Residency · Residency · Respirology

Acute Respiratory Distress Syndrome

Definition and Epidemiology

Berlin Definition (2012)

The Berlin Definition, published in 2012 by an international expert panel, established the current diagnostic framework for acute respiratory distress syndrome. The definition requires four criteria to be satisfied. First, the timing criterion stipulates that respiratory failure must develop within one week of a known clinical insult or the onset of new or worsening respiratory symptoms. Second, the imaging criterion requires bilateral opacities on chest radiograph or CT that are not fully explained by effusions, lobar or lung collapse, or nodular disease. Third, the origin of edema criterion mandates that the respiratory failure not be fully explained by cardiac failure or fluid overload, and an objective assessment such as echocardiography should be performed to exclude hydrostatic edema if no identifiable risk factor for ARDS is present. Fourth, the oxygenation severity criterion stratifies patients into three categories, all assessed while the patient is receiving a minimum of 5 cmH2O of positive end-expiratory pressure: mild ARDS corresponds to a PaO2/FiO2 ratio of 200 to 300 mmHg, moderate ARDS to a PaO2/FiO2 ratio of 100 to 200 mmHg, and severe ARDS to a PaO2/FiO2 ratio of 100 mmHg or less. It is worth noting that a 2023 Global Definition update has been proposed, which would expand the criteria to include patients on high-flow nasal cannula at 30 L/min or greater, incorporate SpO2/FiO2 criteria for settings where arterial blood gas analysis is unavailable, and encompass non-ventilated patients, thereby improving applicability in resource-limited environments.

Epidemiology

ARDS represents a significant burden of critical illness. Approximately 10% of all intensive care unit admissions and 23% of mechanically ventilated patients meet criteria for ARDS. The landmark LUNG SAFE study published in 2016, a multinational observational investigation, revealed troubling gaps in clinical practice: clinician recognition of ARDS occurred in only approximately 60% of cases, and only 65% of recognized patients received tidal volumes of 8 mL/kg ideal body weight or less, indicating substantial underrecognition and undertreatment. Mortality rates correlate with severity classification from the Berlin definition validation cohort: mild ARDS carries a mortality of approximately 27%, moderate ARDS approximately 32%, and severe ARDS approximately 45%. Overall mortality from ARDS has decreased from greater than 60% in the pre-lung-protective ventilation era to approximately 35-40% in the modern era, largely attributable to the adoption of low tidal volume ventilation and other evidence-based management strategies.

Risk Factors and Etiology

The risk factors for ARDS are broadly categorized into direct (pulmonary) and indirect (extrapulmonary) causes. Direct pulmonary insults include pneumonia, which is the most common cause of ARDS and accounts for approximately 60% of cases, aspiration of gastric contents, inhalational injury from smoke or toxic gases, pulmonary contusion from thoracic trauma, and near-drowning. Indirect extrapulmonary causes include non-pulmonary sepsis, acute pancreatitis, trauma with hemorrhagic shock, massive transfusion triggering transfusion-related acute lung injury (TRALI), extensive burns, and disseminated intravascular coagulation. The distinction between direct and indirect causes has pathophysiologic implications: direct injury primarily damages the alveolar epithelium, while indirect injury initially affects the pulmonary endothelium, although in both cases the final common pathway is diffuse alveolar damage.

Pathophysiology

Phases of ARDS

The pathological evolution of ARDS follows a characteristic temporal sequence through three overlapping phases. The exudative phase, spanning approximately days 1 through 7, is characterized by diffuse alveolar damage (DAD), the histopathologic hallmark of ARDS. During this phase, disruption of both the endothelial and epithelial barriers leads to flooding of the alveolar spaces with protein-rich edema fluid. Hyaline membranes, composed of precipitated plasma proteins and cellular debris, form along the denuded alveolar surfaces. Neutrophils infiltrate the interstitium and alveolar spaces, releasing proteases and reactive oxygen species that amplify tissue injury. Surfactant dysfunction, resulting from both type II pneumocyte damage and inactivation by leaked plasma proteins, leads to increased alveolar surface tension and progressive atelectasis.

The proliferative phase, occurring approximately from days 7 through 21, represents the transition toward repair. Type II pneumocytes proliferate to cover the denuded basement membrane, eventually differentiating into type I pneumocytes to restore the alveolar epithelial barrier. Fibroblast proliferation begins, and the hyaline membranes become organized by ingrowth of fibrous tissue. In many patients, resolution commences during this phase, with reabsorption of alveolar edema and restoration of barrier integrity.

