Etiological factors and triggers
The development of adult respiratory distress syndrome is always secondary and provoked by severe systemic or local catastrophes. All causes can be divided into several main groups:
- Infectious and traumatic factors: sepsis ranks first (primarily caused by Gram-negative bacteria). The syndrome is also frequently triggered by severe multiple or traumatic brain injury and fat embolism.
- Aspiration syndrome: penetration of aggressive fluids into the respiratory tract, such as acidic gastric contents or water during drowning.
- Toxic effects: poisoning with narcotic substances (e.g., heroin), side effects of certain medications, as well as endogenous intoxication in severe uremia.
- Other causes: severe acute pancreatitis and intravascular volume overload (hypertransfusion) leading to the failure of compensatory mechanisms.
Pathogenesis: from membrane damage to hypoxemia
The development of the syndrome is based on non-cardiogenic pulmonary edema. This process is fundamentally different from classic cardiac edema and proceeds in several stages:
- Trigger mechanism. Toxins, inflammatory mediators (cytokines), and activated neutrophils massively attack the alveolar-capillary membranes.
- Impairment of barrier function. A sharp, uncontrolled increase in the permeability of these membranes becomes the key link.
- Morphological changes. As a result of increased permeability, the alveoli rapidly fill with the liquid part of blood, plasma proteins, and cellular debris. Concurrently, the surfactant system — a substance that prevents alveolar collapse — is destroyed.
- Shunt formation. Fluid-filled and collapsed alveoli cease to participate in gas exchange. Intrapulmonary blood shunting occurs (blood flows through the lungs without being enriched with oxygen), ultimately leading to the development of severe hypoxemia.
The most important hemodynamic characteristic of ARDS is the complete absence of signs of pulmonary hypertension or congestive heart failure in the initial stages. Pulmonary artery wedge pressure remains below 18 mm Hg.
Clinical presentation and diagnostics
The disease is characterized by fulminant dynamics. Respiratory failure progresses rapidly, literally before one's eyes.
The patient experiences pronounced dyspnea, and accessory muscles are actively recruited into the breathing process. On auscultation, the physician hears the classic picture of non-cardiogenic edema — numerous moist rales of various calibers are detected in the lungs.
Instrumental and laboratory methods are used to confirm the diagnosis:
- Chest X-ray: reveals diffuse infiltrative changes corresponding to the picture of interstitial and subsequently massive alveolar pulmonary edema.
- Blood gas analysis: progressive hypoxemia (decreased oxygen levels) is registered, which is joined by hypercapnia (carbon dioxide retention) as compensatory forces become exhausted.
Complications
The course of the syndrome is often complicated by the addition of secondary life-threatening conditions. The most formidable complications include the development of fatal heart failure, a full-blown picture of disseminated intravascular coagulation (DIC syndrome), and severe infectious processes. Due to this cascade of disorders, the overall prognosis for the patient's life remains extremely unfavorable.
Briefly: answers to topical questions
What pathomorphological phases (stages) are distinguished in the classic course of ARDS?
Three successive pathomorphological stages are distinguished in the classic course of adult respiratory distress syndrome.
- Preclinical stage — characterized by initial morphological signs of damage to the capillaries of the alveolar septa.
- Acute stage — develops within the first week, manifested by intra-alveolar and interstitial edema, accumulation of leukocytes and fibrin, formation of hyaline membranes, and development of atelectasis.
- Stage of exudate organization and proliferation of type II pneumocytes — starts on the 2nd–3rd day of the disease and ultimately leads to the development of interstitial fibrosis.
What parameters and mechanical ventilation modes are used to treat patients with ARDS?
In the provided materials, specific mechanical ventilation parameters for ARDS are indicated for the «Lung rest» mode against the background of ECMO, as well as for the situation of permissive hypercapnia.
- «Lung rest» mode against the background of ECMO: ultra-small tidal volume — 3–6 ml/kg of ideal body weight.
- Respiratory rate against the background of ECMO: 10–14 per minute.
- PEEP against the background of ECMO: moderate — 5–10 cm H2O, to prevent alveolar collapse without overdistension.
- Permissive hypercapnia in ARDS: up to 70 mm Hg, if normocapnia is unattainable at mechanical ventilation parameters: tidal volume 6 ml/kg of ideal body weight and respiratory rate 30 per minute.
For quick reproduction of the pathogenesis, use the logical chain: Cytokines ➔ Membrane permeability ➔ Fluid in alveoli ➔ Surfactant breakdown ➔ Blood shunting ➔ Hypoxemia.
Class and exam questions
What is the main hemodynamic difference between ARDS and cardiogenic pulmonary edema?
In ARDS, there are no signs of heart failure and pulmonary hypertension, and the pulmonary artery wedge pressure is within normal limits (< 18 mm Hg).
What auscultatory picture will the doctor hear?
A mass of moist rales of various calibers is heard over all lung fields, which is characteristic of «wet lung» syndrome.
How do blood gas parameters change in this condition?
Steadily progressive hypoxemia (drop in oxygen) and increasing hypercapnia (carbon dioxide retention) are observed.
What else to study on this topic
- Mechanisms of surfactant destruction by cellular debris and plasma proteins.
- The role of activated neutrophils and cytokines in alveolar-capillary membrane damage.
- Pathophysiology of intrapulmonary blood shunting.
- Differential X-ray diagnosis of interstitial and alveolar edema.
- The connection between Gram-negative sepsis and the development of disseminated intravascular coagulation (DIC).
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