ARDS -Acute Respiratory Distress Syndrome
Diffuse inflammatory lung injury causing refractory hypoxemia. The cornerstone of management is lung-protective ventilation -low tidal volumes, prone positioning, and avoiding further barotrauma.
🔍 Overview
Definition
ARDS is acute, diffuse inflammatory lung injury causing increased alveolar-capillary permeability, pulmonary edema, and severe hypoxemia not explained by cardiac failure. Definition uses the Berlin Definition of ARDS (2012).
Memory trick: ARDS = "Stiff, flooded lungs" → SPP: Sepsis, Pneumonia, Protective ventilation
Berlin Diagnostic Criteria
| Criterion | Requirement |
|---|---|
| Timing | Within 1 week of clinical insult |
| Imaging | Bilateral opacities on CXR/CT (not explained by effusion/collapse) |
| Origin of edema | Not fully explained by heart failure or fluid overload |
| Oxygenation (P/F) | Used to grade severity (see below), with PEEP ≥ 5 cmH₂O |
ARDS Severity Classification
| Severity | PaO₂/FiO₂ (P/F Ratio) | Mortality |
|---|---|---|
| Mild | 200–300 mmHg | ~27% |
| Moderate | 100–200 mmHg | ~32% |
| Severe | < 100 mmHg | ~45% |
All measured with PEEP ≥ 5 cmH₂O. Most common patient profile: critically ill adult with sepsis or pneumonia, often requiring mechanical ventilation.
Most Common Causes of ARDS
| Cause | Approx % |
|---|---|
| Sepsis | ~40–50% |
| Pneumonia | ~30–40% |
| Aspiration | ~10–15% |
| Trauma / contusion | ~5–10% |
| Pancreatitis | ~3–5% |
| Massive transfusion (TRALI) | ~2–5% |
| Inhalational injury | < 2% |
Direct (Pulmonary) Causes
- Pneumonia (bacterial, viral, fungal)
- Aspiration of gastric contents
- Pulmonary contusion, inhalation injury
- Near-drowning
Indirect (Extrapulmonary) Causes
- Sepsis (most common overall cause)
- Pancreatitis
- Massive transfusion / TRALI
- Burns, trauma, DIC
- Drug overdose (heroin, aspirin, cocaine)
Stepwise Pathophysiology
ARDS occurs through three overlapping phases. The key physiologic problem is shunt hypoxemia -blood passes through non-ventilated alveoli, so oxygen therapy alone often fails.
1. Exudative Phase (First 7 Days)
Initial inflammatory injury.
| Step | Pathology |
|---|---|
| Lung insult | Infection, aspiration, trauma |
| Macrophage activation | Cytokine release (TNF-α, IL-1, IL-6) |
| Neutrophil recruitment | Endothelial damage |
| ↑ Capillary permeability | Protein-rich edema floods alveoli |
| Alveolar flooding | Impaired gas exchange, ↓ compliance, shunt physiology |
Results: ↓ compliance, severe hypoxemia, shunt physiology. Lungs become stiff, small (baby lung concept -only ~30% of lung is recruitable).
2. Proliferative Phase (7–21 Days)
Repair begins.
| Process | Effect |
|---|---|
| Type II pneumocyte proliferation | Surfactant restoration |
| Fibroblast activation | Collagen deposition |
| Alveolar repair | Partial recovery |
3. Fibrotic Phase (≥ 21 Days)
Occurs in severe/prolonged cases. Pulmonary fibrosis → stiff lungs → reduced compliance → prolonged ventilation. Not all patients progress to this phase.
Imaging Findings
Chest X-Ray
- Diffuse bilateral infiltrates
- "White lungs" in severe cases
CT Scan
- Dependent consolidation (gravity-dependent atelectasis)
- Ground glass opacities
- Better characterizes distribution -not required for diagnosis
Ventilator-Induced Lung Injury (VILI)
The ventilator itself can worsen ARDS. Preventing VILI is the rationale for lung-protective ventilation.
| Mechanism | Meaning |
|---|---|
| Volutrauma | Excessive tidal volume → alveolar overdistension |
| Barotrauma | High airway pressure → pneumothorax, pneumomediastinum |
| Atelectrauma | Repeated alveolar collapse and reopening → shear injury |
| Biotrauma | Mechanical injury → systemic cytokine release → MODS |
Presentation & Red Flags
- Acute onset dyspnea, tachypnea, worsening hypoxemia
- Bilateral crackles, diffuse infiltrates on CXR
- Refractory hypoxemia -O₂ sats not improving with supplemental O₂
- Decreased lung compliance (stiff lungs on vent -high plateau pressures)
- P/F < 150 despite FiO₂ 1.0 → severe ARDS, consider prone positioning immediately
- pH < 7.15 despite optimal vent → consider ECMO consultation
Oxygenation Targets
ARDS strategy allows permissive hypoxemia -deliberately accepting lower oxygen levels to avoid toxic ventilator settings.
| Parameter | Target |
|---|---|
| SpO₂ | 88–95% |
| PaO₂ | 55–80 mmHg |
Permissive Hypercapnia
Low tidal volume may cause CO₂ retention. This is acceptable as long as pH > 7.20. Contraindicated in raised ICP and sickle cell crisis.
