## Distinguishing ARDS from Cardiogenic Pulmonary Edema ### Key Pathophysiologic Difference **Key Point:** The defining discriminator between ARDS and cardiogenic pulmonary edema is the pulmonary capillary wedge pressure (PAOP). ARDS is characterized by increased pulmonary vascular permeability (non-cardiogenic), while cardiogenic edema results from elevated hydrostatic pressure. ### Comparison Table | Feature | ARDS | Cardiogenic Pulmonary Edema | |---------|------|----------------------------| | **PAOP (gold standard)** | ≤18 mmHg | >18 mmHg | | **Mechanism** | Increased capillary permeability | Elevated hydrostatic pressure | | **Bilateral infiltrates** | Present | Present | | **Onset timing** | Within 1 week of insult | Variable, often gradual | | **PaO₂/FiO₂ ratio** | <300 mmHg | Usually >300 initially | | **Sputum character** | Protein-rich (edema fluid) | Frothy, pink (low protein) | ### Why PAOP Matters **High-Yield:** PAOP ≤18 mmHg is a mandatory criterion in the Berlin Definition of ARDS (2012). If PAOP >18 mmHg, the diagnosis is cardiogenic pulmonary edema, not ARDS, regardless of other findings. **Clinical Pearl:** In practice, echocardiography (EF assessment, diastolic dysfunction) or BNP/NT-proBNP elevation can support cardiogenic etiology when PAC is unavailable. However, the question explicitly asks for the best discriminating feature, which is the hemodynamic measurement. ### Diagnostic Algorithm ```mermaid flowchart TD A[Acute hypoxemia + bilateral infiltrates]:::outcome --> B{PAOP measurement}:::decision B -->|≤18 mmHg| C[ARDS]:::outcome B -->|>18 mmHg| D[Cardiogenic pulmonary edema]:::outcome B -->|Unavailable| E{Assess cardiac function}:::decision E -->|Reduced EF or elevated BNP| F[Likely cardiogenic]:::outcome E -->|Normal EF, low BNP| G[Likely ARDS]:::outcome ``` [cite:Harrison 21e Ch 297]
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