CCM Notes
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Critical Care · Pulmonology
Mechanical Ventilation — ARDS
ARDSnet Lung-Protective Ventilation · FiO₂/PEEP Table · Driving Pressure · Prone Positioning
🫁 Berlin Definition of ARDS (JAMA 2012)
Mild
200–300
PaO₂/FiO₂ on PEEP/CPAP ≥5 cmH₂O
Moderate
100–200
PaO₂/FiO₂ on PEEP ≥5 cmH₂O
Severe
<100
PaO₂/FiO₂ on PEEP ≥5 cmH₂O
Required: Acute onset (<1 week) · Bilateral opacities on CXR/CT · Respiratory failure not fully explained by fluid overload or cardiac failure · PaO₂/FiO₂ ratio as above on PEEP ≥5
⚙️ ARDSnet Lung-Protective Initial Ventilator Settings
| Parameter | Target / Setting | Rationale |
|---|
| Mode | Volume Control AC (VC-AC) | Ensures consistent tidal volume delivery |
| Tidal Volume (Vt) | 6 mL/kg IBW (range 4–8 mL/kg) | ARDSnet trial: reduced mortality vs 12 mL/kg |
| Respiratory Rate | 14–35 breaths/min | Titrate to maintain pH goal; avoid air trapping |
| PEEP | 5–24 cmH₂O (per FiO₂/PEEP table) | Alveolar recruitment; prevent derecruitment |
| Plateau Pressure (Pplat) | ≤30 cmH₂O | Limits barotrauma; check every 4h and after changes |
| Driving Pressure (ΔP) | ≤15 cmH₂O (ΔP = Pplat − PEEP) | ΔP >15 independently predicts mortality (Amato 2015) |
| I:E Ratio | 1:1 to 1:3 | Allow adequate expiration; avoid auto-PEEP |
| FiO₂ | 0.3–1.0 (per table below) | Titrate to SpO₂ 88–95%, PaO₂ 55–80 mmHg |
| pH Target | 7.30–7.45 | Permissive hypercapnia accepted (pH >7.20) |
| PaO₂ Target | 55–80 mmHg | SpO₂ 88–95% acceptable in ARDS |
📊 ARDSnet FiO₂/PEEP Combination Table
Lower PEEP Strategy
| FiO₂ | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 |
|---|
| PEEP | 5 | 5–8 | 8–10 | 10 | 10–14 | 14 | 14–18 | 18–24 |
Higher PEEP Strategy (Moderate–Severe)
| FiO₂ | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 |
|---|
| PEEP | 5–14 | 14–16 | 16–18 | 20 | 22 | 22 | 22–24 | 22–24 |
🔄 Prone Positioning (PROSEVA 2013)
Indication: PaO₂/FiO₂ <150 mmHg on FiO₂ ≥0.6, PEEP ≥5
Duration: ≥16 hours/day continuous sessions
Mortality benefit: 28-day mortality ↓ 32.8% vs 16%
Contraindications: unstable spine, elevated ICP, open abdomen, facial trauma, pregnancy (relative)
Complications: pressure sores, ETT displacement, facial edema
Revert to supine if PaO₂/FiO₂ >150 x 4h (PEEP ≤10, FiO₂ ≤0.6)
🆘 Rescue Therapies (Refractory Hypoxemia)
●Inhaled Nitric Oxide (iNO): 5–40 ppm. Improves V/Q mismatch — no mortality benefit
●Neuromuscular Blockade (NMB): Cisatracurium 37.5 mg/h; 48h course may reduce VILI in moderate-severe ARDS
●Recruitment Maneuvers: Use cautiously — sustained inflation or CPAP 40 cmH₂O × 40 sec (risk: hemodynamic instability)
●ECMO (VV): PaO₂/FiO₂ <80 despite optimization ≥3h, or uncompensated hypercapnia (pH <7.25)
●Corticosteroids: Dexamethasone 20 mg/day × 5d → 10 mg × 5d may reduce ventilator days (DEXA-ARDS, 2020)