Clinical Significance
- ARDS mortality still 30–45%
- Most common cause is sepsis
- Low tidal volume ventilation saves lives ARDSNet, 2000
- Prone positioning improves survival PROSEVA, 2013
- Preventing ventilator-induced lung injury is critical
🧪 Workup & Diagnosis
Diagnostic Workup
| Test | Purpose |
|---|---|
| ABG | Calculate P/F ratio (PaO₂ ÷ FiO₂). P/F < 300 = ARDS by Berlin criteria. Most important diagnostic step. |
| CXR | Bilateral opacities not explained by effusion or collapse (Berlin criterion) |
| Echo (bedside) | Rule out cardiogenic pulmonary edema (PCWP > 18 argues against ARDS). Fastest way to exclude cardiac cause. |
| BNP / NT-proBNP | Further differentiates cardiogenic vs non-cardiogenic edema |
| CBC, CMP, coags, lactate, procalcitonin | Assess severity, organ dysfunction, identify sepsis |
| Blood cultures | Identify infectious cause (sepsis = most common etiology) |
| CT chest | Dependent consolidation, ground glass opacities. Rules out PE/effusion. Not required for diagnosis. |
| BAL / sputum cultures | Identify infectious precipitant; obtain after securing airway |
ARDS vs Cardiogenic Pulmonary Edema
| Feature | ARDS | Cardiogenic |
|---|---|---|
| PCWP | Normal (≤ 18 mmHg) | High (> 18 mmHg) |
| BNP | Normal or mildly elevated | High |
| Cause | Lung injury (sepsis, pneumonia, aspiration) | Heart failure |
| Edema fluid | Protein-rich exudate | Transudate |
| CXR pattern | Diffuse bilateral, peripheral | Perihilar "bat wing", Kerley B lines |
| Response to diuresis | Minimal improvement | Significant improvement |
| Heart size | Normal | Often enlarged |
Key differentiator: ARDS = normal filling pressures, no response to diuretics, diffuse bilateral disease. Cardiogenic = elevated filling pressures, responds to diuresis, often perihilar pattern. Echo is the fastest way to differentiate at the bedside.
🚨 Management
Lung-Protective Ventilation ARDSNet, 2000
This is the only intervention proven to reduce mortality in ARDS. ARDSNet, 2000 showed a 9% absolute mortality reduction with 6 mL/kg vs 12 mL/kg PBW. Low tidal volumes save lives. Do not let anyone override this for "patient comfort" without a very good reason.
Tidal Volume
6 mL/kg of Predicted Body Weight (PBW) -NOT actual body weight. Start at 6–8 mL/kg, titrate down to 6 mL/kg. SSC, 2021: strong recommendation, high quality evidence.
PBW male = 50 + 2.3 × (height in inches − 60)
PBW female = 45.5 + 2.3 × (height in inches − 60)
If Pplat remains > 30 after reducing to 6 mL/kg, reduce further to as low as 4 mL/kg PBW. Increase RR to max 35 to maintain minute ventilation.
PBW male = 50 + 2.3 × (height in inches − 60)
PBW female = 45.5 + 2.3 × (height in inches − 60)
If Pplat remains > 30 after reducing to 6 mL/kg, reduce further to as low as 4 mL/kg PBW. Increase RR to max 35 to maintain minute ventilation.
Plateau Pressure
Keep Pplat ≤ 30 cmH₂O SSC, 2021 -strong recommendation, moderate quality evidence. If above, reduce TV further (down to 4 mL/kg).
Driving pressure (Pplat − PEEP) < 12–15 cmH₂O may be advantageous in patients without spontaneous breathing LUNG SAFE, 2016. Higher plateau pressures correlated with mortality; the relationship was strongest above 29 cmH₂O.
Driving pressure (Pplat − PEEP) < 12–15 cmH₂O may be advantageous in patients without spontaneous breathing LUNG SAFE, 2016. Higher plateau pressures correlated with mortality; the relationship was strongest above 29 cmH₂O.
PEEP Strategy
SSC, 2021: suggests higher PEEP over lower PEEP in moderate-severe ARDS (weak recommendation, moderate evidence). Patient-level meta-analysis PEEP Meta-Analysis, 2010 found mortality benefit with higher PEEP only in P/F ≤ 200.
ARDSNet Lower PEEP / FiO₂ Table:
ARDSNet Higher PEEP / FiO₂ Table (moderate-severe):
Target: SpO₂ 88–95% or PaO₂ 55–80 mmHg.
