2.1 ARDS Definition, Pathophysiology & Phenotypes

ABIM Critical Care Medicine Study Guide Exam

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1 Chapter 1: Exam Orientation: Format, Blueprint & Study Strategy

1.1 ABIM Critical Care Medicine Exam: Format, Blueprint & Study Strategy

2 Chapter 2: Acute Respiratory Failure & ARDS

2.1 ARDS Definition, Pathophysiology & Phenotypes 2.2 Lung-Protective Mechanical Ventilation 2.3 Refractory ARDS: Prone Positioning, Neuromuscular Blockade & VV-ECMO

3 Chapter 3: Obstructive Airways, Pulmonary Vascular & Pleural Emergencies

3.1 Severe Asthma & COPD Exacerbations in the ICU 3.2 Massive & Submassive Pulmonary Embolism 3.3 Pulmonary Hypertension & Right Ventricular Failure in the ICU 3.4 Massive Hemoptysis, Complex Pleural Effusions & Thoracic Drainage

4 Chapter 4: Airway, Parenchymal & Nonthrombotic Pulmonary Emergencies

4.1 Airway Management, Upper-Airway Obstruction & Artificial-Airway Complications 4.2 Diffuse Alveolar Hemorrhage, Interstitial Lung Disease, Pulmonary Vasculitis & Noncardiogenic Edema 4.3 Severe Community-Acquired & Viral Pneumonia 4.4 Nonthrombotic Embolism (Air, Fat, Amniotic Fluid, Tumor, Septic) & Acute Chest Syndrome

5 Chapter 5: Mechanical Ventilation Modes, Dyssynchrony & Liberation

5.1 Mechanical Ventilator Modes & Advanced Breath Delivery 5.2 Patient-Ventilator Dyssynchrony & Waveform Analysis 5.3 Ventilator Liberation, Spontaneous Breathing Trials & Post-Extubation Support

6 Chapter 6: Shock Syndromes & Hemodynamic Principles

6.1 Shock Classification, Cellular Pathophysiology & Biomarkers 6.2 Hemodynamic Monitoring: Arterial Lines, CVP & Pulmonary Artery Catheter 6.3 Dynamic Assessment of Fluid Responsiveness & Resuscitation Targets

7 Chapter 7: Vasoactive Therapies & Mechanical Circulatory Support

7.1 Vasopressors & Inotropes: Pharmacology & Clinical Titration 7.2 Temporary Mechanical Circulatory Support: IABP, Impella & TandemHeart 7.3 Venoarterial ECMO (VA-ECMO) & Differential Hypoxemia 7.4 Post-Cardiac Arrest Care, Targeted Temperature Management & Neuroprognostication

8 Chapter 8: Acute Kidney Injury & Renal Replacement Therapy

8.1 KDIGO AKI Classification, Etiologies & Diagnostic Evaluation 8.2 Continuous Renal Replacement Therapy (CRRT) vs Intermittent Hemodialysis 8.3 Indications & Timing for RRT in Critical Illness: AKIKI & STARRT-AKI

9 Chapter 9: Electrolyte, Acid-Base & Endocrine Emergencies

9.1 Severe Dysnatremias: Hyponatremia, Hypernatremia & Osmotic Demyelination 9.2 Potassium, Calcium, Magnesium & Phosphate Disorders 9.3 Complex & Mixed Acid-Base Disorders: Boston vs Copenhagen vs Stewart 9.4 Endocrine Crises: DKA, HHS, Thyroid Storm, Myxedema Coma & Adrenal Crisis

10 Chapter 10: Glycemic Control, Hyperammonemia, Pituitary & Adrenal Disorders

10.1 ICU Glycemic Control & Severe Hypoglycemia 10.2 Non-Hepatic Hyperammonemia & Metabolic Encephalopathy 10.3 Pituitary Emergencies, Adrenal Excess & Critical Illness-Related Corticosteroid Insufficiency

11 Chapter 11: Acute Coronary Syndromes & Cardiac Arrhythmias

11.1 Acute Coronary Syndromes & Mechanical Complications of Myocardial Infarction 11.2 Critical Tachyarrhythmias: Atrial Fibrillation with RVR, VT & VF 11.3 Critical Bradyarrhythmias, Conduction Blocks & Temporary Cardiac Pacing