The fibrotic phase, developing after approximately day 21, is characterized by extensive collagen deposition and architectural distortion of the lung parenchyma. In patients who progress to this stage, fibroproliferative ARDS ensues, leading to prolonged ventilator dependence, significantly impaired gas exchange, and poor clinical outcomes. Not all patients progress through all three phases; many resolve during the exudative or early proliferative stages, and the proportion progressing to fibrosis depends on the severity and duration of the initial insult as well as ventilator management strategies.

Pathophysiologic Consequences

The pathophysiologic derangements in ARDS produce several clinically significant consequences. Massive ventilation-perfusion (V/Q) mismatch and intrapulmonary shunt develop as blood perfuses non-aerated, edema-filled, or atelectatic alveoli, resulting in refractory hypoxemia that responds poorly to supplemental oxygen alone. Lung compliance is markedly reduced as the alveoli fill with edema and inflammatory debris, producing the characteristic "stiff lung" on the ventilator. Pulmonary hypertension develops through a combination of hypoxic pulmonary vasoconstriction, vascular remodeling from inflammation and endothelial injury, and formation of microthrombi within the pulmonary vasculature.

The "baby lung" concept, introduced by Gattinoni and colleagues, is fundamental to understanding ARDS ventilatory management. CT imaging studies demonstrated that in ARDS, only a small proportion of the total lung volume remains normally aerated, with the remainder consisting of consolidated, atelectatic, or edematous tissue. This small aerated region, termed the "baby lung," functions as the effective gas-exchanging compartment. The critical implication is that tidal volumes must be targeted to this small functional lung rather than the anatomical lung size, because delivering a "normal" tidal volume based on the patient's body weight concentrates the entire delivered volume into a fraction of the expected lung, producing dangerously high alveolar distending pressures and perpetuating ventilator-induced lung injury.

<image>A three-panel illustration showing the histopathological phases of ARDS. Panel 1 (Exudative, days 1-7): show a damaged alveolus with disrupted epithelium (denuded Type I cells), hyaline membranes lining the alveolar surface, protein-rich edema fluid in the alveolar space, activated neutrophils, and damaged capillary endothelium with red blood cells leaking into the alveolus. Panel 2 (Proliferative, days 7-21): show Type II pneumocyte proliferation attempting to cover the basement membrane, early fibroblast activity, organizing hyaline membranes, and reduced edema. Panel 3 (Fibrotic, days 21+): show dense collagen deposition, architectural distortion with obliteration of normal alveolar structure, thickened interstitium, and neovascularization. Label all key structures in each phase. Include a timeline bar across the bottom.</image>

Mechanical Ventilation in ARDS

Lung-Protective Ventilation (Core Strategy)

Lung-protective ventilation represents the cornerstone of ARDS management and is the single intervention with the most robust evidence for mortality reduction. The strategy centers on low tidal volume ventilation at 6 mL/kg of ideal body weight (IBW). Ideal body weight is calculated using standardized formulas: for males, IBW = 50 + 2.3 multiplied by the difference between height in inches and 60; for females, IBW = 45.5 + 2.3 multiplied by the difference between height in inches and 60. The landmark ARDSNet ARMA trial, published in 2000, randomized patients with ARDS to receive either 6 mL/kg or 12 mL/kg IBW and demonstrated a 22% relative mortality reduction (31% versus 40% absolute mortality) in the low tidal volume group. This trial remains one of the most practice-changing studies in critical care medicine. A critical and common error is the use of actual body weight rather than ideal body weight for tidal volume calculation, particularly in obese patients where this leads to dangerously excessive tidal volumes.

Plateau pressure (Pplat), measured with an inspiratory hold maneuver, reflects the elastic recoil pressure of the lung and chest wall at end-inspiration and should be maintained at 30 cmH2O or less. Driving pressure, calculated as the difference between plateau pressure and PEEP (deltaP = Pplat - PEEP), represents the pressure distending the functional lung relative to its resting volume. A retrospective analysis by Amato and colleagues published in 2015 identified driving pressure as the ventilator variable most strongly associated with survival in ARDS, with values exceeding 14 cmH2O correlating with increased mortality. Although this finding has not yet been validated in a prospective randomized controlled trial, driving pressure of 14 cmH2O or less has become a widely adopted ventilator target.