ARDSNet Lower PEEP / FiO₂ Table:
| 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 |
| FiO₂ | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 |
|---|---|---|---|---|---|---|---|---|
| PEEP | 12 | 14 | 16 | 18 | 18 | 20 | 22 | 22–24 |
Permissive Hypercapnia
Acceptable to allow PaCO₂ to rise (up to 50–60 mmHg) to achieve lung-protective goals. pH > 7.20 is generally tolerated. Contraindicated in raised ICP, sickle cell crisis. No single mode (VC vs PC) has demonstrated superiority as long as lung-protective principles are respected.
Prone Positioning (P/F < 150)
Prone ≥ 16 hours/day PROSEVA, 2013 -reduced 28-day mortality from 32.8% to 16% in severe ARDS. SSC, 2021: strong recommendation, moderate quality evidence for > 12 hours daily.
Improves V/Q matching, recruits dependent lung, reduces compression by mediastinum. Meta-analysis confirmed benefit when applied within 36 hours of intubation for > 12 hrs/day in P/F < 200 Munshi, 2017. Associated with increased pressure sores (RR 1.22) but no increase in accidental extubation.
Improves V/Q matching, recruits dependent lung, reduces compression by mediastinum. Meta-analysis confirmed benefit when applied within 36 hours of intubation for > 12 hrs/day in P/F < 200 Munshi, 2017. Associated with increased pressure sores (RR 1.22) but no increase in accidental extubation.
🔄 Updated Practice: Old teaching: prone positioning is a rescue maneuver for refractory hypoxemia. Current practice: prone positioning is now standard of care for moderate-severe ARDS (P/F <150). PROSEVA (2013) showed 16 hours/day of proning reduced 28-day mortality from 33% to 16%, one of the largest mortality benefits ever seen in ARDS. Initiate proning early (within 12-24h), not as a last resort.
Conservative Fluid Strategy
After initial resuscitation, target even to negative fluid balance. FACTT, 2006: conservative fluid strategy reduced vent days by ~2.5 days without worsening renal function or shock. Use CVP or PAOP-guided diuresis. Avoid fluid overload -it worsens pulmonary edema and prolongs mechanical ventilation.
Neuromuscular Blockade
ACURASYS, 2010: 48h cisatracurium infusion in P/F < 150 showed 90-day mortality benefit vs deep sedation alone (RR 0.71). ROSE, 2019: no benefit vs light sedation with PRN boluses (RR 0.99).
SSC, 2021: suggests intermittent NMBA boluses over continuous infusion (weak recommendation, moderate evidence). Overall, NMBAs reduce barotrauma (RR 0.55). ATS, 2024: conditional recommendation for NMBAs in early (<48h) severe ARDS (P/F ≤ 100). Cisatracurium preferred, may have anti-inflammatory pleiotropic effects. Typical dosing: 15 mg bolus then 37.5 mg/hr × 48h.
If NMBAs used, ensure adequate sedation and analgesia.
SSC, 2021: suggests intermittent NMBA boluses over continuous infusion (weak recommendation, moderate evidence). Overall, NMBAs reduce barotrauma (RR 0.55). ATS, 2024: conditional recommendation for NMBAs in early (<48h) severe ARDS (P/F ≤ 100). Cisatracurium preferred, may have anti-inflammatory pleiotropic effects. Typical dosing: 15 mg bolus then 37.5 mg/hr × 48h.
If NMBAs used, ensure adequate sedation and analgesia.
Recruitment Maneuvers
SSC, 2021: suggests traditional recruitment maneuvers (sustained CPAP 30–40 cmH₂O × 30–40 sec) in moderate-severe ARDS -improved 28-day mortality (RR 0.79).
AVOID incremental PEEP titration recruitment -ART, 2017: increased 28-day mortality (RR 1.12). SSC, 2021: strong recommendation against.
🔄 Updated Practice: Old teaching: set PEEP as high as possible to recruit alveoli in ARDS. Current practice: driving pressure (plateau pressure minus PEEP) is the ventilatory variable most strongly associated with survival (Amato, NEJM 2015). Target driving pressure <15 cmH2O. Higher PEEP is beneficial in moderate-severe ARDS (P/F <200) but not in mild ARDS (PEEP Meta-Analysis, 2010). The ART trial (2017) showed aggressive recruitment maneuvers increased mortality.
ECMO Referral
Consider VV-ECMO if P/F < 80 despite optimal settings, pH < 7.15, or plateau pressure > 35 cmH₂O. EOLIA, 2018: supports VV-ECMO as rescue in severe ARDS at experienced centers. SSC, 2021: weak recommendation, low quality evidence. ATS, 2024: conditional recommendation for VV-ECMO in selected severe ARDS (pooled RR 0.76; 95% CI 0.60–0.95; moderate certainty). Refer early -do not wait until irreversible organ failure.