12 Chapter 12: Acute Heart Failure, Valvular & Pericardial Emergencies

12.1 Acute Decompensated Heart Failure & Cardiogenic Pulmonary Edema 12.2 Acute Valvular Regurgitation & Prosthetic Valve Emergencies 12.3 Pericardial Tamponade & Acute Aortic Syndromes

13 Chapter 13: Myocardial Disease, Hypertensive & Cardiac Device Emergencies

13.1 Fulminant Myocarditis, Stress & Peripartum Cardiomyopathy, and Hypertrophic Cardiomyopathy 13.2 Hypertensive Emergencies & Ruptured Abdominal Aortic Aneurysm 13.3 Implanted Cardiac Devices, LVAD Emergencies & Electrical Storm

14 Chapter 14: Sepsis & Systemic Infectious Diseases

14.1 Sepsis-3 Definitions & Surviving Sepsis Campaign 2026 Guidelines 14.2 Refractory Septic Shock & Adjunctive Therapies 14.3 Multidrug-Resistant Pathogens & Antimicrobial Stewardship in the ICU

15 Chapter 15: ICU-Acquired & Severe Focal Infections

15.1 Hospital-Acquired & Ventilator-Associated Pneumonia 15.2 Central Line-Associated Bloodstream Infections & Catheter-Associated UTIs 15.3 Intra-Abdominal Sepsis, Necrotizing Soft Tissue Infections & CNS Infections

16 Chapter 16: Focal, Special-Host & Systemic Infections

16.1 CNS Infections Beyond Bacterial Meningitis: Encephalitis, Brain Abscess & Epidural Abscess 16.2 Infections in the Immunocompromised Host, Tropical & Bioterror Infections 16.3 Infective Endocarditis, Device Infections & Other Focal Cardiovascular Infections 16.4 Antimicrobial Pharmacokinetics, Allergy, Toxicity & Infection Prevention 16.5 Gastrointestinal & Hepatobiliary Infections: Fulminant C. difficile, Typhlitis, CMV Colitis & Abscesses

17 Chapter 17: Neurocritical Care & Coma

17.1 Coma, Altered Mental Status & Acute Delirium in the ICU 17.2 Status Epilepticus & Non-Convulsive Status Epilepticus 17.3 Acute Ischemic Stroke & Intracranial Hemorrhage

18 Chapter 18: Cerebrovascular Syndromes, Encephalopathy & Withdrawal States

18.1 Cerebral Venous Sinus Thrombosis, PRES & Reversible Cerebral Vasoconstriction Syndrome 18.2 Alcohol & Sedative Withdrawal, Wernicke Encephalopathy & Drug-Induced Encephalopathy

19 Chapter 19: Intracranial Hypertension & Neuromuscular Weakness

19.1 Intracranial Pressure Monitoring, Cerebral Perfusion & Tiered ICP Management 19.2 Acute Neuromuscular Respiratory Failure, ICU-Acquired Weakness & Brain Death Determination

20 Chapter 20: Traumatic Brain Injury, Spinal Cord Injury & Toxin-Mediated Paralysis

20.1 Traumatic Brain Injury Management & Paroxysmal Sympathetic Hyperactivity 20.2 Acute Spinal Cord Injury, Neurogenic Shock & Autonomic Dysreflexia 20.3 Toxin-Mediated Paralysis & Neuromuscular Crises: Botulism, Tetanus, Tick Paralysis & Myasthenic Crisis

21 Chapter 21: Gastrointestinal, Hepatic & Abdominal Emergencies

21.1 Acute Upper & Lower Gastrointestinal Bleeding in the ICU 21.2 Acute Liver Failure & Severe Acute Pancreatitis 21.3 Abdominal Compartment Syndrome & Acute Mesenteric Ischemia

22 Chapter 22: Luminal GI Emergencies & Complications of Cirrhosis

22.1 Esophageal, Gastric & Bowel Emergencies: Perforation, Obstruction, Ogilvie Syndrome & Colonic Ischemia 22.2 Complications of Cirrhosis: Alcohol-Associated Hepatitis, Hepatorenal Syndrome & Pulmonary Vascular Syndromes

