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Pathophysiology
Clinical meaning
Acute Respiratory Distress Syndrome (ARDS) is a syndrome of non-cardiogenic pulmonary edema and refractory hypoxemia caused by diffuse alveolar damage. The Berlin definition classifies ARDS by severity based on the PaO2/FiO2 ratio: mild (200 to 300 mmHg), moderate (100 to 200 mmHg), and severe (less than 100 mmHg), all with bilateral opacities on chest imaging not fully explained by cardiac failure or fluid overload. While the Berlin definition treats ARDS as a single entity, accumulating evidence demonstrates that ARDS comprises distinct phenotypes with different pathobiology, treatment responses, and outcomes, making phenotype recognition increasingly important for nursing care.
The most clinically actionable phenotypic distinction is between focal and diffuse ARDS, identified by CT morphology. Focal ARDS (approximately 30% of cases) is characterized by heterogeneous lung involvement: areas of consolidation and atelectasis concentrated in the dependent (posterior in supine patients) lung regions, with relatively preserved aeration in non-dependent (anterior) regions. Diffuse ARDS (approximately 70% of cases) shows homogeneous bilateral involvement with widespread ground-glass opacities and consolidation throughout all lung regions. This distinction matters because the mechanical behavior of the lungs differs fundamentally between phenotypes, directly affecting how ventilator management should be optimized.
The pathophysiology of diffuse ARDS involves widespread injury to the alveolar-capillary membrane across both lungs. The alveolar epithelial barrier (composed of flat type I pneumocytes covering 95% of the alveolar surface and cuboidal type II pneumocytes that produce surfactant) is damaged by direct insults (pneumonia, aspiration, inhalation injury) or indirect insults (sepsis, pancreatitis, transfusion-related acute lung injury). Neutrophils are recruited to the alveolar space where they release reactive oxygen species, proteases, and neutrophil extracellular traps (NETs) that further damage the epithelium. The resultant increased permeability allows protein-rich edema fluid to flood the alveoli, inactivating surfactant and causing alveolar collapse. Because the injury is widespread, most lung units are recruited or recruitable, and the lung behaves like a small, stiff (low compliance) but relatively homogeneous organ.
In focal ARDS, the injury is concentrated in dependent lung regions. Gravitational forces, the weight of the overlying edematous lung, and the compressive effect of abdominal contents (especially in obesity or abdominal hypertension) cause dependent atelectasis and consolidation. Non-dependent lung regions remain aerated and relatively compliant. This creates significant heterogeneity: the lung behaves as a small aerated portion (the baby lung) that receives all tidal volume, surrounded by non-aerated dependent regions. If inappropriate ventilatory strategies are applied, the aerated regions can be subjected to overdistension (volutrauma) while collapsed regions remain unrecruited.
Two biologic sub-phenotypes have also been identified through latent class analysis of clinical trial data: hyperinflammatory (phenotype 2) and hypoinflammatory (phenotype 1). The hyperinflammatory phenotype is characterized by higher plasma levels of inflammatory biomarkers (interleukin-6, interleukin-8, soluble TNF receptor-1, plasminogen activator inhibitor-1), lower serum bicarbonate, higher prevalence of sepsis as the inciting cause, more vasopressor use, higher mortality (approximately 44% vs 23%), and, critically, different responses to interventions. Hyperinflammatory patients benefit from higher PEEP strategies and conservative fluid management, while hypoinflammatory patients may not benefit from or may even be harmed by the same interventions.
Ventilatory management differs by phenotype. Lung-protective ventilation (tidal volume 6 mL/kg predicted body weight, plateau pressure less than 30 cmH2O) remains standard for all ARDS. However, the optimal PEEP differs: in diffuse ARDS, higher PEEP (12 to 20 cmH2O) recruits collapsed alveoli across the widespread injury, improving oxygenation and reducing atelectrauma (cyclic opening and closing of unstable alveoli). In focal ARDS, higher PEEP may overdistend the already-aerated non-dependent regions without recruiting the consolidated dependent regions, potentially causing hemodynamic compromise and volutrauma. The driving pressure (plateau pressure minus PEEP) has emerged as the variable most strongly associated with mortality; keeping driving pressure below 15 cmH2O is a key target.
Prone positioning for at least 16 hours per day has demonstrated mortality benefit in moderate-to-severe ARDS (PaO2/FiO2 less than 150 mmHg). The physiologic rationale differs by phenotype: in focal ARDS, pronation redistributes ventilation to the dorsal (previously dependent) consolidated regions, improving V/Q matching and recruitment. In diffuse ARDS, pronation creates more homogeneous distribution of transpulmonary pressure, reducing overdistension of ventral alveoli. Neuromuscular blockade with cisatracurium improves oxygenation by eliminating patient-ventilator dyssynchrony and reducing oxygen consumption by the respiratory muscles, though its mortality benefit remains debated.
Exam Focus
Exam relevance
Risk factors:
- Sepsis (most common indirect cause, accounts for approximately 40% of ARDS cases, often hyperinflammatory phenotype)
- Pneumonia (most common direct cause: bacterial, viral including COVID-19, aspiration)
- Aspiration of gastric contents (chemical pneumonitis causing direct alveolar epithelial injury)
- Multiple blood transfusions (transfusion-related acute lung injury, TRALI, or transfusion-associated circulatory overload, TACO)
- Pancreatitis (systemic inflammatory response triggering indirect lung injury)
- Trauma with pulmonary contusion, long bone fractures (fat embolism), or massive transfusion
- Inhalation injury (smoke, chemical gases, near-drowning causing direct epithelial damage)
Diagnostics:
- Arterial blood gas: calculate PaO2/FiO2 ratio to classify severity (mild 200-300, moderate 100-200, severe less than 100); monitor serial ABGs to assess response to ventilatory changes
- Chest CT to distinguish focal versus diffuse morphology: focal shows dependent consolidation with non-dependent aeration; diffuse shows bilateral ground-glass opacities throughout all regions
- Echocardiography to exclude cardiogenic pulmonary edema (left atrial pressure less than 18 mmHg or absence of left ventricular dysfunction) per Berlin criteria
- Inflammatory biomarkers (IL-6, IL-8, procalcitonin, CRP, ferritin) to help characterize hyperinflammatory versus hypoinflammatory phenotype
- Ventilator mechanics: monitor plateau pressure (goal less than 30 cmH2O), driving pressure (plateau minus PEEP, goal less than 15 cmH2O), static compliance, and auto-PEEP
- Daily assessment of lung recruitment potential: response to incremental PEEP trials, change in compliance and oxygenation with PEEP adjustments
Core concept
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Clinical scenario
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Takeaways
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