Oxygenation Strategy Before Intubation
High-Flow Nasal Cannula (HFNC)
SSC, 2021: suggests HFNC over NIV for sepsis-induced hypoxemic respiratory failure (weak recommendation, low quality evidence). FLORALI, 2015: HFNC improved 90-day survival vs NIV (OR 0.42) and reduced intubation in severe hypoxemia (P/F ≤ 200): 35% vs 58%. HFNC provides flows up to 60 L/min, FiO₂ up to 100%, with washout of nasopharyngeal dead space and modest PEEP effect (~3–5 cmH₂O).
NIV in ARDS: SSC, 2021 issued no recommendation for NIV vs invasive ventilation in sepsis-induced hypoxemic respiratory failure. LUNG SAFE, 2016: NIV used in 15% of ARDS with higher failure rates in severe disease. NIV failure is an independent mortality risk factor. If trialing NIV: monitor closely for Vt, work of breathing, and need for intubation.
Corticosteroids in ARDS
Steroids have an evolving role in ARDS -evidence now supports early dexamethasone in moderate-to-severe ARDS.
Early dexamethasone in moderate-to-severe ARDS (P/F ≤ 200): DEXA-ARDS, 2020: dexamethasone 20 mg IV daily × 5 days → 10 mg daily × 5 days. Reduced 60-day mortality (21% vs 36%, p=0.0047) and increased ventilator-free days. Start within 30h of ARDS diagnosis. This applies to non-COVID ARDS. Supported by CoDEX, 2020, which also showed dexamethasone increased ventilator-free days in moderate-severe ARDS.
Unresolving/fibroproliferative ARDS (after day 7): ARDS Steroid Trial, 2007: methylprednisolone 1 mg/kg/day with slow taper improved LIS score and reduced mechanical ventilation days. Do NOT start after day 14 -ARDSNet LaSRS, 2006: late initiation associated with increased 60- and 180-day mortality.
COVID-19 ARDS: RECOVERY, 2020: dexamethasone 6 mg/day × 10 days reduced 28-day mortality in patients on mechanical ventilation (29.3% vs 41.4%). CAPE COVID, 2020 found that low-dose hydrocortisone did not significantly reduce treatment failure in COVID pneumonia. REMAP-CAP, 2021 showed IL-6 inhibitors (tocilizumab, sarilumab) improved organ support-free days and survival in severe COVID when combined with corticosteroids.
Early dexamethasone in moderate-to-severe ARDS (P/F ≤ 200): DEXA-ARDS, 2020: dexamethasone 20 mg IV daily × 5 days → 10 mg daily × 5 days. Reduced 60-day mortality (21% vs 36%, p=0.0047) and increased ventilator-free days. Start within 30h of ARDS diagnosis. This applies to non-COVID ARDS. Supported by CoDEX, 2020, which also showed dexamethasone increased ventilator-free days in moderate-severe ARDS.
Unresolving/fibroproliferative ARDS (after day 7): ARDS Steroid Trial, 2007: methylprednisolone 1 mg/kg/day with slow taper improved LIS score and reduced mechanical ventilation days. Do NOT start after day 14 -ARDSNet LaSRS, 2006: late initiation associated with increased 60- and 180-day mortality.
COVID-19 ARDS: RECOVERY, 2020: dexamethasone 6 mg/day × 10 days reduced 28-day mortality in patients on mechanical ventilation (29.3% vs 41.4%). CAPE COVID, 2020 found that low-dose hydrocortisone did not significantly reduce treatment failure in COVID pneumonia. REMAP-CAP, 2021 showed IL-6 inhibitors (tocilizumab, sarilumab) improved organ support-free days and survival in severe COVID when combined with corticosteroids.
🆕 ATS 2024 Update: The ATS now formally recommends corticosteroids for ARDS (conditional recommendation, moderate certainty). Pooled analysis: RR 0.84 (95% CI 0.73–0.96) for mortality reduction. Optimal regimen remains unknown, DEXA-ARDS protocol (above) is the most studied. ATS ARDS Guideline, 2024
What Does NOT Work in ARDS
| Therapy | Outcome |
|---|---|
| Late steroids (> day 14) | ↑ mortality when started after day 14 ARDSNet LaSRS, 2006. Early dexamethasone (within 30h) IS beneficial -see above. |
| Inhaled nitric oxide (iNO) | Transient oxygenation improvement only. No mortality benefit. Increases renal dysfunction. |
| Surfactant | Effective in neonatal RDS. Ineffective in adult ARDS. |
| High-frequency oscillation (HFOV) | Increased mortality. Trial stopped early. OSCILLATE, 2013 |
| Incremental PEEP recruitment | Increased 28-day mortality (RR 1.12). ART, 2017 ATS, 2024: strong recommendation against prolonged recruitment maneuvers. Higher PEEP WITHOUT recruitment maneuvers is conditionally recommended for moderate-severe ARDS. |
| IV beta-agonists (albuterol) | Increased mortality. BALTI-2, 2012 |
💊 Medications & Doses
Sedation & Analgesia
Analgesia-first, target light sedation (RASS 0 to −2). PADIS, 2018 guidelines recommend protocolized sedation with daily awakening trials. PADIS Update, 2025: dexmedetomidine now suggested over propofol when light sedation and/or delirium reduction are priorities. Deep sedation (RASS −4/−5) only when NMBAs required.
