Acute Respiratory Distress Syndrome: Disease Process

Introduction

Acute respiratory distress syndrome (ARDS) is a life-threatening form of acute lung injury in which inflammation damages the alveolar–capillary barrier. The resulting pulmonary edema, loss of functional alveoli, and impaired gas exchange cause severe hypoxemia that is not primarily explained by heart failure or fluid overload. ARDS can develop after direct lung injury, such as pneumonia or aspiration, or after an indirect systemic insult, such as sepsis, trauma, shock, or transfusion-related lung injury.

Initiating injury

The disease process begins when an inflammatory or physical insult injures either the pulmonary epithelium, the pulmonary vascular endothelium, or both. In direct lung injury, pathogens, gastric contents, smoke, or other substances initially damage the alveolar epithelial surface. In indirect lung injury, inflammatory mediators circulating in the blood—particularly during sepsis or severe shock—first activate and injure the pulmonary endothelium.

Alveolar macrophages and injured epithelial cells respond by releasing cytokines and chemokines, including tumor necrosis factor-α, interleukin-1β, and interleukin-6. These signals recruit neutrophils and other immune cells into the pulmonary circulation and alveolar spaces. Although this response is intended to eliminate the original threat, it becomes excessive and causes secondary damage to normal lung tissue.

Breakdown of the alveolar–capillary barrier

Normally, the alveolar–capillary barrier is extremely thin, allowing oxygen and carbon dioxide to diffuse efficiently while preventing large amounts of fluid from entering the air spaces. It is formed mainly by:

• Type I alveolar epithelial cells, which cover most of the alveolar surface and support gas exchange
• Type II alveolar epithelial cells, which produce surfactant and help repair damaged epithelium
• Pulmonary capillary endothelial cells, which line the blood vessels
• Junctions and basement membranes that maintain barrier integrity

In ARDS, inflammatory mediators, neutrophil enzymes, reactive oxygen species, and mechanical stress disrupt this barrier. Endothelial junctions separate, epithelial cells undergo apoptosis or necrosis, and the normally tight barrier becomes highly permeable. Protein-rich fluid then leaks from the pulmonary capillaries into the interstitial tissue and alveoli.

Pulmonary edema and surfactant dysfunction

The leaked fluid accumulates inside the alveoli, producing noncardiogenic pulmonary edema. This fluid contains plasma proteins that interfere with surfactant, while injury to type II alveolar cells reduces new surfactant production. Because surfactant normally lowers surface tension, its loss makes alveoli more likely to collapse, producing atelectasis.

The combination of alveolar flooding and collapse reduces the amount of lung available for ventilation. Blood may continue flowing through poorly ventilated or collapsed alveoli, creating a ventilation–perfusion mismatch and intrapulmonary shunt. Oxygen is therefore unable to move effectively from the alveoli into the bloodstream, producing severe hypoxemia.

Impaired fluid clearance

Healthy alveoli actively remove excess fluid through epithelial ion channels and transport proteins. Sodium is transported out of the alveolar space, and water follows osmotically. In ARDS, epithelial injury disrupts these ion channels and transport systems. The damaged lung therefore loses both its ability to prevent fluid entry and its ability to remove fluid that has already accumulated.

This creates a self-reinforcing cycle:

Inflammation \rightarrow barrier injury \rightarrow alveolar edema \rightarrow epithelial dysfunction \rightarrow impaired fluid clearance

As edema increases, oxygen diffusion becomes more difficult and carbon dioxide removal may also deteriorate.

Neutrophils, oxidative stress, and coagulation

Neutrophils are important contributors to ARDS progression. After entering the lung, they release proteases, reactive oxygen species, and neutrophil extracellular traps. These substances can destroy pathogens, but they can also injure epithelial and endothelial cells. Oxidative stress damages cell membranes, proteins, and DNA, further increasing vascular permeability.

At the same time, endothelial injury activates coagulation. Platelets adhere to activated endothelium, and small clots may develop within pulmonary microvessels. These microthrombi obstruct portions of the pulmonary circulation, increase dead-space ventilation, and worsen the mismatch between ventilation and perfusion. Thus, ARDS involves not only inflammation and edema but also abnormalities in pulmonary blood flow.

Clinical phases

ARDS is often described in overlapping phases:

Exudative phase

The early phase is dominated by inflammation, endothelial and epithelial injury, alveolar flooding, and hyaline membrane formation. Hyaline membranes are made from protein-rich edema fluid and cellular debris along the damaged alveolar walls. They further impair oxygen diffusion.

Proliferative phase

During the intermediate phase, type II alveolar cells proliferate in an attempt to restore the epithelial lining. Fibroblasts may also become activated, and the lung begins to reorganize damaged tissue. Some patients recover substantially during this phase, while others continue to deteriorate.

Fibrotic phase

In severe or persistent ARDS, excessive fibroblast activity and collagen deposition can produce pulmonary fibrosis. The lungs become stiffer, compliance decreases, and the work required to breathe increases. Not every patient develops prominent fibrosis, but persistent inflammation and mechanical ventilation can contribute to this progression.

How the disease produces respiratory failure

The major physiological consequences of ARDS are:

1. Reduced oxygen diffusion: Fluid and cellular debris increase the distance oxygen must cross.
2. Alveolar collapse: Surfactant dysfunction and increased surface tension cause atelectasis.
3. Intrapulmonary shunting: Blood passes through poorly ventilated alveoli and remains inadequately oxygenated.
4. Reduced lung compliance: Edema, collapse, and fibrosis make the lungs stiff.
5. Increased work of breathing: Respiratory muscles must generate greater pressure to expand the lungs.
6. Possible carbon dioxide retention: Severe disease, fatigue, or extensive dead-space ventilation can impair carbon dioxide removal.

The patient may initially compensate with rapid breathing, but progressive lung injury can lead to respiratory muscle fatigue, worsening hypoxemia, and the need for ventilatory support.

Summary

ARDS is best understood as a syndrome of uncontrolled inflammatory lung injury. An initial pulmonary or systemic insult activates immune cells and damages the alveolar epithelium and pulmonary endothelium. Breakdown of the alveolar–capillary barrier allows protein-rich fluid to enter the alveoli, while surfactant loss and impaired epithelial fluid clearance promote alveolar collapse and edema. Neutrophil-mediated injury, oxidative stress, and pulmonary microthrombi amplify the damage. The final result is severe ventilation–perfusion mismatch, stiff noncompliant lungs, and refractory hypoxemic respiratory failure.

Current treatment is primarily supportive and focuses on treating the underlying cause while limiting additional ventilator-induced injury through lung-protective strategies.