WHO Classification of Pulmonary Hypertension

Pulmonary hypertension—elevated blood pressure within the pulmonary artery—is a serious condition that has many causes. For doctors, understanding the cause of a person’s pulmonary hypertension is a critical first step in deciding on optimal therapy.

Since the causes of pulmonary hypertension are so varied, it helps to classify them into discrete categories. In an effort to standardize the classification of pulmonary hypertension—so that when doctors and researchers discuss the various causes they will be speaking the same language—the World Health Organization (WHO) has published a new system of classification that has rapidly become the standard.

Prior to this new system of classification, pulmonary hypertension was commonly divided into only two broad categories: primary or idiopathic (in which no underlying cause can be identified), and secondary (in which an underlying cause has been found). However, so many types of secondary pulmonary hypertension have been identified that this simple system of classification became inadequate.

The new WHO classification system attempts to be more specific about the mechanism of pulmonary hypertension, and the expected response to treatment. It is a useful classification system for experts on pulmonary hypertension, but for patients and for some doctors it can seem a little confusing. This is especially true because this new classification system is not really strictly organized according to the mechanism of the underlying cause of pulmonary hypertension, but rather, seems somewhat arbitrary. 

The WHO classification system divides pulmonary hypertension into five categories:

• Group 1: This group includes disorders in which the pulmonary hypertension is associated with abnormalities in the small branches of the pulmonary artery, the arterioles. Several types of pulmonary hypertension are in Group 1, including idiopathic pulmonary hypertension, and pulmonary hypertension due to several types of infection such as HIV, connective tissue disorders such as scleroderma, toxins such as fenfluramine, amphetamines and cocaine; and some types of congenital heart disease.
• Group 2: This group includes pulmonary hypertension caused by left-sided heart diseases such as cardiomyopathy, diastolic dysfunction, mitral stenosis, mitral regurgitation, aortic stenosis, and aortic regurgitation.
• Group 3: This group includes pulmonary hypertension due to lung problems, such as chronic obstructive lung disease.
• Group 4: This group includes pulmonary hypertension due to pulmonary embolus (blood clots that travel to the lungs) or pulmonary thrombosis (blood clots that form in the lungs).

The WHO classification of pulmonary hypertension is useful to specialists who deal frequently with patients who have pulmonary hypertension. But it is a complex classification system with little conceptual coherence (so it must be memorized rather than "figured out"). If you have pulmonary hypertension, you should not worry about the classification system. The important thing for you is not how your pulmonary hypertension is classified, but rather, that your doctor does whatever is necessary to find out the specific cause of your condition so that your treatment can be tailored appropriately.