| Drug | Dose | Indication | Notes |
|---|---|---|---|
| Fentanyl (Sublimaze) | 25–200 mcg/hr IV | Analgesia (first-line) | Analgesia-first approach. Less histamine release and hypotension than morphine. Accumulates with hepatic/renal failure. |
| Propofol (Diprivan) | 5–50 mcg/kg/min IV | Sedation (first-line) | Rapid on/off -ideal for daily awakening trials. Monitor triglycerides q48h. Propofol infusion syndrome risk if > 80 mcg/kg/min > 48h. |
| Dexmedetomidine | 0.2–1.5 mcg/kg/hr IV | Light sedation (no NMBAs) | Preserves respiratory drive. Less delirium than benzos. May cause bradycardia/hypotension. Not appropriate for deep sedation. |
| Midazolam (Versed) | 0.02–0.1 mg/kg/hr IV | Sedation (2nd-line) | Accumulates in renal/hepatic failure → prolonged wake-up. Associated with more delirium. Avoid long-term use. |
Neuromuscular Blockade
| Drug | Dose | Indication | Notes |
|---|---|---|---|
| Cisatracurium (bolus) | 0.1–0.2 mg/kg IV PRN | Intermittent use in mod-severe ARDS SSC, 2021 | Preferred strategy per SSC 2021. Monitor ventilator synchrony. Does not require train-of-four for intermittent dosing. |
| Cisatracurium (infusion) | 0.1–0.2 mg/kg/hr IV × 48h | Severe ARDS with refractory dyssynchrony | ACURASYS, 2010: mortality benefit vs deep sedation. ROSE, 2019: no benefit vs light sedation + boluses. Monitor with train-of-four (target 1–2/4). |
Other ARDS Medications
| Drug | Dose | Indication | Notes |
|---|---|---|---|
| Methylprednisolone (Solu-Medrol) | 1 mg/kg/day IV → taper over 2 wks | Unresolving ARDS (fibroproliferative phase, day 7–14) | ARDS Steroid Trial, 2007. Do NOT start after day 14 ARDSNet LaSRS, 2006: late steroids = ↑ mortality. |
| Dexamethasone (early) | 20 mg IV daily × 5d → 10 mg × 5d | Moderate-severe ARDS (P/F ≤ 200), start within 30h | DEXA-ARDS, 2020: reduced 60-day mortality (21% vs 36%) and ↑ ventilator-free days in non-COVID ARDS. |
| Dexamethasone (COVID) | 6 mg IV/PO daily × 10 days | COVID-19 ARDS | RECOVERY, 2020: NNT 8 in ventilated COVID patients. |
| Furosemide (Lasix) | Titrated to even/negative fluid balance | Conservative fluid strategy | FACTT, 2006. Start after resuscitation phase. Monitor Cr and K⁺ closely. CVP or PAOP guidance if available. |
| Sodium Bicarbonate | 1–2 mEq/kg IV slow push or infusion | pH < 7.15 with permissive hypercapnia | Use sparingly. Does not improve outcomes. Temporizing for severe acidemia only. |
📋 On Rounds
On Rounds
Pimp Questions
What tidal volume should you use in ARDS and why?
6 mL/kg of ideal body weight. Higher tidal volumes cause volutrauma (overdistension) and worsen alveolar injury. ARDSnet 2000: 22% relative reduction in mortality with 6 vs 12 mL/kg PBW -the single most impactful ARDS trial ever done.
When do you prone a patient and what is the evidence?
P/F ratio < 150 despite optimal ventilator settings. PROSEVA 2013 prone ≥ 16 hrs/day reduced 28-day mortality from 32.8% to 16% -one of the most dramatic results in critical care medicine.
What is permissive hypercapnia and why do we accept it?
Allowing PaCO₂ to rise above normal (up to ~60 mmHg) as a trade-off for achieving lung-protective tidal volumes. Increasing minute ventilation to normalize CO₂ would require higher tidal volumes that cause further lung injury. pH > 7.20 is generally safe. Contraindicated in raised ICP and sickle cell crisis.
What is driving pressure, and why does it matter more than plateau pressure alone?
Driving pressure = Pplat − PEEP. It reflects the cyclic strain on functional lung tissue (the "baby lung"). A patient-level meta-analysis Driving Pressure Meta-Analysis, 2015 found driving pressure < 15 cmH₂O was the strongest independent predictor of survival in ARDS -better than Vt or Pplat alone. It accounts for both the tidal volume and the functional lung size.
ACURASYS vs ROSE -how do you reconcile conflicting NMBA data?