23 Chapter 23: Trauma, Burns & Perioperative Critical Care

23.1 Polytrauma Resuscitation, Damage Control & Massive Transfusion 23.2 Severe Burn Injury, Inhalation Injury & Environmental Emergencies 23.3 Perioperative Critical Care for High-Risk Surgical Patients

24 Chapter 24: Thoracic Trauma, Environmental Injury & Obstetric Critical Care

24.1 Thoracic Trauma: Flail Chest, Pulmonary Contusion, Great Vessel & Tracheobronchial Injury 24.2 Environmental Injury: Drowning, Diving, Altitude, Heat, Cold, Electrical & Radiation 24.3 Obstetric & Genitourinary Emergencies and the Physiology of Pregnancy

25 Chapter 25: Transplantation, Donor Management & Post-Cardiac Surgery Care

25.1 Solid Organ Transplant Recipients & Deceased Donor Management 25.2 Post-Cardiac Surgery Critical Care: Bleeding, Tamponade, Low Output & Vasoplegia

26 Chapter 26: Hematologic Emergencies, Toxicology, Critical Ultrasound & ICU Liberation

26.1 Hematologic Emergencies in the ICU: HIT, DIC, TMA & Anticoagulant Reversal 26.2 Critical Care Toxicology, Toxidromes & Specific Antidotes 26.3 Critical Care Point-of-Care Ultrasound (POCUS) 26.4 ICU Liberation, Analgosedation, Goals of Care & Critical Care Ethics

27 Chapter 27: Transfusion Medicine, Hematologic & Oncologic Disorders

27.1 Transfusion Medicine: Products, Thresholds, Reactions & Apheresis 27.2 Red and White Cell Disorders, Hemoglobinopathies, Factor Deficiencies & Hypercoagulable States 27.3 Oncologic Emergencies, Hematopoietic Cell Transplant Complications & Cellular Therapy Toxicity

28 Chapter 28: Clinical Pharmacology, Adverse Drug Reactions & Specific Poisonings

28.1 Pharmacokinetic Principles, Dose Adjustment & Drug Interactions in Critical Illness 28.2 Adverse Drug Reactions: Anaphylaxis, Severe Cutaneous Reactions & Hyperthermic Syndromes 28.3 Specific Poisonings: Sodium-Channel Blockers, Lithium, Valproate, Theophylline, Iron, Sulfonylureas & Methemoglobinemia

29 Chapter 29: Ethics, Research Methods, Administration & Abdominal Ultrasound

29.1 Advance Care Planning, Nonbeneficial Treatment, Informed Consent & Confidentiality 29.2 Critical Care Research Methods, Statistics, IRB Oversight & Quality Improvement 29.3 Abdominal, Vascular & Procedural Point-of-Care Ultrasound

2.1 ARDS Definition, Pathophysiology & Phenotypes

Key Takeaways Explain This Topic AI

The Berlin definition requires onset within 7 days of a known clinical insult, bilateral radiographic opacities not fully explained by effusions, collapse, or nodules, and non-cardiogenic pulmonary edema confirmed by objective evaluation excluding hydrostatic etiology.

Severity is stratified by PaO2/FiO2 on PEEP >= 5 cmH2O: Mild (201-300 mmHg, ~27% mortality), Moderate (101-200 mmHg, ~32% mortality), and Severe (<= 100 mmHg, ~45% mortality).

Diffuse alveolar damage (DAD) progresses through three overlapping histological phases: an early exudative phase with alveolar-capillary barrier disruption and hyaline membranes, a proliferative phase with type II pneumocyte hyperplasia, and a late fibrotic phase marked by microvascular obliteration and high dead space.

Biological subphenotyping distinguishes a hyperinflammatory subphenotype (elevated IL-6, IL-8, TNFR-1, low bicarbonate, shock, higher mortality) from a hypoinflammatory subphenotype; secondary analyses suggest differing responses to PEEP and fluid strategy.

The 2024 Global Definition of ARDS adds non-intubated patients on high-flow nasal oxygen >= 30 L/min, accepts SpO2/FiO2 <= 315 (with SpO2 <= 97%), and allows lung ultrasound for the imaging criterion.

