RESPIRATORY CONDITIONS

Acute Respiratory Distress Syndrome

Etiology and Risk Factors

ARDS may follow a direct pulmonary injury or an indirect systemic inflammatory insult.

Direct pulmonary causes include:

• Pneumonia
• Aspiration of gastric contents
• Toxic inhalation
• Near drowning
• Pulmonary contusion

Indirect causes include:

• Sepsis
• Pancreatitis
• Shock
• Severe nonthoracic trauma
• Multiple blood-product transfusions

Sepsis is an important precipitating condition because systemic inflammatory activation can injure the pulmonary capillary endothelium even when the original infection is outside the lungs (Sisson et al., 2019).

Pathophysiology

An injurious event activates the innate immune response. Alveolar macrophages and other immune cells recognize microbial products or signals released by damaged tissue. Proinflammatory mediators, including interleukin-1 beta, tumor necrosis factor, interleukin-6, and interleukin-8, recruit and activate neutrophils and monocytes within the lungs (Sisson et al., 2019).

Activated inflammatory cells release proteases, reactive oxygen species, eicosanoids, cytokines, and other mediators that damage the alveolar epithelium and pulmonary capillary endothelium. Loss of barrier integrity allows protein-rich fluid to enter the pulmonary interstitium and alveolar spaces (Sisson et al., 2019).

Alveolar fluid interferes with surfactant activity. Increased surface tension promotes alveolar instability, collapse, and atelectasis. Alveolar filling and collapse reduce functional lung volume and compliance, increase the work of breathing, and create extensive low ventilation-perfusion ratios and intrapulmonary shunting (Norris, 2020; Sisson et al., 2019).

Blood continues to perfuse poorly ventilated or unventilated alveoli. As the shunt fraction increases, supplemental oxygen becomes progressively less effective. This mechanism accounts for the severe and sometimes refractory hypoxemia associated with ARDS (Sisson et al., 2019).

Lung injury is heterogeneous. Relatively preserved alveoli may lie next to collapsed or consolidated lung units. Mechanical ventilation can overdistend the remaining aerated alveoli when excessive volumes or pressures are used, contributing to additional lung injury (Sisson et al., 2019).

Clinical Manifestations

Early manifestations may include:

• Acute or progressively worsening dyspnea
• Tachypnea
• Increased work of breathing
• Hypoxemia
• Tachycardia
• Inspiratory crackles
• Respiratory deterioration following a recognized clinical insult

Chest imaging may initially appear normal. Bilateral opacities can develop as interstitial and alveolar edema progresses. Severe disease can produce profound hypoxemia, respiratory-muscle fatigue, diffuse pulmonary opacification, and the need for mechanical ventilation (Sisson et al., 2019).

PaCO₂ may initially be normal or decreased because tachypnea increases minute ventilation. Hypercapnia can develop later as respiratory-muscle fatigue, reduced effective ventilation, and increasing physiologic dead space impair carbon dioxide elimination (Sisson et al., 2019).

Diagnostic Evaluation

ARDS is recognized through the combined assessment of timing, imaging, the likely origin of pulmonary edema, and the degree of impaired oxygenation.

Findings that support the diagnosis include:

• New or worsening respiratory symptoms within 1 week of a known clinical insult
• Bilateral pulmonary opacities not adequately explained by pleural effusion, atelectasis, or pulmonary nodules
• Respiratory failure not fully explained by cardiac failure or fluid overload
• Impaired oxygenation in a compatible clinical setting

The PaO₂/FiO₂ ratio compares arterial oxygen tension with the fraction of inspired oxygen. A decreasing ratio indicates worsening oxygen-transfer impairment. The ratio should be interpreted with the patient’s ventilatory support, hemodynamic status, imaging, and overall clinical presentation rather than used as a stand-alone diagnostic test (Norris, 2020; Sisson et al., 2019).

Oxygenation Severity

Severity	PaO₂/FiO₂ ratio
Mild	Greater than 200 through 300 mm Hg
Moderate	Greater than 100 through 200 mm Hg
Severe	100 mm Hg or less

These categories derive from the Berlin classification and are intended for use within its complete diagnostic framework, including the applicable positive-pressure requirement. They should not be applied solely from an isolated arterial blood gas result.

Diagnostic definitions continue to evolve. Current institutional critical-care standards should be used when applying ARDS criteria in clinical practice.

