Etiology and mortality statistics
ARDS is not an independent disease; it develops as a severe complication of other pathological processes. Clinically and morphologically, this syndrome is analogous to infant respiratory distress syndrome.
The main causes of shock lung include:
- Aspiration of gastric contents (has the highest mortality rate at 93.8%).
- Infectious pulmonary diseases, primarily severe pneumonias (mortality around 60%).
- Various types of shock (including endotoxic) and sepsis (mortality 77.8%).
- DIC syndrome.
- Inhalation of toxic substances, including excess oxygen.
- Narcotic drug overdose.
- Radiation exposure.
- Open-heart surgery using cardiopulmonary bypass.
Overall, the total mortality rate for this syndrome exceeds 60–65%.
Pathogenesis and biochemical mechanisms
The core link in pathogenesis is direct damage to the capillary endothelium and type I pneumocytes, which form the air-blood barrier. This inevitably leads to acute respiratory failure.
In the early period, a key role is played by polymorphonuclear leukocytes (neutrophils), as well as alveolar macrophages and the endothelium itself. Their activation triggers the release of a cascade of damaging factors:
- Proteolytic lysosomal enzymes.
- Free oxygen radicals.
- Nitric oxide.
- Arachidonic acid derivatives (leukotrienes and prostaglandins), which activate phospholipase A.
In parallel, platelets are involved in the process. Under the action of platelet-activating factor, they aggregate and sequester within the pulmonary vascular bed (clinically manifested as thrombocytopenia). Platelets begin to produce growth factor (PDGF), which strongly stimulates sclerosis.
Furthermore, damage to type II pneumocytes impairs the synthesis of surfactant. Deprived of surface-active material, the alveoli collapse, leading to the development of multiple atelectases.
Morphological stages
The morphogenesis of adult respiratory distress syndrome is generally divided into three consecutive stages:
- Preclinical stage. Characterized by initial morphological signs of capillary damage in the alveolar septa. There are no prominent clinical manifestations yet.
- Acute stage. Develops within the first week after exposure to the damaging factor. During this period, interstitial and intra-alveolar edema increases. Microscopy reveals a large number of fibrin and polymorphonuclear leukocytes in the exudate and infiltrates. Characteristic hyaline membranes form on the inner surfaces of the alveoli, and areas of atelectasis appear.
- Stage of exudate organization and proliferation. Organization processes are triggered very early — as early as the 2nd–3rd day of the disease. Active proliferation of type II pneumocytes begins, and the exudate is replaced by connective tissue. The outcome of this stage is the development of interstitial fibrosis.
Clinical presentation and outcomes
Clinically, ARDS manifests as refractory hypoxemia — a critical decrease in blood oxygen levels that cannot be corrected even by increasing the fraction of inspired oxygen in the breathing mixture. The patient's vital capacity (VC) is significantly reduced.
Radiological studies show symptoms of pronounced pulmonary edema. However, there is a crucial differential sign: unlike cardiogenic edema, intracapillary and oncotic pressures are unchanged in ARDS.
As the disease progresses, total interstitial pulmonary fibrosis develops. The proliferation of connective tissue and the obliteration of the vascular bed create insurmountable resistance for the right ventricle of the heart. As a result, the direct cause of death in such patients is cor pulmonale (cardiopulmonary failure).
Briefly: answers to topical questions
What macroscopic changes are observed in the lungs during the acute stage of ARDS?
- Areas of atelectasis.
- Serous-hemorrhagic edema.
- Precipitation of fibrin strands with the formation of hyaline membranes.
To memorize the pathogenesis, use the "P-T-S" triad: Pneumocytes (type I cells die, surfactant from type II cells drops) — Platelets (aggregate, release PDGF growth factor) — Sclerosis (outcome in interstitial fibrosis).
Class and exam questions
How to distinguish ARDS from pulmonary edema in heart failure?
The main differential sign is that in ARDS, intracapillary hydrostatic and oncotic pressures remain within normal limits, whereas in cardiogenic edema they are primarily altered.
Why doesn't oxygen inhalation help in shock lung?
Refractory hypoxemia develops. Due to intra-alveolar edema, the formation of dense hyaline membranes, and alveolar collapse (atelectasis), oxygen physically cannot diffuse across the damaged air-blood barrier.
What role do platelets play in the outcome of ARDS?
Platelets accumulate in the pulmonary capillaries and release platelet-derived growth factor (PDGF). This factor directly stimulates sclerotic processes, ultimately leading to interstitial pulmonary fibrosis.
What else to study on this topic
- Morphology and composition of hyaline membranes in acute alveolar injury
- Mechanisms of free radical and nitric oxide generation by neutrophils
- Synthesis of pulmonary surfactant by type II pneumocytes
- Pathogenesis of cor pulmonale development in fibrosis
- The role of arachidonic acid derivatives in pulmonary inflammatory reactions
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