ARDS Basics
Understanding acute respiratory distress syndrome — from definition and diagnosis to lung-protective ventilation strategies and supportive care.
What Is ARDS?
Acute Respiratory Distress Syndrome (ARDS) is a life-threatening inflammatory condition of the lungs characterized by diffuse alveolar damage, severe hypoxemia, and decreased lung compliance. It represents the most severe form of acute lung injury and carries a significant mortality rate, particularly in severe cases.
ARDS is not a disease itself but a syndrome — a common final pathway for a wide variety of insults to the lung, both direct (pulmonary) and indirect (extrapulmonary). The resulting inflammatory cascade leads to increased vascular permeability, alveolar flooding with protein-rich fluid, surfactant dysfunction, and widespread atelectasis.
For respiratory therapists, ARDS represents one of the most challenging and consequential scenarios encountered in the ICU. The ventilator strategy chosen directly impacts patient outcomes, and the evidence base for lung-protective ventilation in ARDS is among the strongest in critical care medicine.
The Berlin Definition
The 2012 Berlin Definition replaced older ARDS definitions and requires all four of the following criteria:
Timing
Acute onset within 1 week of a known clinical insult or new/worsening respiratory symptoms
Chest Imaging
Bilateral opacities on CXR or CT not fully explained by effusions, collapse, or nodules
Pulmonary Edema Origin
Respiratory failure not fully explained by cardiac failure or fluid overload (requires objective assessment when no risk factor present)
Oxygenation (P/F Ratio)
Mild: 200 < PaO₂/FiO₂ ≤ 300 mmHg with PEEP ≥ 5 cmH₂O | Moderate: 100 < PaO₂/FiO₂ ≤ 200 | Severe: PaO₂/FiO₂ ≤ 100
The P/F ratio (PaO₂ ÷ FiO₂) is the primary tool for classifying severity. A P/F ratio of 150 at an FiO₂ of 0.8 indicates more severe impairment than a ratio of 150 at FiO₂ of 0.4.
Common Causes of ARDS
Direct (Pulmonary) Causes
- Pneumonia (bacterial, viral, fungal)
- Aspiration of gastric contents
- Near-drowning
- Inhalation injury (smoke, toxic gases)
- Pulmonary contusion
Indirect (Extrapulmonary) Causes
- Sepsis (most common indirect cause)
- Severe trauma and shock
- Major burns
- Pancreatitis
- Massive blood transfusion (TRALI)
- Drug overdose
Lung-Protective Ventilation
The ARDSNet trial published in 2000 established that lower tidal volumes significantly reduce mortality in ARDS. The key insight was that the ventilator itself can injure the lung through ventilator-induced lung injury (VILI) — including barotrauma (pressure injury), volutrauma (volume injury), atelectrauma (repeated opening and closing of alveoli), and biotrauma (inflammatory mediator release).
ARDSNet Lung-Protective Protocol Targets
Permissive hypercapnia — allowing PaCO₂ to rise above normal to achieve low tidal volumes — is an accepted trade-off in ARDS management. Most patients tolerate mild to moderate hypercapnia (PaCO₂ up to 50–60 mmHg) if the pH remains above 7.20.
PEEP in ARDS
PEEP prevents end-expiratory alveolar collapse, recruits atelectatic lung units, reduces the shunt fraction, and improves oxygenation. In ARDS, appropriate PEEP is essential for lung protection — too little allows cyclic de-recruitment, while too much causes overdistension and cardiovascular compromise.
PEEP-FiO₂ tables (ARDSNet Low PEEP and High PEEP tables) provide guidance for coordinating PEEP increases with FiO₂ to achieve oxygenation goals while minimizing oxygen toxicity. The optimal approach to PEEP titration in ARDS remains an active area of research.
In moderate-to-severe ARDS, higher PEEP strategies (10–20 cmH₂O) are often used, guided by plateau pressure limits and hemodynamic response. Esophageal pressure monitoring can help assess transpulmonary pressure and individualize PEEP selection.
Adjunct Therapies
Prone Positioning
In severe ARDS (P/F < 150), prone positioning for 16+ hours/day reduces mortality. Improves V/Q matching and recruits dorsal lung zones. Requires a coordinated team including RT for ventilator management during repositioning.
Conservative Fluid Strategy
Avoiding excessive fluid administration reduces pulmonary edema in ARDS. Daily fluid balance and diuresis are managed collaboratively between the ICU team and RT.
Neuromuscular Blockade
Early short-term NMBA (cisatracurium) was shown to improve outcomes in moderate-severe ARDS in some studies by improving synchrony and reducing VILI. Current evidence is mixed — institutional protocols guide use.
Recruitment Maneuvers
Brief application of high sustained airway pressure (40 cmH₂O × 40 seconds) to open atelectatic units. Benefit is temporary and must be followed by adequate PEEP to prevent de-recruitment. Should be used cautiously in hemodynamically unstable patients.
Frequently Asked Questions
What is driving pressure and why does it matter?
Driving pressure is plateau pressure minus PEEP. It represents the pressure applied to the functional lung tissue with each breath. A driving pressure > 15 cmH₂O is associated with increased mortality in ARDS. Reducing driving pressure by optimizing PEEP or reducing tidal volume may improve outcomes.
How is P/F ratio calculated?
P/F ratio = PaO₂ (mmHg) ÷ FiO₂ (as a decimal). Example: PaO₂ of 80 on FiO₂ of 0.60 = P/F ratio of 133, which falls in the moderate ARDS category.
What is the difference between ARDS and pneumonia?
Pneumonia is a specific infection that can cause ARDS, but ARDS itself is a syndrome defined by hypoxemia and bilateral lung infiltrates from any cause. Not all pneumonia causes ARDS, and ARDS has many causes beyond infection.
When is prone positioning indicated?
The 2013 PROSEVA trial showed a survival benefit for prone positioning in severe ARDS (P/F < 150) for at least 16 hours per day. It is most beneficial when initiated early (within 36–48 hours) and should be performed by experienced teams.
Summary
- ARDS is defined by the Berlin criteria: acute onset, bilateral infiltrates, non-cardiogenic edema, PaO₂/FiO₂ ≤ 300.
- Lung-protective ventilation (4–6 mL/kg IBW, plateau ≤ 30) is the foundation of ARDS management.
- PEEP maintains alveolar recruitment and is essential for oxygenation in ARDS.
- Prone positioning reduces mortality in severe ARDS (P/F < 150).
- Permissive hypercapnia is an accepted trade-off for achieving low tidal volume targets.
- ARDS management requires close collaboration between RT, nursing, and the ICU team.
Advanced References in RTB2
RTB2 offers faster bedside references and advanced tools inside the mobile app — including ARDSNet tables, PEEP titration references, and ventilator management guides.
Related articles