PEEP is applied to prevent end-expiratory alveolar derecruitment and maintain the functional residual capacity of the baby lung. The selection of optimal PEEP remains a subject of ongoing investigation. Permissive hypercapnia is accepted as a consequence of lung-protective tidal volumes, with PaCO2 values up to 60-70 mmHg tolerated provided the arterial pH remains above 7.20. This approach should be avoided in patients with traumatic brain injury, where intracranial pressure management takes precedence. FiO2 is titrated to maintain a PaO2 of 55 to 80 mmHg or an SpO2 of 88% to 95%, with both hyperoxia and severe hypoxemia being avoided.

PEEP Strategies

The optimal approach to PEEP titration in ARDS remains one of the most debated topics in critical care. The ARDSNet low PEEP/FiO2 table and high PEEP/FiO2 table provide protocolized PEEP selection based on FiO2 requirements, but meta-analyses comparing these strategies have not demonstrated a consistent mortality difference. An individual patient data meta-analysis by Briel and colleagues in 2010 suggested that higher PEEP may benefit patients with moderate to severe ARDS, while conferring no benefit or potential harm in mild ARDS. The ART trial, which evaluated an aggressive open-lung strategy combining a maximal recruitment maneuver with titrated high PEEP, demonstrated increased mortality in the intervention group, establishing that recruitment maneuvers using sustained high pressures are not recommended.

Physiologically guided PEEP strategies have been investigated as alternatives to empiric tables. Electrical impedance tomography (EIT) provides real-time imaging of regional ventilation distribution and can identify the PEEP level that balances overdistension and collapse. Esophageal pressure-guided PEEP titration, using an esophageal balloon catheter to estimate transpulmonary pressure, is physiologically rational as it accounts for chest wall mechanics. However, the EPVent-2 trial demonstrated that esophageal pressure-guided PEEP did not improve mortality compared with empiric high PEEP, tempering enthusiasm for this approach as a routine strategy while acknowledging its potential utility in specific clinical scenarios such as morbid obesity.

Prone Positioning

Prone positioning has emerged as one of the most impactful interventions in ARDS management. The PROSEVA trial, published in 2013, demonstrated that prone positioning for 16 hours or more per day in patients with moderate to severe ARDS (PaO2/FiO2 less than 150 mmHg) reduced 28-day mortality from 32.8% to 16.0%, yielding a number needed to treat of approximately 6. This near-halving of mortality established prone positioning as a practice-changing intervention with one of the strongest treatment effects observed in critical care medicine.

The physiologic mechanisms underlying the benefit of prone positioning are multifaceted. Proning improves V/Q matching by redistributing perfusion more uniformly relative to ventilation. The dorsal lung regions, which are the largest zones of the lung and are compressed by the weight of the heart and edematous lung tissue in the supine position, are recruited when the patient is turned prone. Critically, prone positioning reduces ventilator-induced lung injury by creating a more homogeneous distribution of mechanical stress across the lung parenchyma, decreasing the transpulmonary pressure gradient from ventral to dorsal regions.

Regarding timing and duration, prone positioning should be initiated early, within 12 to 24 hours of meeting criteria, and continued until the patient demonstrates sustained improvement in oxygenation, defined as a PaO2/FiO2 ratio greater than 150 mmHg on PEEP of 10 cmH2O or less and FiO2 of 0.60 or less when returned to the supine position. Contraindications include hemodynamic instability, spinal instability, open abdomen, and recent facial or tracheal surgery. Successful implementation requires an experienced nursing team with specific training in prone positioning protocols, as the procedure involves coordination of multiple team members to ensure safe repositioning and prevention of complications including pressure injuries, endotracheal tube displacement, and vascular access disruption.

Neuromuscular Blockade

The role of neuromuscular blockade in ARDS has evolved with conflicting trial results. The ACURASYS trial, published in 2010, demonstrated that a 48-hour infusion of cisatracurium initiated early in severe ARDS improved 90-day survival (hazard ratio 0.68) compared with placebo. However, the larger ROSE trial, published in 2019, compared cisatracurium infusion for 48 hours with a strategy of light sedation without paralysis and found no difference in 90-day mortality. The discrepancy between these trials may be explained by differences in sedation depth in the control groups: ACURASYS used deep sedation in both arms, whereas ROSE employed a light sedation strategy in the control arm, which itself may have provided benefit. Current practice favors a nuanced approach: short-course neuromuscular blockade for 24 to 48 hours is considered for patients with severe ARDS who exhibit significant ventilator dyssynchrony or refractory hypoxemia despite optimized sedation, but routine use is not recommended.

