ARDS and Respiratory Failure

Direct (pulmonary) injury — insult reaches the alveolar epithelium first

• Pneumonia: the single most common direct cause; bacterial, viral (influenza, SARS-CoV-2), Pneumocystis, or fungal. Epithelial destruction precedes endothelial leak, so opacities are often asymmetric early.
• Aspiration of gastric contents: low-pH chemical pneumonitis (Mendelson syndrome) injures type I pneumocytes within minutes; classic in obtundation, seizure, intoxication, or post-anesthesia stems.
• Inhalation and thermal injury, near-drowning, pulmonary contusion: mechanical or oxidant destruction of the alveolar–capillary barrier; contusion appears in the blunt-trauma stem within hours.

Indirect (extrapulmonary) injury — systemic mediators reach the lung via the pulmonary circulation

• Sepsis: the most common cause overall, and the highest-risk single precipitant; circulating TNF-α/IL-1 drive endothelial-side permeability, so infiltrates are typically diffuse and symmetric.
• Acute pancreatitis: circulating phospholipase A2, trypsin, and free fatty acids degrade surfactant — hypoxemia is a graded component of the Ranson/APACHE scores.
• Massive transfusion and TRALI: donor anti-HLA/anti-neutrophil antibodies prime recipient neutrophils; onset within 6 hours of a blood product with fever and hypotension.
• Major trauma, burns, fat embolism, cardiopulmonary bypass, drug overdose (opioids, salicylates), and eclampsia.

Modifiable risk factors (examiners plant these to hint at causation or prevention)

• Chronic alcohol use disorder: glutathione depletion in alveolar lining fluid impairs antioxidant defense and raises ARDS risk in at-risk patients.
• Cigarette smoking and vaping.
• Injurious ventilation: high tidal volume and high plateau pressure can cause ARDS in a previously uninjured lung.
• Liberal fluid resuscitation and unnecessary transfusion; hypoalbuminemia lowering oncotic pressure.
• Obesity, hyperoxia, and delayed source control of infection.

Non-modifiable risk factors

• Advanced age and severity of the underlying illness (higher APACHE/SOFA), which dominate prognosis.
• Genetic polymorphisms in surfactant protein B and ACE, and non-pulmonary organ failure at presentation.
• Chronic liver disease, which independently worsens ARDS outcome.

ARDS Pathophysiology (typically progresses through phases)

• Exudative phase (Days 0-7): Direct or indirect insult triggers release of pro-inflammatory mediators (TNF-α, IL-1, IL-6, IL-8) leading to increased pulmonary capillary permeability; loss of tight junctions in the alveolar epithelium and endothelium allows protein-rich fluid to leak into alveolar spaces, forming characteristic hyaline membranes; pulmonary edema is NON-CARDIOGENIC (wedge pressure ≤18 mmHg) and results from increased permeability, not hydrostatic pressure

• Neutrophil recruitment and damage: Inflammatory cascade recruits activated neutrophils to lung tissue; these cells release proteases, elastase, and reactive oxygen species (ROS) causing further epithelial and endothelial injury; activation of complement cascade and coagulation cascade perpetuates inflammation

• Surfactant dysfunction: Damage to pneumocytes and inactivation of surfactant proteins A and D (opsonins with anti-inflammatory function) reduces surface tension reduction; surfactant deposition with fibrin forms hyaline membranes visible on histology; loss of surfactant leads to alveolar collapse and atelectasis

• Ventilation-perfusion (V/Q) mismatch and intrapulmonary shunting: Atelectasis and pulmonary edema create areas of low V/Q and true shunt (perfused but not ventilated areas); shunting explains the profound hypoxemia poorly responsive to supplemental oxygen alone; refractory hypoxemia is a hallmark feature

• Fibroproliferative phase (Days 7-21): If patient survives, type II pneumocytes attempt regeneration; fibroblasts proliferate and deposit collagen; risk of developing pulmonary fibrosis and chronic lung disease