Last updated: October 2026

Diagnostic Evolution and the Berlin Definition

Acute Respiratory Distress Syndrome (ARDS) was first described by Ashbaugh and colleagues in 1967 in a landmark case series of 12 intensive care patients exhibiting severe dyspnea, tachypnea, cyanosis refractory to supplemental oxygen, decreased static respiratory compliance, and diffuse alveolar infiltrates on chest radiography. In 1994, the American-European Consensus Conference (AECC) formalized diagnostic criteria, establishing categories for Acute Lung Injury (ALI; PaO 2 / FiO 2 ≤ 300 mmHg \text{PaO}_2/\text{FiO}_2 \le 300\text{ mmHg} PaO 2 ​ / FiO 2 ​ ≤ 300 mmHg ) and ARDS ( PaO 2 / FiO 2 ≤ 200 mmHg \text{PaO}_2/\text{FiO}_2 \le 200\text{ mmHg} PaO 2 ​ / FiO 2 ​ ≤ 200 mmHg ). However, the AECC definition exhibited significant limitations: it lacked criteria for positive end-expiratory pressure (PEEP), relied on subjective interpretations of bilateral infiltrates, had undefined timing for "acute" onset, and mandated a pulmonary artery wedge pressure (PAWP) ≤ 18 mmHg \le 18\text{ mmHg} ≤ 18 mmHg or the absence of left atrial hypertension, which overlooked the frequent clinical coexistence of left ventricular dysfunction or fluid overload with inflammatory lung injury.

To address these limitations, the European Society of Intensive Care Medicine convened an international panel in 2011, establishing the 2012 Berlin Definition . The Berlin definition retired the term "acute lung injury" and established four explicit, evidence-based diagnostic criteria:

Timing : Respiratory symptoms must originate within 1 week of a known clinical insult (e.g., severe sepsis, pneumonia, gastric aspiration, major trauma, pancreatitis) or represent new or worsening respiratory symptoms over the past 7 days.

Chest Imaging : Bilateral opacities visible on chest radiography or computed tomography (CT) that are not fully explained by pleural effusions, lobar or lung atelectasis, or pulmonary nodules.

Origin of Edema : Respiratory failure not fully explained by cardiac failure or fluid overload. An objective evaluation (e.g., transthoracic or transesophageal echocardiography, lung ultrasound, or invasive hemodynamic monitoring) is mandatory to exclude hydrostatic pulmonary edema if no clear ARDS risk factor is present.

Oxygenation Impairment : Arterial hypoxemia categorized while ventilated with a minimum PEEP or continuous positive airway pressure (CPAP) ≥ 5 cmH 2 O \ge 5\text{ cmH}_2\text{O} ≥ 5 cmH 2 ​ O :

Severity Category Oxygenation Criterion (PEEP ≥ 5 cmH 2 O \ge 5\text{ cmH}_2\text{O} ≥ 5 cmH 2 ​ O ) Hospital Mortality (Berlin validation cohort) Median Ventilation Duration in Survivors

Mild 200 mmHg < PaO 2 / FiO 2 ≤ 300 mmHg 200\text{ mmHg} < \text{PaO}_2/\text{FiO}_2 \le 300\text{ mmHg} 200 mmHg < PaO 2 ​ / FiO 2 ​ ≤ 300 mmHg ~27% 5 days

Moderate 100 mmHg < PaO 2 / FiO 2 ≤ 200 mmHg 100\text{ mmHg} < \text{PaO}_2/\text{FiO}_2 \le 200\text{ mmHg} 100 mmHg < PaO 2 ​ / FiO 2 ​ ≤ 200 mmHg ~32% 7 days

Severe PaO 2 / FiO 2 ≤ 100 mmHg \text{PaO}_2/\text{FiO}_2 \le 100\text{ mmHg} PaO 2 ​ / FiO 2 ​ ≤ 100 mmHg ~45% 9 days

Note

A crucial feature of the Berlin definition is the requirement that PaO 2 / FiO 2 \text{PaO}_2/\text{FiO}_2 PaO 2 ​ / FiO 2 ​ be assessed with a standardized minimum PEEP of at least 5 cmH 2 O 5\text{ cmH}_2\text{O} 5 cmH 2 ​ O . A patient breathing through non-rebreather oxygen or high-flow nasal cannula without continuous positive airway pressure cannot formally be classified under the strict 2012 Berlin criteria.