Adjunctive Therapies

Fluid Management

Conservative fluid management after the initial resuscitation phase is supported by the FACTT trial, published in 2006. This study compared a conservative fluid strategy (targeting central venous pressure less than 4 mmHg or pulmonary artery wedge pressure less than 8 mmHg) with a liberal fluid strategy and demonstrated that the conservative approach improved oxygenation and increased ventilator-free days without a statistically significant difference in mortality, although trends favored conservative management. The concept of deresuscitation, involving active fluid removal after initial hemodynamic stabilization, has gained traction in clinical practice. This is typically achieved with loop diuretics such as furosemide, with or without concomitant albumin infusion to maintain oncotic pressure and facilitate fluid mobilization from the interstitium.

Corticosteroids

Corticosteroids have assumed an increasingly important role in ARDS management based on accumulating evidence. The DEXA-ARDS trial, published in 2020, randomized patients with established moderate to severe ARDS to receive dexamethasone 20 mg intravenously daily for 5 days followed by 10 mg daily for 5 days or placebo. The dexamethasone group experienced significantly more ventilator-free days (12.3 versus 7.5) and reduced 60-day mortality (21% versus 36%). The Meduri protocol, using methylprednisolone for early and unresolving ARDS, has also shown supportive evidence, though the data are not as robust as those from DEXA-ARDS.

The COVID-19 pandemic generated additional corticosteroid evidence relevant to ARDS. The RECOVERY trial established dexamethasone 6 mg daily for 10 days as the standard of care for patients with COVID-19 requiring supplemental oxygen or mechanical ventilation, demonstrating reduced 28-day mortality. Regarding timing, the benefit of corticosteroids is greatest when therapy is initiated early in the course of ARDS, within the first 14 days of onset. Importantly, the ARDSNet LaSRS trial demonstrated that initiating corticosteroids after day 14 of ARDS may be harmful, highlighting the importance of appropriate timing.

ECMO (Extracorporeal Membrane Oxygenation)

Venovenous (VV) ECMO provides extracorporeal gas exchange for patients with refractory hypoxemia in severe ARDS. The EOLIA trial, published in 2018, randomized patients with very severe ARDS (PaO2/FiO2 less than 50 mmHg for more than 3 hours, or PaO2/FiO2 less than 80 mmHg for more than 6 hours, or arterial pH less than 7.25 with PaCO2 of 60 mmHg or greater for more than 6 hours) to VV-ECMO versus conventional management. The trial demonstrated a 60-day mortality of 35% in the ECMO group versus 46% in the control group, but the p-value of 0.09 did not reach conventional statistical significance. Interpretation was further complicated by a high crossover rate, with 28% of the control group receiving rescue ECMO. Bayesian analyses and post hoc investigations suggest a probable survival benefit, and ECMO is now widely considered a rescue therapy for appropriately selected patients.

A critical practical consideration is the importance of early referral to an ECMO center. Transfer of a patient with severe ARDS who is deteriorating despite optimal conventional therapy, including prone positioning, should not be delayed until the patient is too sick to transport safely. During ECMO support, ultra-protective ventilation is employed, with tidal volumes reduced to as low as 2 to 4 mL/kg, respiratory rates below 10 breaths per minute, and low plateau pressures, effectively allowing the injured lungs to rest while the extracorporeal circuit provides gas exchange.

<image>A comprehensive ARDS management algorithm. Start with ARDS diagnosis (Berlin criteria: bilateral opacities, P/F ratio, PEEP >= 5, not cardiogenic). Stratify by severity: Mild (P/F 200-300), Moderate (P/F 100-200), Severe (P/F <= 100). For ALL: lung-protective ventilation (TV 6 mL/kg IBW, Pplat <= 30, driving pressure <= 14), conservative fluid management, treat underlying cause. For Moderate-Severe: prone positioning >= 16 hours/day (PROSEVA), consider dexamethasone (DEXA-ARDS), consider short-course NMB if severe dyssynchrony. For Severe refractory (P/F < 80 despite proning + optimal ventilation): early ECMO referral (EOLIA criteria), inhaled vasodilators (NO, epoprostenol) as bridge. Show a rescue therapy ladder ascending from bottom to top: lung-protective vent -> prone -> NMB -> steroids -> inhaled vasodilators -> ECMO. Include ventilator settings targets at each level. Use green-yellow-red color coding for mild-moderate-severe.</image>