• Loss of hypoxic pulmonary vasoconstriction (HPV): Inflammatory mediators impair the normal mechanism that redirects blood away from poorly ventilated areas; pulmonary hypertension develops (elevated RV afterload), worsening right heart strain

• Type II respiratory failure component: In severe ARDS, work of breathing increases dramatically; respiratory muscle fatigue develops alongside severe hypoxemia; hypercapnia may develop indicating ventilatory failure

Common precipitants of ARDS

• Direct lung injury: Pneumonia (bacterial, viral, fungal), aspiration, pulmonary contusion, inhalation injury, radiation
• Indirect lung injury: Sepsis, pancreatitis, major trauma, massive transfusion, fat/air embolism, drug reactions, transfusion-related acute lung injury (TRALI)

• Dyspnea and tachypnea: Rapid onset of severe dyspnea within hours to days of triggering insult; respiratory rate typically >30 breaths/minute; patient appears anxious and distressed with use of accessory muscles

• Hypoxemia refractory to supplemental oxygen: PaO₂/FiO₂ ratio (P/F ratio) <300 is part of diagnostic criteria; profound hypoxemia occurs despite high-flow oxygen (FiO₂ 0.5 or higher), distinguishing ARDS from simple hypoxemia; this refractory nature reflects the shunting physiology

• Bilateral infiltrates on imaging: Chest X-ray shows diffuse bilateral opacities ("white-out" appearance in severe cases) described as ground-glass opacities or consolidation; infiltrates are peripheral and dependent; NOT unilateral (rules out simple pneumonia or pulmonary edema in one lung field)

• Absence of primary cardiac cause: Patient does NOT have elevated pulmonary artery occlusion pressure (PAOP ≤18 mmHg by Swan-Ganz catheter if measured); echocardiography typically shows normal left ventricular function and normal filling pressures; clinical history and findings rule out cardiogenic pulmonary edema

• Altered mental status and agitation: Severe hypoxemia and hypercapnia impair cognition; anxiety from air hunger; encephalopathy may precede obvious respiratory symptoms

• Cyanosis: Central cyanosis with SpO₂ <88% despite supplemental oxygen is common; peripheral cyanosis may be present

• Rales and decreased breath sounds: Auscultation may reveal crackles (from pulmonary edema and atelectasis) or decreased breath sounds (from consolidation); findings may be subtle initially or widespread depending on severity

• Clinical pearl: ARDS typically develops 12-48 hours after the precipitating insult; delayed presentation (days 5-7) suggests alternative diagnosis or secondary complication

• Berlin Criteria (2012) — Current gold standard for ARDS diagnosis (requires ALL four criteria):
• Timing: Respiratory symptoms within 1 week of known clinical insult or new/worsening respiratory symptoms during hospitalization
• Bilateral opacities: Bilateral radiographic infiltrates on chest X-ray or CT (not fully explained by effusions, collapse, or nodules)
• Origin of edema: Respiratory failure NOT explained by cardiac failure or fluid overload; PAOP ≤18 mmHg if measured, or absence of left heart disease on clinical assessment
• Hypoxemia severity (distinguishes mild, moderate, severe):
• Mild ARDS: 200 < P/F ratio ≤300 on PEEP ≥5 cm H₂O
• Moderate ARDS: 100 < P/F ratio ≤200 on PEEP ≥5 cm H₂O
• Severe ARDS: P/F ratio ≤100 on PEEP ≥5 cm H₂O
• P/F ratio = PaO₂ (mmHg) / FiO₂ (as decimal); measured from arterial blood gas

• Arterial blood gas (ABG): Essential for diagnosis and classification; reveals hypoxemia (PaO₂ typically <60 mmHg on room air) and assesses for concurrent hypercapnia (may be normal or elevated); respiratory alkalosis common early, metabolic acidosis may develop

