Pulmonary edema

Etiology and main types

Pulmonary edema is not a standalone disease; it is a severe syndrome accompanying various pathologies. Depending on the initiating mechanism, two main types are distinguished:

• Hemodynamic (cardiogenic). Develops against the background of heart failure (left ventricular or total).
• Membranogenic. Occurs due to exposure to toxic substances or an active inflammatory process.

Toxic factors include carbon monoxide, organophosphates, combat chemical agents (e.g., phosgene), and pure oxygen under high pressure. The inflammatory process is most commonly associated with pneumonia.

Pathogenesis of hemodynamic edema

Cardiogenic edema is triggered by a decrease in left ventricular myocardial contractility. Acute left ventricular failure develops, progressing through the following cascade:

1. Total cardiac output drops.
2. Residual systolic blood volume increases in the left ventricular cavity.
3. End-diastolic volume naturally increases, along with elevated pressure.
4. Blood pressure in the pulmonary circulation vessels rises rapidly.
5. Effective hydrodynamic pressure in the pulmonary microvessels increases.

At the capillary level, a dual mechanism comes into play: fluid filtration sharply increases in the arterial segment, while its reabsorption critically decreases in the venous segment. The result is massive transudation of fluid into the alveolar cavity.

Foam formation and consequences

Due to high membrane permeability, the exuded edema fluid contains a large amount of protein. During respiration, this protein mass is physically whipped up in the alveoli, forming a persistent foam. The foam rapidly fills the airways, blocking airflow. To combat this phenomenon, antifoaming agents (specifically, ethanol) are used in therapy via inhalation.

The accumulation of fluid and foam leads to severe consequences:

• Critical impairment of gas exchange in the alveoli.
• Development of respiratory hypoxia, which compounds the pre-existing circulatory hypoxia (caused by heart failure).
• Formation of pronounced acidosis.

Features of membranogenic edema

Unlike cardiogenic edema, the initiating factor here is direct damage to the microvessel walls. Their permeability increases pathologically, leading to the leakage of fluid and proteins into the interstitial space and alveoli.

Special attention is paid to iatrogenic oxygen-induced damage. The use of 100% oxygen during mechanical ventilation causes destruction of capillary endothelium and alveolocytes, provoking pronounced interstitial and alveolar edema. Therefore, safe gas mixtures with an oxygen concentration strictly within 30–50% are used to treat hypoxia.

In inflammation (pneumonia), inflammatory mediators increase the permeability of the alveolocapillary membrane, leading to the accumulation of exudate. Membranogenic edema develops extremely rapidly and is fraught with acid-base disorders.

For comparison: pathogenesis of renal edema

Unlike localized pulmonary edema, kidney pathology causes generalized edema. In nephrosis (non-inflammatory parenchymal destruction), nephrotic syndrome develops.

Its basis is autoaggressive immune damage to the basement membrane and glomerular epithelium. This leads to massive protein loss (greater than 3.5 g/day, predominantly albumin) and hypoproteinemia. The initiating factor here is oncotic. A drop in plasma oncotic pressure causes fluid to shift into the interstitium.

Hypovolemia occurs, cardiac output and renal perfusion decrease. This activates the RAS, causing secondary aldosteronism. Meanwhile, the secretion of natriuretic factor does not increase due to reduced venous return. Over time, hydrostatic and lymphogenic factors join the oncotic factor, forming vicious cycles.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of development of secondary aldosteronism in hypovolemia.
• The role of inflammatory mediators in altering the permeability of the alveolocapillary membrane.
• Changes in acid-base balance (ABB) against the background of acute respiratory hypoxia.
• Pathogenesis of nephrotic syndrome: autoaggressive glomerular damage.
• Formation of vicious cycles in generalized edema (lymphogenic and hydrostatic factors).

Other topics in "Chapter 11. Disorders of Water Metabolism. Edema"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.