The 2024 Global Definition of ARDS (Matthay et al., Am J Respir Crit Care Med 2024) keeps the Berlin timing, imaging, and edema-origin criteria but adds a category for non-intubated patients receiving high-flow nasal oxygen at ≥ 30 L/min \ge 30\text{ L/min} ≥ 30 L/min or NIV/CPAP with at least 5 cmH 2 O 5\text{ cmH}_2\text{O} 5 cmH 2 ​ O end-expiratory pressure. It accepts SpO 2 / FiO 2 ≤ 315 \text{SpO}_2/\text{FiO}_2 \le 315 SpO 2 ​ / FiO 2 ​ ≤ 315 (when SpO 2 ≤ 97 % \text{SpO}_2 \le 97\% SpO 2 ​ ≤ 97% ) as an alternative to PaO 2 / FiO 2 ≤ 300 \text{PaO}_2/\text{FiO}_2 \le 300 PaO 2 ​ / FiO 2 ​ ≤ 300 , allows lung ultrasound to document bilateral opacities, and, in resource-limited settings, requires neither PEEP nor a minimum oxygen flow rate.

Pathophysiology and Histopathologic Phases

The morphological hallmark of ARDS is diffuse alveolar damage (DAD) , which represents a coordinated, stereotypic response of pulmonary tissue to severe inflammatory insult. Histopathologically and clinically, ARDS evolves through three classically defined, overlapping phases:

1. The Exudative Phase (Days 0 to 7)

The exudative phase begins within hours of the inciting injury. Direct alveolar insults (e.g., gastric aspiration, bacterial or viral pneumonia) or indirect systemic inflammatory insults (e.g., sepsis, acute pancreatitis, severe burn injury, massive transfusion) activate resident alveolar macrophages and vascular endothelial cells. The release of pro-inflammatory cytokines (tumor necrosis factor-alpha [TNF- α \alpha α ], interleukin-1-beta [IL-1 β \beta β ], interleukin-6 [IL-6], and interleukin-8 [IL-8]) stimulates extensive neutrophil marginization, activation, and migration across the alveolar-capillary membrane.

Neutrophils release cytotoxic granular enzymes, myeloperoxidase, reactive oxygen species, and neutrophil extracellular traps (NETs), severely disrupting the capillary endothelium and the tight junctions of type I alveolar epithelial cells. The resulting increase in vascular permeability leads to the flooding of interstitial spaces and alveoli with protein-rich exudative fluid, red blood cells, and cellular debris.

Two critical pathophysiological consequences emerge during this phase:

Surfactant Depletion and Inactivation : Type II pneumocyte injury diminishes surfactant production, while intra-alveolar plasma proteins (particularly fibrinogen and monomeric fibrin) directly inactivate residual surfactant. This leads to dramatically increased surface tension, diffuse microatelectasis, and a severe right-to-left intrapulmonary shunt ( V A / Q = 0 V_A/Q = 0 V A ​ / Q = 0 ).

Hyaline Membrane Formation : Denuded basement membranes become coated with dense, eosinophilic hyaline membranes composed of polymerized fibrin, condensed plasma proteins, and necrotic epithelial remnants. Gas diffusion is severely impaired, and static respiratory compliance ( C rs C_{\text{rs}} C rs ​ ) drops precipitously.

2. The Proliferative Phase (Days 7 to 14)

The proliferative phase represents an early, coordinated attempt at tissue repair and restoration of barrier integrity. During this phase, surviving type II alveolar pneumocytes undergo vigorous cellular division, spreading over the denuded basal lamina and gradually differentiating into flat type I pneumocytes to re-establish the epithelial barrier. Type II cells actively express apical epithelial sodium channels (ENaC) and basolateral Na + / K + \text{Na}^+/\text{K}^+ Na + / K + -ATPase pumps, creating an osmotic gradient that drives fluid reabsorption from the alveolar lumen back into the interstitial lymphatics.

Simultaneously, myofibroblasts and fibroblasts proliferate within the provisional fibrin scaffold, depositing fibronectin and early collagen. Neutrophilic infiltration recedes, replaced predominantly by mononuclear lymphocytes and macrophages that phagocytose cellular debris. Clinically, this phase is marked by dynamic heterogeneity: some patients experience progressive gas exchange improvement, radiographic clearing, and successful liberation from mechanical ventilation, whereas others transition toward unconstrained fibroproliferation.