Expected Diagnostic Findings

Findings commonly associated with ARDS include:

• Acute hypoxemic respiratory failure
• Bilateral pulmonary opacities
• Reduced PaO₂/FiO₂ ratio
• Reduced lung compliance
• Increasing oxygen or ventilatory requirements
• A recent serious pulmonary or systemic insult
• Lack of a cardiac or volume-overload explanation sufficient to account for the respiratory failure

Histologically, ARDS is associated with diffuse alveolar damage, inflammatory-cell infiltration, epithelial injury, and hyaline-membrane formation. Lung biopsy is not generally required to recognize the clinical syndrome (Sisson et al., 2019).

Differential Diagnosis

Cardiogenic Pulmonary Edema

Both disorders can produce hypoxemia, crackles, and bilateral pulmonary opacities. Cardiogenic edema is driven primarily by increased pulmonary hydrostatic pressure. ARDS results from inflammatory disruption of the alveolar-capillary barrier. Cardiac history, examination findings, fluid status, imaging, and cardiac evaluation help determine the dominant mechanism (Norris, 2020; Sisson et al., 2019).

Multifocal Pneumonia

Pneumonia may mimic ARDS or precipitate it. The distinction depends on the distribution and progression of pulmonary injury, oxygenation impairment, microbiologic findings, and whether diffuse respiratory failure develops beyond the original infection (Sisson et al., 2019).

Pulmonary Embolism

Pulmonary embolism can cause abrupt hypoxemia, tachypnea, pleuritic pain, and respiratory distress. It primarily creates ventilation without adequate perfusion, increasing physiologic dead space. ARDS predominantly causes low ventilation-perfusion ratios and shunting through alveolar filling and collapse (Sisson et al., 2019).

Treatment and Management

Management focuses on treating the precipitating condition while supporting oxygenation, ventilation, and organ perfusion.

Clinical priorities include:

• Identify and treat the underlying cause, such as sepsis, pneumonia, aspiration, shock, or another serious insult.
• Provide supplemental oxygen and escalate respiratory support according to hypoxemia and work of breathing.
• Initiate mechanical ventilation when progressive hypoxemia, fatigue, or ventilatory failure prevents adequate spontaneous gas exchange.
• Use a ventilatory strategy that limits excessive alveolar volume and pressure because heterogeneous lung injury places aerated alveoli at risk for overdistention.
• Apply positive end-expiratory pressure as clinically indicated to support alveolar recruitment and oxygenation while monitoring pulmonary and hemodynamic effects.
• Evaluate cardiac function, intravascular volume, and tissue perfusion.
• Reassess oxygenation, ventilation, mental status, work of breathing, airway pressures, hemodynamics, urine output, and other indicators of organ function (Norris, 2020; Sisson et al., 2019).

This page provides a pathophysiology and diagnostic-reasoning foundation rather than a complete critical-care treatment protocol.

Complications and Red Flags

Urgent escalation is required for:

• Rapidly increasing oxygen requirements
• Persistent or worsening hypoxemia
• Exhaustion or declining mental status
• Inability to protect the airway
• Rising PaCO₂ with acidemia
• Hemodynamic instability
• Progressive organ dysfunction
• Evidence of barotrauma or other complications of mechanical ventilation

ARDS may accompany broader multiorgan dysfunction. Survivors may require months for functional recovery, and some develop persistent pulmonary fibrosis or reactive airway disease (Norris, 2020; Sisson et al., 2019).

Clinical Reasoning Priorities

The central diagnostic task is to recognize acute diffuse lung injury, determine whether cardiac failure or fluid overload adequately explains the findings, evaluate the severity of oxygenation impairment, and identify the precipitating condition.

Bilateral pulmonary opacities should not automatically be attributed to cardiogenic pulmonary edema. Integrate the time course, oxygenation impairment, cardiac findings, fluid status, imaging pattern, and exposure to recognized ARDS triggers.

Severe hypoxemia that responds poorly to increasing inspired oxygen suggests substantial intrapulmonary shunting and advanced gas-exchange impairment.

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References

Norris, T. L. (2020). Porth’s essentials of pathophysiology (5th ed.). Wolters Kluwer.

Sisson, T. H., Claar, D., Chesnutt, M. S., & Prendergast, T. J. (2019). Pulmonary disease. In G. D. Hammer & S. J. McPhee (Eds.), Pathophysiology of disease: An introduction to clinical medicine (8th ed.). McGraw Hill Education.