitment exceeds overdistention)
• Recruitment-to-Inflation (R/I) ratio: Bedside index of lung recruitability; R/I >0.5 suggests significant recruitable lung → may benefit from higher PEEP
• Oesophageal pressure-guided PEEP (EPVent-2 trial): No universal benefit but may help in obese patients and those with high chest wall elastance
• Aggressive recruitment manoeuvres + very high PEEP: ART and PHARLAP trials showed harm — not recommended routinely

IV. MECHANICAL POWER — New Concept (ATS 2024)

• Integrates all injurious variables: TV, RR, driving pressure, PEEP, flow
• Target: MP <17 J/min (higher values associate with VILI and mortality)
• ATS 2024 highlights mechanical power as an emerging guide for ventilator optimisation, though RCT evidence is still awaited
• Currently used as a monitoring parameter to identify cumulative injury risk

Miller's Anaesthesia 10e: "Mechanical power, as an index of rate of energy dissipation, can be used to assess the risk of developing ventilator-induced lung injury."

V. PRONE POSITIONING — Strengthened Recommendation

• PROSEVA trial (2013): ≥16 hours/day prone → 28-day mortality 16% vs 32.8% in P/F <150
• 2023 Global ARDS Guidelines: Strong recommendation — prone positioning ≥16 hours/day for moderate-severe ARDS (P/F <150); initiate within 36 hours of ARDS diagnosis
• ATS 2024: Reaffirms prone positioning as standard of care (not a rescue manoeuvre) for P/F <150; recommends early initiation
• Mechanism: Homogenises ventilation distribution, reduces dependent atelectasis, decreases VILI, improves V/Q matching, reduces RV afterload
• PRONE-SICU trial (2022–2023): Awake prone positioning in non-intubated COVID-ARDS showed reduction in intubation rates

VI. NEUROMUSCULAR BLOCKADE (NMB)

• ACURASYS (2010): Early 48-hour cisatracurium → improved 90-day survival in severe ARDS
• ROSE trial (2019): NMB no benefit over light sedation strategy
• 2023 Global Guideline / ATS 2024:
  • Routine NMB: NOT recommended in moderate-severe ARDS
  • Conditional use in: severe patient-ventilator asynchrony, breath stacking, very large inspiratory efforts causing P-SILI (Patient Self-Inflicted Lung Injury), refractory hypoxaemia
  • P-SILI is now a recognised indication: spontaneous large-effort breathing in ARDS can amplify lung injury — monitor with oesophageal pressure or respiratory muscle EMG

VII. PERMISSIVE HYPERCAPNIA

• Accept elevated PaCO₂ to avoid injurious ventilator settings
• pH Goal: 7.30–7.45
• If pH <7.15 despite RR 35/min → may increase TV (safety over targets)
• ATS 2024: Hypercapnia acceptable as long as haemodynamics and CNS are stable

VIII. RESCUE STRATEGIES FOR REFRACTORY HYPOXAEMIA (P/F <100)

IX. FLUID MANAGEMENT (FACTT Trial)

• Conservative strategy reduces ventilator days and ICU stay without worsening renal/organ function
• Target: euvolaemia after resuscitation phase; CVP 4–6 mmHg
• ATS 2024 reaffirms: conservative fluid balance once haemodynamic stability achieved

X. ADJUNCTS — 2023/2024 Guideline Updates

XI. LIBERATION FROM VENTILATION

• Daily SBT (pressure support 5–8 cmH₂O, PEEP 5)
• Criteria: FiO₂ ≤0.40, PEEP ≤8, haemodynamic stability, adequate cough
• ATS 2024: Protocol-driven weaning superior to physician-directed weaning; early SBT reduces ICU stay
• Avoid prolonged NMB — ICU-acquired weakness delays extubation

Summary — Key Targets (2024)

Key Trials Reference Table

• Driving pressure is now a primary ventilator target, not just a secondary metric
• Mechanical power is a new monitoring concept for cumulative VILI risk
• Awake prone positioning on HFNO is a formal recommendation
• ECMO receives conditional recommendation for ultra-severe ARDS (P/F <80)
• ARDS definition itself expanded — non-intubated patients, SpO₂/FiO₂ ratio, ultrasound accepted
• Corticosteroids — conditional recommendation for early moderate-severe ARDS