Understanding the Causes of Pulmonary Hypertension

Pulmonary hypertension, once referred to by various names, has now been standardized and can be understood in a similar context to general hypertension, but specifically affecting the arteries in the lungs. This condition has multiple causes, and based on underlying mechanisms and etiology, pulmonary hypertension is broadly categorized into five main groups.

Type 1: Pulmonary Arterial Hypertension (PAH)

This category refers to hypertension caused directly by disease within the pulmonary arteries themselves. The most common subtypes include idiopathic pulmonary arterial hypertension, which has no known cause, pulmonary hypertension linked to connective tissue disorders, and cases resulting from congenital heart defects. These conditions lead to narrowing or obstruction of the small pulmonary arteries, increasing pressure within the lung vasculature.

Type 2: Pulmonary Hypertension Due to Left Heart Disease

This form of pulmonary hypertension is closely associated with dysfunction of the left side of the heart. It commonly occurs in patients suffering from left-sided heart failure or valvular diseases such as mitral valve regurgitation or aortic stenosis. The increased pressure in the left heart chambers is transmitted back into the pulmonary circulation, leading to elevated pulmonary artery pressures.

Type 3: Pulmonary Hypertension Induced by Lung Diseases and Hypoxia

Chronic lung conditions and prolonged low blood oxygen levels can lead to this type of pulmonary hypertension. It is often seen in individuals with chronic obstructive pulmonary disease (COPD), interstitial lung disease, or developmental abnormalities of the lungs. The reduced oxygen levels cause the pulmonary blood vessels to constrict, increasing pressure over time.

Type 4: Chronic Thromboembolic Pulmonary Hypertension (CTEPH)

This type develops as a result of chronic or recurring blood clots in the pulmonary arteries. Unlike acute pulmonary embolism, CTEPH involves unresolved clots that can cause permanent blockages and increased pressure in the pulmonary circulation. It can occur after repeated episodes of pulmonary embolism or in individuals with clotting disorders.

Type 5: Pulmonary Hypertension with Unclear or Multifactorial Causes

This category includes pulmonary hypertension that arises from a variety of systemic or metabolic conditions. One of the more commonly observed causes in this group is pulmonary hypertension associated with thyroid disorders, such as hyperthyroidism or hypothyroidism. Other metabolic, hematologic, or systemic diseases can also contribute to this type of pulmonary hypertension, often through complex and poorly understood mechanisms.