Lung size determined by height/sex — NOT actual weight. Volutrauma causes VILI.
Plateau Pressure
≤30 cmH2O
<28 cmH2O
>30 cmH2O
Barotrauma risk above 30 cmH2O. Reduce Vt by 1 mL/kg steps if exceeded.
Driving Pressure
≤15 cmH2O
<13 cmH2O
>15 cmH2O
Amato 2015 (NEJM): driving pressure most strongly associated with ARDS survival.
Static Compliance
>50 mL/cmH2O
>60 mL/cmH2O
<40 mL/cmH2O
Low compliance = stiff lungs. ARDS typically 15–40 mL/cmH2O. Guides PEEP titration.
PEEP
≥5 cmH2O (Berlin ARDS criterion)
Per ARDSNet table
<5 cmH2O in ARDS
Prevents alveolar derecruitment. Titrate per PEEP-FiO2 table to SpO2 88–95%.
SpO2 Target
88–95%
88–95%
<88% or >97%
Avoid hyperoxia. Above 95% offers no additional benefit and may cause harm.
Respiratory Rate
14–35/min
14–25/min
>35/min or pH <7.20
Adjust to achieve pH 7.30–7.45. Permissive hypercapnia (pH ≥7.25) acceptable in ARDS.
ARDSNet PEEP-FiO₂ Table — Lower & Higher PEEP Strategies
Lower PEEP / Higher FiO₂
FiO₂
PEEP (cmH₂O)
0.30
5
0.40
5–8
0.50
8–10
0.60
10
0.70
10–14
0.80
14
0.90
14–18
1.00
18–24
Higher PEEP / Lower FiO₂
FiO₂
PEEP (cmH₂O)
0.30
5–14
0.40
14–16
0.50
16–20
0.60
20
0.70
20
0.80
20–22
0.90
22
1.00
22–24
ARDSNet PEEP-FiO₂ tables. Target: SpO₂ 88–95% or PaO₂ 55–80 mmHg using the lowest FiO₂ achieving target. Higher PEEP strategy preferred in moderate-severe ARDS (P/F <200).
Lung-protective ventilation (LPV) is the cornerstone of ARDS management and the primary intervention proven to reduce ARDS mortality.
The landmark ARDSNet trial (NEJM 2000) demonstrated a 22% reduction in mortality using 6 mL/kg predicted body weight (PBW) tidal volumes versus traditional 12 mL/kg — the largest mortality reduction ever demonstrated in ARDS.
LPV prevents Ventilator-Induced Lung Injury (VILI) by limiting volutrauma (overdistension), barotrauma (excessive pressure), and atelectrauma (repeated alveolar collapse).
Why Predicted Body Weight — Not Actual Weight?
Lung size is determined by the patient's height and biological sex — not by adipose tissue.
An obese patient (actual weight 120 kg, height 170 cm, male) has a PBW of approximately 70 kg.
Using actual weight would set a tidal volume of 720 mL (12 mL/kg × 60 actual kg above PBW) — causing severe volutrauma.
Using PBW correctly targets 420 mL (6 mL/kg × 70 kg PBW).
Always use PBW for tidal volume calculations in mechanically ventilated patients — actual weight is used only for drug dosing, not ventilator management.
Driving Pressure — The Most Important Ventilator Variable
Amato et al. (NEJM 2015) analysed 3,562 ARDS patients across 9 trials and found that driving pressure (Pplat − PEEP) was the variable most strongly associated with survival — more predictive than tidal volume or plateau pressure alone.
Driving pressure represents the actual stress applied to the recruitable lung — it is a direct measure of lung strain.
A driving pressure >15 cmH₂O should prompt immediate ventilator adjustment: reduce tidal volume or increase PEEP (if compliance permits).
Static Compliance — Reading Lung Stiffness
Static compliance = Tidal Volume ÷ (Pplat − PEEP) = Vt ÷ Driving Pressure.
Normal compliance is 60–100 mL/cmH₂O.
In ARDS, compliance is typically 15–40 mL/cmH₂O — severely reduced due to diffuse alveolar damage and non-aerated lung.
A compliance that improves with increasing PEEP indicates alveolar recruitment — the PEEP level is beneficial.
A compliance that worsens with increasing PEEP indicates overdistension — the PEEP is too high for that patient.
Serial compliance monitoring guides PEEP optimization and is the best bedside tool for assessing PEEP response.
What is permissive hypercapnia in ARDS ventilation?
Permissive hypercapnia is the deliberate acceptance of an elevated PaCO₂ (and mild respiratory acidosis) in order to maintain low tidal volumes for lung protection. When tidal volumes are reduced from 12 to 6 mL/kg PBW, CO₂ elimination decreases and PaCO₂ rises. The ARDSNet protocol accepts this trade-off — lung protection takes priority over normalising CO₂. The typical acceptable range is pH ≥7.25 (minimum) with PaCO₂ up to 60–70 mmHg. Sodium bicarbonate infusion may be used if pH falls below 7.15. Permissive hypercapnia is contraindicated in: raised intracranial pressure (ICP), severe right heart failure (CO₂ causes pulmonary vasoconstriction), and metabolic acidosis.
When should I use the higher PEEP strategy vs lower PEEP?
The higher PEEP / lower FiO₂ strategy is generally preferred in patients with moderate-to-severe ARDS (P/F ratio <200 mmHg) where significant alveolar recruitment potential exists — indicated by improving compliance as PEEP increases. The lower PEEP / higher FiO₂ strategy may be appropriate for mild ARDS (P/F 200–300) or when high PEEP causes hemodynamic compromise (right ventricular failure), worsening compliance (overdistension), or is not tolerated. Monitor compliance and hemodynamics with every PEEP change — if compliance falls and cardiac output decreases, the PEEP is too high.
What does high PIP with normal Pplat indicate?
A large gap between Peak Inspiratory Pressure (PIP) and Plateau Pressure (Pplat) — typically PIP − Pplat >5 cmH₂O — indicates elevated airway resistance, not reduced lung compliance. Common causes: bronchospasm (asthma, COPD exacerbation), endotracheal tube (ETT) kinking or obstruction, secretions in ETT, biting the ETT, or ETT too small for patient. Normal Pplat with high PIP means the alveolar pressure is acceptable — the high PIP is a flow-resistance problem, not a stiffness problem. Reassess tube position, suction ETT, and consider bronchodilator therapy.
How do I calculate the recommended tidal volume for a specific patient?
Step 1: Calculate PBW from height and sex: Male PBW = 50 + 0.91 × (height cm − 152.4); Female PBW = 45.5 + 0.91 × (height cm − 152.4). Step 2: Multiply PBW by 6 mL/kg for the ARDSNet target. Step 3: Check that plateau pressure is ≤30 cmH₂O and driving pressure is ≤15 cmH₂O. If plateau is >30 or driving pressure >15, reduce Vt by 1 mL/kg steps (down to minimum 4 mL/kg) until targets are met. Example: 170 cm female: PBW = 45.5 + 0.91×(170−152.4) = 45.5 + 16.0 = 61.5 kg. Target Vt = 6 × 61.5 = 369 mL (acceptable range 246–492 mL at 4–8 mL/kg).