Etiology and Main Types
Pulmonary edema is not a distinct disease, but a severe syndrome accompanying various pathologies. Depending on the initial mechanism, two main types are distinguished:
- Hemodynamic (cardiogenic). Develops against the background of heart failure (left ventricular or generalized).
- Non-cardiogenic (membrane-mediated). Arises from exposure to toxic agents or an active inflammatory process.
Toxic factors include carbon monoxide, organophosphates, chemical warfare agents (such as phosgene), and high-pressure pure oxygen. The inflammatory process is most commonly associated with pneumonia.
Pathogenesis of Cardiogenic Edema
Cardiogenic edema is triggered by a reduction in myocardial contractility of the left ventricle. Acute left ventricular failure develops according to the following cascade:
- Total cardiac output decreases.
- Residual systolic blood volume increases within the left ventricular cavity.
- End-diastolic volume naturally increases, raising pressure.
- Blood pressure in the pulmonary circulation rises sharply.
- Effective hydrodynamic pressure in the pulmonary microvessels increases.
At the capillary level, a dual mechanism is triggered: fluid filtration increases sharply in the arterial segment, while its reabsorption drops critically in the venous segment. The end result is massive transudation of fluid into the alveolar spaces.
Foam Formation and Consequences
Due to high membrane permeability, the transuded edema fluid contains a large amount of proteins. During breathing, this proteinaceous mass is physically whipped up within the alveoli, forming a stable foam. The foam rapidly fills the airways, blocking airflow. To combat this phenomenon, anti-foaming agents (specifically, ethanol) are administered 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 Non-Cardiogenic Edema
Unlike cardiogenic edema, the primary initiating factor here is direct damage to the microvascular 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 toxicity. The use of 100% oxygen during mechanical ventilation causes destruction of capillary endothelium and alveolocytes, provoking severe 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 alveolar-capillary membrane, leading to the accumulation of exudate. Non-cardiogenic 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 auto-aggressive immune damage to the glomerular basement membrane and epithelium. This leads to massive protein loss (over 3.5 g/day, predominantly albumin) and hypoproteinemia. The initiating factor here is oncotic. The drop in plasma oncotic pressure causes fluid to shift into the interstitium.
This results in hypovolemia, decreased cardiac output, and reduced renal perfusion. This activates the renin-angiotensin-aldosterone system (RAAS), causing secondary aldosteronism. Meanwhile, natriuretic peptide secretion does not increase due to reduced venous return. Over time, hydrostatic and lymphogenic factors join the oncotic factor, creating vicious cycles.