ACURASYS, 2010 compared 48h cisatracurium infusion vs deep sedation without NMBAs → mortality benefit. ROSE, 2019 compared infusion vs light sedation with PRN boluses → no benefit. The difference was the control arm: it's the deep sedation that was harmful, not the NMBAs that were magical. The lesson: avoid deep sedation; use light sedation with PRN NMBA boluses as first-line SSC, 2021.
When should you start thinking about ECMO?
P/F < 80 despite optimal vent settings (PEEP optimized, Pplat ≤ 30, TV 6 mL/kg PBW), prone positioning, and trial of NMBAs. Also consider for pH < 7.15 from respiratory acidosis or Pplat > 35 despite TV reduction to 4 mL/kg. EOLIA, 2018 showed benefit when referred early to experienced ECMO centers. Key: refer early -don't wait for irreversible multi-organ failure.
Why should you avoid fluid overload in ARDS, and what trial supports conservative fluids?
FACTT, 2006 randomized 1,000 ARDS patients to conservative vs liberal fluid management after initial resuscitation. Conservative strategy: ~2.5 fewer ventilator days, improved oxygenation index, no increase in shock or renal failure. Mechanism: reduced hydrostatic pressure across damaged pulmonary capillaries → less alveolar edema. Target even or negative daily fluid balance once resuscitation is complete.
What is the definition of ARDS?
Acute diffuse inflammatory lung injury → increased alveolar-capillary permeability → pulmonary edema → severe hypoxemia. Diagnosed by Berlin criteria: acute onset ≤ 1 week, bilateral CXR opacities, not cardiogenic, P/F ratio determines severity.
What are the Berlin severity classifications?
Mild: P/F 200–300 (~27% mortality). Moderate: P/F 100–200 (~32% mortality). Severe: P/F < 100 (~45% mortality). All with PEEP ≥ 5 cmH₂O.
What is the most common cause of ARDS?
Sepsis (~40–50%), followed by pneumonia (~30–40%). Sepsis is the most common indirect cause; pneumonia is the most common direct (pulmonary) cause.
Why does hypoxemia occur in ARDS?
Shunt physiology -blood passes through non-ventilated, fluid-filled alveoli without participating in gas exchange. This is why supplemental O₂ alone often fails to correct the hypoxemia, and why PEEP (to recruit collapsed alveoli) is critical.
What plateau pressure should you target?
Pplat ≤ 30 cmH₂O. Additionally, driving pressure (Pplat − PEEP) < 15 cmH₂O is the strongest independent predictor of survival Driving Pressure Meta-Analysis, 2015.
How do you differentiate ARDS from cardiogenic pulmonary edema?
ARDS: normal PCWP (≤ 18), normal BNP, protein-rich exudate, diffuse bilateral CXR, no response to diuretics. Cardiogenic: high PCWP, high BNP, transudative, perihilar "bat wing" pattern, responds to diuresis. Bedside echo is the fastest differentiator.
Clinical Examples
📋 Case 1, Pneumonia-Induced ARDS
Patient: 55M, PMH DM2. Severe CAP (Strep pneumoniae). Intubated for hypoxemic respiratory failure.
Key findings: P/F 120 (moderate ARDS). Bilateral infiltrates. BNP 45. TV 420 mL (6 mL/kg PBW), PEEP 12, FiO2 0.7. Pplat 28, driving pressure 16.
Management:
- Lung-protective ventilation: TV 6 mL/kg PBW, Pplat ≤ 30 ARDSNet, 2000
- Driving pressure 16, reduce below 15 Amato et al., 2015
- Conservative fluid strategy FACTT, 2006
- P/F < 150, prone ≥ 16h/day PROSEVA, 2013
- Light sedation (RASS -1 to -2), daily SAT/SBT assessment
Teaching point: Driving pressure (Pplat - PEEP) < 15 is the strongest predictor of survival in ARDS.
📋 Case 2, Severe ARDS Requiring Proning
Patient: 40F, previously healthy. Influenza A pneumonia. P/F 72 (severe) despite FiO2 1.0, PEEP 16.
Key findings: TV 330 mL (6 mL/kg PBW), Pplat 30, driving pressure 14. SpO2 85% supine.
Management:
- Prone immediately ≥ 16h/day PROSEVA, 2013
- Cisatracurium PRN boluses for dyssynchrony, not routine infusion ROSE, 2019
- Light sedation (RASS -2), avoid deep sedation
- If P/F remains < 80 after proning, consult ECMO center EOLIA, 2018
- Oseltamivir 75 mg BID for influenza
Teaching point: PROSEVA reduced mortality from 32.8% to 16%. ACURASYS vs ROSE: deep sedation was harmful, not paralytics magical. Use light sedation + PRN NMBA boluses.
📋 Case 3, ARDS vs Cardiogenic Pulmonary Edema
Patient: 68M, PMH HFrEF (EF 25%). Bilateral infiltrates, intubated. P/F 180. Is this ARDS or decompensated HF?
Key findings: BNP 2800. Echo: EF 20%, dilated LV. Responds to IV furosemide. Cardiogenic pulmonary edema, NOT ARDS.