• Chest imaging: CXR shows bilateral infiltrates; High-resolution CT shows patchy or diffuse ground-glass opacities and consolidation with dependent distribution; CT helps exclude alternative diagnoses (pneumothorax, pleural effusion predominance)

• Pulmonary artery catheter (Swan-Ganz) — rarely performed now but historically important: If placed for other reasons, documents PAOP ≤18 mmHg (normal to low-normal), confirming non-cardiogenic mechanism; elevated PAOP suggests cardiogenic pulmonary edema instead

• Echocardiography: Assesses left ventricular function and fills pressures to exclude primary cardiac cause; normal EF and normal diastolic function support ARDS diagnosis

• Complete blood count, comprehensive metabolic panel, coagulation studies: Assess for sepsis (elevated WBC, left shift), organ dysfunction (elevated creatinine, bilirubin); coagulopathy may develop (DIC in severe cases)

• Blood and respiratory cultures: If infectious trigger suspected (pneumonia, sepsis); culture results guide antimicrobial therapy but don't alter ARDS diagnosis

• Lactate: Elevated lactate suggests tissue hypoxia and poor prognosis; serial trends guide resuscitation adequacy

• Important diagnostic pearls:
• ARDS is a **clinical diagnosis

Immediate stabilization

• Secure oxygenation and airway: high-flow nasal cannula or a trial of noninvasive ventilation may be used in mild disease, but persistent hypoxemia, rising work of breathing, or altered mentation mandate intubation — delayed intubation with self-inflicted lung injury worsens outcome.
• Treat the precipitant: this is the only truly "definitive" therapy. Per the Surviving Sepsis Campaign, obtain cultures, give broad-spectrum antimicrobials early (for example an antipseudomonal beta-lactam such as piperacillin-tazobactam ± vancomycin when MRSA is a concern), and achieve source control. Vancomycin is dosed to a 24-hour AUC/MIC of 400–600 per the 2020 IDSA/ASHP consensus.

First-line: lung-protective ventilation (ARDSNet/ARMA; ATS/ESICM/SCCM 2017 guideline, strong recommendation)

• Low tidal volume: 6 mL/kg of predicted body weight (calculated from height and sex, never actual weight) to limit volutrauma.
• Plateau pressure ≤30 cm H₂O and attention to driving pressure; increase respiratory rate — not tidal volume — for CO₂ clearance.
• Permissive hypercapnia: tolerate a modestly acidotic pH rather than abandon low tidal volumes.
• PEEP titrated by an ARDSNet FiO₂/PEEP table; higher PEEP is suggested for moderate-to-severe disease to recruit collapsed alveoli and reduce atelectrauma.
• Conservative fluid strategy once shock has resolved (FACTT): fewer ventilator days, no increase in organ failure.

Escalation for refractory hypoxemia

• Prone positioning ≥12–16 hours/day for P/F <150 (PROSEVA); recruits dorsal lung, homogenizes stress, and improves survival — recommended by ATS/ESICM/SCCM.
• Neuromuscular blockade (cisatracurium) for severe dyssynchrony; routine early use was not beneficial in ROSE.
• Inhaled pulmonary vasodilators (nitric oxide, epoprostenol): rescue oxygenation only, no mortality benefit.
• VV-ECMO at an experienced center for refractory hypoxemia or uncontrollable hypercapnia (EOLIA; ELSO guidance).
• Corticosteroids: dexamethasone is standard in COVID-19 ARDS requiring oxygen per NIH COVID-19 Treatment Guidelines; non-COVID use remains selective.

Ventilator-induced lung injury (the central iatrogenic hazard)

• Barotrauma → pneumothorax, pneumomediastinum, subcutaneous emphysema: alveolar rupture from high transpulmonary pressure. Signals: sudden rise in both peak and plateau pressure, unilateral absent breath sounds, tracheal deviation, and hypotension. Tension pneumothorax is an emergency — needle decompression followed by tube thoracostomy before imaging.
• Volutrauma and atelectrauma: overdistension and repetitive opening/closing shear generate biotrauma, systemic cytokine spillover, and distant organ failure — the mechanism by which non-protective ventilation kills.
• Auto-PEEP/dynamic hyperinflation: incomplete exhalation raises intrathoracic pressure, drops venous return, and produces hypotension that improves when the patient is briefly disconnected from the circuit.

