An Overview of PH and PAH

A comprehensive overview of pulmonary hypertension and pulmonary arterial hypertension, particularly regarding WHO classifications and functional classes.


Hilary M. DuBrock, MD: Pulmonary hypertension [PH] is a broad umbrella term that refers to an elevated mean pulmonary artery [PA] pressure that can develop from a variety of causes. We define PH as a mean PA pressure of greater than 20 mm Hg. This definition was recently changed at the 6th World Symposium on Pulmonary Hypertension. Previously, PH had been defined as a mean PA pressure of greater than or equal to 25 mm Hg, but since we now know that a mean PA pressure of greater than 20 mm Hg is abnormal, we have changed the definition of pulmonary hypertension. Pulmonary arterial hypertension [PAH] is a subgroup of pulmonary hypertension that’s characterized by an elevated mean pulmonary artery pressure and elevated pulmonary vascular resistance greater than or equal to 3 Wood units and a normal wedge pressure less than or equal to 15 mm Hg. In addition to these hemodynamic criteria for PAH, patients with PAH are categorized as having World Health Organization Group 1 pulmonary hypertension. This means that they have pulmonary hypertension that is idiopathic, heritable, associated with drug and toxin use, or associated with certain conditions such as connective tissue disease, portal hypertension, congenital heart disease, or other conditions.

Patients with PH and PAH are a heterogeneous group of individuals. There’s not 1 typical patient profile that fits all patients with pulmonary hypertension. We do know, however, that there are consistent risk factors, such as female sex, that are associated with an increased risk of pulmonary hypertension. Women are about 2 times more likely to develop pulmonary hypertension than males. Additionally, in the past, we thought that PAH typically affected young individuals, particularly young females. We now know from more recent US and European registries of pulmonary hypertension, however, that PAH is affecting more and more older individuals in addition to younger individuals.

There are 5 different World Health Organization—defined groups that classify patients with pulmonary hypertension. We divide patients into these 5 groups because there are different etiologies as well as treatment approaches for different types of pulmonary hypertension. Patients with Group 1 pulmonary arterial hypertension have either idiopathic, heritable, or drug and toxin–induced pulmonary arterial hypertension; or PAH associated with certain conditions like connective tissue disease, congenital heart disease, or portal hypertension.

Group 2 pulmonary hypertension refers to pulmonary hypertension associated with left heart disease, which is typically heart failure with preserved ejection fraction, heart failure with reduced ejection fraction, or patients with valvular heart disease. Patients with Group 2 PH typically have an elevated pulmonary capillary wedge pressure and postcapillary pulmonary hypertension, as opposed to pre-capillary pulmonary hypertension, which characterizes Group 1 PAH. Patients with Group 3 pulmonary hypertension have pulmonary hypertension associated with chronic lung diseases and chronic hypoxia, and this can develop in the setting of obstructive or restrictive lung disease or combined obstructive and restrictive lung disease. Group 4 pulmonary hypertension refers to chronic thromboembolic pulmonary hypertension, which develops due to chronic PE [pulmonary embolism] and organized fibrous thrombi that develop with the pulmonary blood vessels that lead to pulmonary hypertension and right heart failure. Group 5 refers to pulmonary hypertension with multifactorial or unknown mechanisms of action, and this includes pulmonary hypertension related to conditions such as sarcoidosis, chronic renal failure, or myeloproliferative disorders.

We use functional class to describe a patient’s physical limitations related to pulmonary hypertension. For example, patients with functional class I symptoms have no limitation in their physical activity related to PAH, whereas patients with functional class II symptoms have slight limitation. Patients with functional class III symptoms have more marked limitation, and patients with functional class IV symptoms have such limitation in their physical activity that they’re unable to perform physical activity without any symptoms. Patients with functional class IV symptoms of pulmonary hypertension often have signs of right heart failure, such as lower extremity edema. They may have exertional syncope as well, and they can have symptoms at rest. We use functional class to both stratify the risk related to pulmonary hypertension and determine treatment approaches.

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