Management:
- IV furosemide drip, target negative fluid balance
- Do NOT apply low-TV strategy reflexively, volume overload, not ARDS
- Optimize HF management: afterload reduction, cardiology consult
- Daily SBT when oxygenation improves
Teaching point: Berlin criteria require bilateral opacities NOT fully explained by cardiac failure. ARDS = normal BNP/PCWP, no diuretic response. Cardiogenic = elevated BNP/PCWP, responds to diuresis.
📋 Sample Presentation
"Mr. Torres is a 52-year-old with no significant PMH who developed ARDS in the setting of severe CAP with Streptococcus pneumoniae. He was intubated on hospital day 1. He is on lung-protective ventilation: TV 6 mL/kg PBW at 390 mL, PEEP 12, FiO₂ 0.7, with a P/F ratio of 148 this morning -consistent with moderate ARDS. He was proned for 16 hours overnight with improvement in P/F to 180. His plateau pressure is 26. He is on propofol and fentanyl drips, RASS −2. We are on day 2 of cisatracurium. Fluid balance is negative 400 mL over the last 24 hours. He remains hemodynamically stable on low-dose norepinephrine. Plan is to continue prone cycling and target daily fluid balance of at least even."
Daily Ventilator Assessment
- P/F ratio -calculate every morning from ABG. Trend drives all major decisions (prone, ECMO).
- Plateau pressure -perform inspiratory hold. Target ≤ 30 cmH₂O.
- Driving pressure (Pplat − PEEP) -target < 15 cmH₂O. Strongest independent predictor of mortality Driving Pressure Meta-Analysis, 2015.
- Tidal volume -confirm 6 mL/kg PBW. Recalculate if height was incorrectly entered.
- Static compliance = Vt / (Pplat − PEEP). Normal ~50–80 mL/cmH₂O. In ARDS typically 20–40.
- SpO₂ target -88–95%. Avoid hyperoxemia ICU-ROX, 2020: conservative O₂ therapy may benefit; liberal O₂ offers no advantage.
- Fluid balance -calculate 24h net. Target even or negative after resuscitation FACTT, 2006.
Daily Checklist (ICU Rounds)
- ABG with P/F ratio
- Vent settings review: TV (mL/kg PBW), RR, FiO₂, PEEP, Pplat, driving pressure
- Sedation assessment: RASS score, daily awakening trial eligibility
- Spontaneous breathing trial (SBT) eligibility if improving
- Prone positioning: hours completed, plan for next cycle if P/F < 150
- Fluid balance: I/O, weight, diuretic needs
- DVT prophylaxis, stress ulcer prophylaxis (if high-risk SUP-ICU, 2018), HOB ≥ 30°
- Nutrition: enteral preferred, start within 48h SSC, 2021
- Source control: is the ARDS precipitant being treated?
When to Escalate
- Worsening P/F despite optimal PEEP + prone → ECMO evaluation
- P/F < 80 on FiO₂ 1.0 → emergent ECMO referral
- pH < 7.15 despite max RR and permissive hypercapnia → ECMO or iNO as bridge
- Pplat > 30 despite TV 4 mL/kg → consider ECMO, rule out tension PTX or abdominal compartment syndrome
- New hemodynamic instability → rule out tension PTX, PE, auto-PEEP, RV failure from high PEEP
- Barotrauma (subcutaneous emphysema, pneumomediastinum, PTX) → reduce PEEP/TV, chest tube if indicated
Weaning Readiness Criteria
Assess daily once P/F > 200 and FiO₂ ≤ 0.4 with PEEP ≤ 8:
- Precipitant resolving or resolved
- Hemodynamically stable (off or low-dose vasopressors)
- Adequate cough and gag reflex
- RASS −1 to +1 (awake, cooperative)
- No planned surgery or procedures requiring deep sedation
- Rapid shallow breathing index (RSBI) < 105 breaths/min/L
SBT Protocol: 30–120 min on PS 5–8 / PEEP 5 or T-piece. Pass = RR < 35, SpO₂ > 90%, HR < 140, no accessory muscle use, no distress. → Extubate. If fails → resume full support, retry in 24h. WIND, 2018: daily SBTs reduce time to extubation.
Complications to Watch
- Ventilator-associated pneumonia (VAP) -new fever, purulent sputum, new infiltrate on CXR after 48h of MV. Maintain HOB ≥ 30°, oral care, subglottic suctioning.
- Barotrauma -pneumothorax, pneumomediastinum, subcutaneous emphysema. Risk increases with Pplat > 30.
- ICU-acquired weakness -from NMBAs, steroids, immobility. Early mobilization when safe Early Mobilization Trial, 2009.
- Delirium -screen daily with CAM-ICU. Minimize benzos, sleep hygiene, early mobilization.
- RV failure -from high PEEP and pulmonary vasoconstriction. Monitor with echo. Reduce PEEP if RV dilation/failure suspected.