Cardiovascular

• Acute cor pulmonale/right ventricular failure: hypoxic vasoconstriction, hypercapnia, and high PEEP raise RV afterload; look for rising CVP, new tricuspid regurgitation, and a dilated RV with septal flattening on bedside echo. Emergency — reduce plateau pressure, correct hypercapnia, avoid volume overload.
• PEEP-induced hypotension: reduced preload; distinguish from sepsis and from tension pneumothorax.

Infectious and ICU-acquired

• Ventilator-associated pneumonia: new infiltrate, purulent secretions, fever, rising oxygen requirement; prevented with head-of-bed elevation and sedation interruption.
• **Catheter-related bloodstream infection, C. difficile, sinusitis**.
• Venous thromboembolism and stress-related GI bleeding — both justify prophylaxis per Surviving Sepsis Campaign bundles.

Late and treatment-specific

• Pulmonary fibrosis from the fibroproliferative phase: persistent restriction, reduced DLCO, traction bronchiectasis on CT.
• Oxygen toxicity from prolonged high FiO₂: absorption atelectasis and free-radical injury.
• ICU-acquired weakness and critical illness polyneuromyopathy: worsened by neuromuscular blockade, corticosteroids, and immobility; failure to wean is the clue.
• Delirium, depression, PTSD, and long-term cognitive impairment (post-intensive care syndrome).
• Prone positioning: pressure ulcers, facial and conjunctival edema, brachial plexus injury, and — the feared event — endotracheal tube or line dislodgement during turning.
• ECMO: hemorrhage from anticoagulation, hemolysis, circuit thrombosis, and limb ischemia.

• Hyaline membranes on lung histology plus diffuse alveolar damage is the pathognomonic pairing; if a vignette shows these after sepsis or aspiration, the answer is ARDS regardless of how the imaging is described.
• Tidal volume is 6 mL/kg of predicted body weight, derived from height and sex — not measured weight. Choosing actual body weight is the classic distractor, and it systematically over-ventilates short and obese patients.
• Rising PaCO₂ on lung-protective settings? Increase the respiratory rate, not the tidal volume. Permissive hypercapnia is accepted; abandoning 6 mL/kg to normalize the CO₂ is always the wrong answer.
• Only three interventions have shown a mortality benefit in ARDS trials: low tidal volume ventilation (ARDSNet/ARMA) and prone positioning for P/F <150 (PROSEVA), with conservative fluid management (FACTT) improving ventilator-free days. Inhaled nitric oxide, surfactant, and beta-agonists improve numbers, not survival — a favorite trap.
• **The single best next step for new hypotension plus a sudden rise in peak and plateau pressure in a ventilated ARDS patient is needle decompression for tension pneumothorax**, before a chest x-ray. If only the peak pressure rises with an unchanged plateau, the problem is airway resistance — mucus plug, bronchospasm, or a kinked tube.
• Distinguish ARDS from cardiogenic pulmonary edema: ARDS has a normal or low wedge pressure, normal ejection fraction, and a low BNP; cardiogenic edema gives cephalization, Kerley B lines, effusions, an S₃, and a markedly elevated BNP. Diuresing a hypotensive septic patient because you mislabeled the edema is the intended error.
• The association examiners test: chronic alcohol use disorder increases ARDS susceptibility through glutathione depletion, and acute pancreatitis causes ARDS through circulating phospholipase A2 degrading surfactant.
• PEEP recruits alveoli but reduces venous return; new hypotension after a PEEP increase is preload failure, not sepsis, until proven otherwise.