Main forms and general characteristics
Pulmonary perfusion pathology manifests as deviations of blood pressure in the lesser circulation vessels from normal values. In clinical practice, two opposite states are distinguished:
- Pulmonary hypertension — persistent elevation of pressure.
- Pulmonary hypotension — persistent decrease in pressure.
Each of these conditions develops under the influence of specific hemodynamic mechanisms affecting the microvasculature, the heart, or the total circulating blood volume.
Pulmonary hypertension: precapillary and postcapillary causes
Pulmonary hypertension is classified into three main forms: precapillary, postcapillary, and mixed.
- The precapillary form is accompanied by a rise in pressure in the precapillaries and capillaries, where systolic values exceed 30 mmHg and diastolic values exceed 12 mmHg. The main trigger here is the spasm of arteriolar smooth muscle cells, provoked by stress, pulmonary embolism (PE), catecholamine release in pheochromocytoma, acidosis, and alveolar hypoxia (Euler-Liljestrand reflex). Another cause can be microvascular obstruction by emboli or compression by tumors and bronchial asthma.
- The postcapillary form is caused by impaired blood outflow into the left atrium, leading to an excess accumulation of blood in the lungs. This is caused by mitral valve stenosis, compression of the pulmonary veins by tumors or lymph nodes, and left ventricular heart failure.
Mixed form and pulmonary hypotension
The mixed form of hypertension occurs as a complication and result of the progression of pre- or postcapillary disorders. For instance, hindered blood outflow into the left atrium triggers reflex narrowing of the pulmonary arterioles. Common manifestations of hypertension include hypoxemia, hypercapnia, acidosis, decreased vital capacity (VC), and signs of heart failure with congestion.
In contrast, pulmonary hypotension is characterized by a persistent drop in pulmonary vascular pressure. It is caused by congenital heart defects with venous blood shunting into the arterial bed (e.g., Tetralogy of Fallot), marked hypovolemia due to vomiting or shock, and systemic arterial hypotension during collapse and cardiac decompensation.
Briefly: answers to topical questions
What is the pathogenesis of the Kitaev reflex in mitral valve stenosis?
The pathogenesis of the Kitaev reflex involves reflex spasm of the pulmonary arterioles. This process produces active (arterial) pulmonary hypertension, which arises as a compensatory mechanism to protect capillaries from edema. The spasm develops in response to the transmission of elevated pressure from the left atrium into the pulmonary veins and capillaries (so-called passive venous hypertension resulting from blood flow obstruction in mitral stenosis).
What is the molecular mechanism of hypoxic pulmonary vasoconstriction (the Euler-Liljestrand reflex)?
According to literature sources, the mechanism of the Euler-Liljestrand reflex is as follows: an acute decrease in the partial pressure of oxygen in inhaled air, i.e., alveolar hypoxia, causes smooth muscle cell spasm in the walls of pulmonary arterioles.
The pathogenesis indicates the production of vasoconstrictor mediators, including:
- catecholamines;
- endothelin;
- thromboxane A2.
What other heart defects, besides Tetralogy of Fallot, lead to pulmonary hypotension due to right-to-left shunting?
In addition to Tetralogy of Fallot, pulmonary valve regurgitation leads to pulmonary hypotension due to right-to-left blood shunting. In such heart defects, there is a direct shunt of venous blood into the arterial system, which causes a persistent decrease in blood pressure in the vessels of the lesser circulation.
Pulmonary hypertension rests on three pillars: Pre (arteriolar spasm and hypoxia), Post (congestion due to the left atrium), and Mixed (their combination as the disease progresses).
Class and exam questions
What criteria define precapillary pulmonary hypertension?
The condition is diagnosed when pressure in the precapillaries and capillaries increases, with systolic pressure exceeding 30 mm Hg and diastolic pressure exceeding 12 mm Hg.
What are the main causes of the postcapillary form of pulmonary hypertension?
This form develops due to impaired blood outflow into the left atrium. The main causes are mitral stenosis, compression of pulmonary veins by lymph nodes or tumors, and left ventricular heart failure.
What causes pulmonary hypotension?
A decrease in pulmonary vascular pressure is caused by heart defects with right-to-left blood shunting (Tetralogy of Fallot), hypovolemia in shock or diarrhea, as well as systemic arterial hypotension and collapse.
What else to study on this topic
- The Euler-Liljestrand reflex and mechanisms of alveolar hypoxia
- The role of endothelin and thromboxane A2 in pulmonary vasoconstriction
- Pathogenesis of respiratory and mixed acidosis in chronic hypertension
- Hemodynamic consequences of right-to-left blood shunting in congenital heart defects
- Impact of left and right ventricular failure on the lesser circulation
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