- Renal failure -common in ARDS. May be from underlying sepsis, contrast, or positive pressure effects on renal perfusion.
⚡ Summary
Summary
Berlin Criteria Define It
Acute onset within 1 week of a known insult, bilateral opacities not fully explained by effusion or collapse, respiratory failure not fully explained by cardiac failure or fluid overload, and impaired oxygenation on at least 5 cmH₂O of PEEP. Severity by P/F ratio: mild 200 to 300, moderate 100 to 200, severe below 100.
Low Tidal Volume Is the Intervention That Saves Lives
6 mL/kg of predicted body weight, calculated from height and sex, not actual weight, because lung size tracks height. ARMA showed a mortality reduction from 40% to 31% with this alone. Keep plateau pressure below 30 cmH₂O and driving pressure below 15, since driving pressure is the variable most closely associated with survival.
Accept Permissive Hypercapnia
Tolerate a rising CO₂ and a pH down to about 7.20 rather than raising the tidal volume. The ventilator injures the lung; the CO₂ largely does not. Raise the respiratory rate first, and use bicarbonate only if the acidemia is causing hemodynamic compromise.
Prone Early in Severe Disease
P/F below 150 for at least 16 h per session. PROSEVA showed a large mortality reduction, from recruitment of dependent lung and more uniform distribution of ventilation. Prone early rather than as a last resort, and treat pressure areas, tube security and line management as the main risks of the procedure.
Use Paralysis and Steroids Selectively
Neuromuscular blockade for severe early ARDS with ventilator dyssynchrony: ACURASYS suggested benefit but ROSE, with lighter sedation in the control arm, did not, so it is not routine. Steroids have evidence in moderate-to-severe ARDS (DEXA-ARDS) and in COVID-related disease, but avoid starting them late in established fibroproliferative ARDS, where harm was seen.
Keep Them Dry Once They Are Stable
A conservative fluid strategy after shock has resolved improves oxygenation and shortens ventilator days (FACTT), without increasing renal failure. Resuscitate the shocked patient first, then dry them out: the sequence matters, and continuing liberal fluids into the stable phase is the commoner error.
Separate It From Cardiogenic Edema
ARDS: no cardiomegaly, peripheral and patchy opacities, normal or low BNP, no response to diuresis alone. Cardiogenic edema: cardiomegaly, central bat-wing distribution, effusions, raised BNP, responds to diuresis. Echocardiography settles it and should be done early rather than after a failed diuretic trial.
Escalate in a Defined Order
Optimize the ventilator, then prone, then consider inhaled pulmonary vasodilators as a temporizing measure, then refer for VV-ECMO. Refer early to an ECMO center (EOLIA), since transferring a patient after multiorgan failure has set in is far less successful. Throughout, treat the underlying cause: ARDS is a syndrome, and the sepsis, pancreatitis or aspiration driving it determines the outcome.
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ARDS
Low tidal volumes save lives. Prone early. Keep the lungs small and protected. Everything else is supportive.
Berlin Criteria -All 4 Required
- Acute onset ≤ 1 week of clinical insult
- Bilateral opacities on CXR/CT
- Not explained by cardiac failure/fluid overload
- P/F < 300 with PEEP ≥ 5
Severity
MildP/F 200–300 · ~27% mortality
ModerateP/F 100–200 · ~32% mortality
SevereP/F < 100 · ~45% mortality
Lung-Protective Ventilation ARDSnet 2000
1
Tidal volume: 6 mL/kg PBW (NOT actual weight). Reduce to 4 mL/kg if Pplat > 30.
2
Plateau pressure: ≤ 30 cmH₂O. Driving pressure: ≤ 15 cmH₂O (Pplat − PEEP).
3
PEEP: Per ARDSnet table. Higher PEEP for more severe disease.
4
Prone position if P/F < 150 → ≥ 16h/day PROSEVA 2013 -mortality halved
5
Conservative fluids after resuscitation -target even/negative balance FACTT 2006
6
ECMO if P/F < 80 or pH < 7.15 despite optimal settings EOLIA 2018
Key Drugs
Propofol5–50 mcg/kg/min
Fentanyl25–200 mcg/hr
Cisatracurium bolus0.1–0.2 mg/kg PRN
FurosemideEven/negative balance
Dexa (COVID only)6 mg/d × 10d
Does NOT Work
- HFOV OSCILLATE, 2013
- Inhaled nitric oxide
- Routine steroids (non-COVID)
- Surfactant (adults)
- Incremental PEEP recruit ART, 2017
- IV albuterol BALTI-2, 2012
Pitfalls
- High TV "for comfort"
- Delaying prone positioning
- Fluid overload post-resus
- Not calculating PBW (predicted body weight) from height
- Deep sedation without indication
- Late ECMO referral
Related Topics
Acute Decompensated Heart FailureAcute Exacerbation of COPD (AECOPD)Acute Ischemic StrokeAcute Mesenteric IschemiaAcute PancreatitisAirway & Ventilator Management