Special Considerations

Ventilator-Induced Lung Injury (VILI)

Ventilator-induced lung injury encompasses several mechanisms by which mechanical ventilation itself can perpetuate and amplify lung damage. Volutrauma, resulting from overdistension of alveoli by excessive tidal volumes, is considered the most important mechanism and provides the rationale for low tidal volume ventilation. Atelectrauma refers to the injury caused by cyclic opening and closing of alveoli during each breath when insufficient PEEP allows end-expiratory alveolar collapse; this repetitive shear stress damages the epithelium and promotes inflammation. Barotrauma describes the macroscopic consequences of elevated airway pressures, including pneumothorax, pneumomediastinum, and subcutaneous emphysema. Biotrauma is the systemic consequence of mechanical stretch, whereby the physical forces applied to lung tissue trigger an inflammatory cascade with release of pro-inflammatory cytokines into both the pulmonary and systemic circulations, contributing to distant organ injury and the development of multiorgan dysfunction syndrome.

Refractory Hypoxemia

The management of refractory hypoxemia in ARDS follows a structured escalation pathway. The first step is to ensure optimization of ventilator settings, confirming that tidal volume is set at 6 mL/kg IBW, PEEP is adequate, and FiO2 is at 1.0. Prone positioning is the single most effective intervention for severe ARDS and should be the first rescue measure employed. Inhaled pulmonary vasodilators, including inhaled nitric oxide at 5 to 40 parts per million or inhaled epoprostenol at 10 to 50 ng/kg/min, selectively dilate the pulmonary vasculature supplying ventilated alveoli, thereby improving V/Q matching and oxygenation. While these agents have not been demonstrated to confer a mortality benefit, they serve as valuable bridging therapies. Neuromuscular blockade reduces oxygen consumption by eliminating the metabolic demands of skeletal muscle contraction and improves chest wall compliance, facilitating more efficient ventilation. VV-ECMO represents the ultimate rescue for refractory hypoxemia when all other interventions have proven insufficient.

Long-Term Outcomes

Survivors of ARDS face a substantial burden of long-term sequelae that extends well beyond hospital discharge. Physical deconditioning and ICU-acquired weakness are common and often debilitating, limiting patients' ability to perform activities of daily living. Cognitive impairment, including deficits in memory, attention, and executive function, affects a significant proportion of survivors. Psychological sequelae are frequent and include post-traumatic stress disorder, depression, and anxiety. Long-term follow-up studies, including the ARDS Long-term Outcomes Study with 5-year follow-up, have demonstrated persistent impairment in exercise capacity and health-related quality of life, with most patients failing to return to their pre-ARDS level of function. Pulmonary function testing at one year typically shows normalization of most parameters, although some patients demonstrate persistent reductions in diffusing capacity (DLCO). Structured follow-up programs and early enrollment in rehabilitation are recommended to optimize long-term recovery.

Key Clinical Pearls

• Always calculate tidal volume using IDEAL body weight (not actual body weight); this is the most common error in ARDS ventilation management
• Prone positioning reduces mortality by nearly 50% in moderate-severe ARDS (PROSEVA: NNT = 6) and should be implemented early (within 12-24 hours) for at least 16 hours/day
• Driving pressure (Pplat - PEEP) <= 14 cmH2O may be a more important ventilator target than Pplat alone; it represents the distending pressure relative to the functional lung size
• Early dexamethasone in moderate-severe ARDS reduces ventilator days and mortality (DEXA-ARDS); corticosteroids started after day 14 of ARDS may be harmful
• ARDS is underrecognized (only 60% clinician recognition in LUNG SAFE) and undertreated; systematic screening for ARDS criteria in all mechanically ventilated patients should be standard practice

References

1. ARDS Definition Task Force, Ranieri VM, Rubenfeld GD, et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526-2533.
2. Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342(18):1301-1308. (ARMA)
3. Guerin C, Reignier J, Richard JC, et al. Prone positioning in severe acute respiratory distress syndrome. N Engl J Med. 2013;368(23):2159-2168. (PROSEVA)
4. Villar J, Ferrando C, Martinez D, et al. Dexamethasone treatment for the acute respiratory distress syndrome: a multicentre, randomised controlled trial. Lancet Respir Med. 2020;8(3):267-276. (DEXA-ARDS)
5. Combes A, Hajage D, Capellier G, et al. Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome. N Engl J Med. 2018;378(21):1965-1975. (EOLIA)

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