3. The Fibrotic Phase (Day 14 and Beyond)

In patients who do not resolve the inflammatory cascade, extensive and dysregulated fibroproliferation leads to irreversible architectural remodeling. Intra-alveolar exudates organize into dense fibrous tissue, driven by sustained expression of transforming growth factor-beta (TGF- β \beta β ) and platelet-derived growth factor (PDGF). Interstitial collagen deposition thickens alveolar septa, obliterating the normal microvascular architecture.

Microvascular thrombosis and progressive capillary obliteration lead to significant pulmonary hypertension, elevated right ventricular afterload, and markedly increased physiological dead space ( V D / V T V_D/V_T V D ​ / V T ​ ). Lung parenchyma develops patchy microcystic honeycombing and bullous emphysematous changes, rendering the lung stiff, highly non-compliant, and exceptionally prone to barotrauma. Patients in the fibrotic phase endure prolonged ventilator dependence and high late mortality.

ARDS Subphenotypes: Biological and Physiological

Recent clinical trials and physiological investigations demonstrate that ARDS is not a homogeneous syndrome, but rather a heterogeneous clinical collection of distinct subphenotypes requiring divergent clinical strategies.

Biological Subphenotypes: Hyperinflammatory vs. Hypoinflammatory

Through latent class analysis of large multicenter trial cohorts (ARMA, ALVEOLI, FACTT, SAILS, and the HARP-2 simvastatin trial), Calfee and colleagues identified two consistently distinct biological subphenotypes:

Hyperinflammatory Subphenotype (~30% of patients) :

Characterized by dramatically elevated plasma concentrations of inflammatory biomarkers, including IL-6, IL-8, TNF receptor-1 (TNFR-1), and soluble intercellular adhesion molecule-1 (ICAM-1).

Demonstrates lower serum bicarbonate, severe metabolic acidosis, significantly higher vasopressor requirements, and a higher prevalence of sepsis as the precipitating risk factor.

Carries a much higher 90-day mortality (~40-50%).

Therapeutic Responsiveness : In secondary analyses there was a significant subphenotype-by-treatment interaction: hyperinflammatory patients in ALVEOLI had lower mortality with higher PEEP , and in FACTT (Famous et al., 2017, as corrected) 90-day mortality was 40% with a conservative fluid strategy vs. 50% with a liberal one. These are hypothesis-generating findings, not yet a basis for treatment allocation.

Hypoinflammatory Subphenotype (~70% of patients) :

Characterized by lower systemic inflammatory markers, less endothelial activation, lower shock incidence, and higher serum bicarbonate.

Exhibits lower 90-day mortality (~20-25%).

Therapeutic Responsiveness : Hypoinflammatory patients derived no benefit from higher PEEP (with a signal toward harm), and in FACTT their 90-day mortality was 26% with conservative vs. 18% with liberal fluids.

• Feature Hyperinflammatory Subphenotype Hypoinflammatory Subphenotype
• Inflammatory Cytokines Markedly elevated (IL-6, IL-8, TNFR-1) Low to normal
• Endothelial Markers High sICAM-1, high Angiopoietin-2 Lower sICAM-1, preserved barrier
• Hemodynamics & Shock High vasopressor requirements, profound shock Low shock incidence, hemodynamically stable
• Metabolic Profile Severe acidosis, low serum bicarbonate Preserved acid-base balance
• 90-Day Mortality High (~40% to 50%) Lower (~20% to 25%)
• High PEEP Response Lower mortality with higher PEEP (secondary analysis) No benefit or potential harm
• Fluid Strategy Response Responsive to conservative fluid balance Tolerant of liberal fluids
• Physiological / Morphological Subphenotypes: Type L vs. Type H

During the COVID-19 pandemic, Gattinoni and colleagues popularized the distinction between two physiological and radiological archetypes. The framework is debated (most patients fall on a continuum between the two), but it is a useful way to reason about recruitability in any ARDS:

Type L (Low Elastance / High Compliance) :

Characterized by low lung elastance (i.e., relatively preserved static respiratory system compliance, typically C rs > 50 mL/cmH 2 O C_{\text{rs}} > 50\text{ mL/cmH}_2\text{O} C rs ​ > 50 mL/cmH 2 ​ O ).

