Knowledge of pulmonary hypertension pathophysiology and the different types of the disease based on it provides greater insight into the disease than a mere definition of it. It provides a more complete answer to the question – what is pulmonary hypertension?
Pulmonary hypertension is higher than normal pressure within the structures of the pulmonary circulation. The pulmonary circulation is the movement of oxygen depleted blood away from the right side of the heart to the lungs through a network of arteries, and then back to the left side of the heart through a network of veins after replenishing of it with oxygen during its passage through the lungs.
It is distinct from systemic circulation which is the flow of blood from the left side of the heart to all the other tissues of the body through a network of arteries and the return of that blood to the right side of the heart through a network of veins. Despite the fact that they are distinct in their function and pressures, they are part of the same closed blood flow circuit. Thus, a dysfunction of one part of the system can impact the other.
The unit for measuring pressure in either system of blood flow is millimeters of mercury (mmHg). A measured number of mm/Hg refers to the height in mm that a column of mercury rises as a result of the pressure against the walls of an artery or other structure due to the flow of blood through it. For example, a systolic right ventricular pressure of 15 mmHg means that the pressure against the walls of the right ventricle when the heart contracts causes the height of a column of mercury to rise 15 mm.
By definition, pulmonary hypertension is a mean pulmonary artery pressure of 25 mmHg or greater at rest. The diagnosis usually requires right heart catheterization to measure the pressure.
Mean arterial pressure is the averaged force exerted against the walls of an artery during the cardiac cycle. The cardiac cycle is the time period and all of the sequence of events from the beginning of one heartbeat to the next. It includes all of systole and diastole. Mean arterial pressure is not a direct measurement but a calculation. It is twice the diastolic pressure plus the systolic pressure all divided by 3. The formula is MAP = (2(DBP) + SBP)/3.
Pulmonary hypertension pathophysiology
Pressure within the pulmonary arteries can increase because the volume of blood in them exceeds normal or the size of their lumens is smaller than normal. Factors that increase pulmonary pressure can act primarily on the network of veins or arteries of the lungs or both jointly.
Factors that act on the veins do so by causing back pressure as a result of decreased outflow of blood from the left side of the heart into the systemic circulation. This leads to an increase in the volume of the veins and subsequently the arteries as well.
Factors that act upon the arterial bed of the lungs affect either or a combination of the three layers of the blood vessels. They are the following:
- tunica media (media) – the middle layer of a blood vessel, composed of smooth muscle
- tunica intima (intima) – the inner layer of a blood vessel, composed of endothelial cells
- tunica adventitia (adventitia) – the outermost layer of connective tissue that covers a blood vessel, organ, or other body structure
Pulmonary Hypertension Classification
The current classification system is a creation of the World Health Organization. It consists of 5 groups. The basis of it is the pathophysiology and pathogenesis of the pulmonary pressure increase and the diseases associated with it. The system is as follows:
Group 1: Pulmonary arterial hypertension
Group 2: Pulmonary hypertension due to left heart disease
Group 3: Pulmonary hypertension due to lung diseases and/or hypoxia
Group 4: Chronic thromboembolic pulmonary hypertension
Group 5: Multifactorial pulmonary hypertension
Classification and Pathophysiology Correlations
Group 1 disease is distinct from the others not just in how and why it develops, but also in name. Its specific name is pulmonary arterial hypertension (PAH) because it seems to start within the arterial bed. Its former name was primary pulmonary hypertension, but that term is now outdated. In most cases the cause is unknown. But in some cases, heredity, connective tissue disease, HIV infection, drugs, toxins and/or schistosomiasis – a specific type of flatworm infection – are involved.
The primary defect in PAH is narrowing of the arteries that carry blood from the right ventricle to the lungs. The narrowing can be the result of vasoconstriction or other mechanisms involving any of the three layers of the arteries. Other types of pathology that might exist and the layers that might be involved are the following:
- Medial hypertrophy – thickening of the muscle of the media
- Intimal proliferation – an increase in the number of cells of the intima
- Adventitial fibrosis – the formation of scar tissue in the adventia
PULMONARY HYPERTENSION DUE TO LEFT HEART DISEASE
Left heart disease which comprises group 2 leads to elevated pressure in the pulmonary vascular bed due to a rise in the fluid volume. As the outflow of blood from the left side of the heart decreases, fluid backs up into the blood vessels of the lungs. The most common diseases in this group are left-sided heart failure and valvular heart disease. The heart failure can be systolic failure, diastolic failure, or a combination of the two.
PULMONARY HYPERTENSION DUE TO LUNG DISEASES AND/OR HYPOXIA
Chronic lung disease can cause a rise in the pressure of the pulmonary arterial bed through various mechanisms, depending upon the disease. They include the following:
- Chronic hypoxia which leads to vasoconstriction
- Destruction of the alveolar capillary bed, such as in emphysema
- Compression of the arteries by surrounding scar tissue, such as in pulmonary fibrosis or interstitial lung disease
CHRONIC THROMBOEMBOLIC PULMONARY HYPERTENSION
The disease in this fourth group is the result of a blood clot in a vein breaking off and lodging chronically in one of the two main pulmonary arteries. It results in obstruction of the vessel. Vasoconstriction then follows and leads to remodeling – changes in the structure – of the more distal arterial bed, such that the vessels become narrow.
MULTIFACTORIAL PULMONARY HYPERTENSION
Group 5 pulmonary hypertension does not fit into any of the other 4 groups in terms of cause or disease linkage. Most times, the mechanism of the disease is not well-understood. It tends to occur with a variety of blood, metabolic, renal, and other disorders. Some of the more common ones are sickle cell disease and sarcoidosis.
Pulmonary Hypertension Common Denominators
Regardless of the cause or type of the disease, persistently increased pressure over time, causes the arteries of the lungs to become narrow and thickened. As flow through them decreases, oxygenation drops due to a number of factors. In addition, long-standing increased pulmonary arterial pressure places a strain on the right ventricle, the result of which tends to be right-sided heart failure. Right-sided heart failure is the most common cause of death due to the disease.