CT imaging shows predominantly subpleural ground-glass opacities with minimal dense dependent consolidation ("low lung weight").

Low recruitability: Because non-aerated lung tissue is minimal, applying high PEEP does not recruit closed alveoli but instead overdistends already aerated units, causing dead-space expansion and right ventricular strain.

Severe hypoxemia is primarily mediated by vasoplegia and loss of hypoxic pulmonary vasoconstriction (HPV), resulting in perfusion of poorly ventilated lung regions.

Type H (High Elastance / Low Compliance) :

Characterized by high elastance (severe reduction in static compliance, typically C rs < 40 mL/cmH 2 O C_{\text{rs}} < 40\text{ mL/cmH}_2\text{O} C rs ​ < 40 mL/cmH 2 ​ O ).

CT demonstrates dense, bilateral dependent consolidations ("high lung weight") with extensive edema.

High recruitability: Non-aerated, consolidated dependent units can be reopened with higher PEEP and prone positioning.

Represents the classic, fully expressed ARDS phenotype in which standard ARDSNet ventilation, higher PEEP, and early prone positioning are most clearly appropriate.

Warning

Applying high PEEP ( > 15 cmH 2 O > 15\text{ cmH}_2\text{O} > 15 cmH 2 ​ O ) indiscriminately to a patient with Type L ARDS or a hypoinflammatory subphenotype does not improve oxygenation. Instead, it overdistends compliant ventral lung tissue, compresses alveolar microvessels, shifts pulmonary blood flow to non-aerated regions, worsens physiological shunt, and induces acute cor pulmonale.

Clinical Traps and Differential Diagnosis

When evaluating acute bilateral pulmonary infiltrates and hypoxemic respiratory failure on the intensive care unit boards, clinicians must avoid critical diagnostic pitfalls:

1. Cardiogenic Pulmonary Edema vs. ARDS

Hydrostatic (cardiogenic) pulmonary edema and non-hydrostatic (permeability) pulmonary edema frequently share identical clinical presentations. Key distinguishing features include:

Echocardiography / POCUS : ARDS classically demonstrates thickened, irregular pleural lines, subpleural consolidations, and non-homogeneous B-lines with interspersed "spared areas" of normal lung sliding. Cardiogenic pulmonary edema displays smooth, thin pleural lines, homogeneous and symmetric dependent B-lines ("lung rockets"), and preserved sliding without anterior consolidation.

Left Atrial Filling Pressures : An elevated mitral E / e ′ E/e' E / e ′ ratio ( > 14 > 14 > 14 ), left atrial dilation, and elevated PAWP ( > 18 mmHg > 18\text{ mmHg} > 18 mmHg ) point toward a hydrostatic etiology. However, under the Berlin definition, the presence of left ventricular dysfunction or fluid overload does not rule out ARDS if an objective inflammatory insult (e.g., bacteremic sepsis, aspiration) is also present.

2. Transfusion-Related Acute Lung Injury (TRALI) vs. TACO

TRALI : Occurs within 6 hours of blood product transfusion (most commonly plasma or platelets) mediated by donor anti-HLA or anti-HNA antibodies that bind recipient neutrophils sequestered in pulmonary capillaries. Characterized by acute permeability edema, normal left atrial pressures, low or normal BNP, and an edema fluid-to-plasma protein ratio of about 0.75 0.75 0.75 or higher.

TACO (Transfusion-Associated Circulatory Overload) : Pure hydrostatic pulmonary edema resulting from rapid volume infusion in patients with compromised cardiovascular reserve. Features elevated central venous pressure, marked BNP elevation, left atrial enlargement, and an edema fluid-to-plasma protein ratio below about 0.65 0.65 0.65 , responding dramatically to diuresis.

3. Acute Eosinophilic Pneumonia (AEP)

Characterized by rapid-onset febrile hypoxemic respiratory failure, diffuse alveolar infiltrates, and absence of peripheral eosinophilia on initial presentation. A history of recent initiation or change in smoking habits (cigarettes, vaping) is classic. Definitive diagnosis requires bronchoalveolar lavage (BAL) demonstrating ≥ 25 % \ge 25\% ≥ 25% eosinophils. It responds promptly to systemic corticosteroids.