Pulmonary Hypertension Unit

What is Pulmonary Hypertension?

Pulmonary arterial hypertension includes a group of diseases incorporating Idiopathic Pulmonary Arterial Hypertension (IPAH) and PAH related to various underlying medical conditions.  (Please see Diagnostic Classification for further details.)

PAH represents diverse population groups that vary greatly in aetiology, disease severity, speed of progression, prognosis and response to treatment.

Idiopathic PAH (disease which presents without any identifiable cause) is reported to generate 1-2 incident cases per million per annum in the USA. The reported PAH incidence rates associated with connective tissue disease (CTD) congenital heart disease (CHD) including HIV and portal hypertension are somewhat higher.

Shortness of breath on exertion is one of the earliest manifestations of disease.

PAH is further associated with increasing fatigue and progressive deterioration in exercise capacity; significantly impacting on quality of life. The underlying disease process is complex. In the early stages of illness the patient may remain asymptomatic with an absence of clinical signs. However, as the course of the illness progresses, the right heart begins to fail and physical symptoms present,  leading to high premature mortality.

Classification of disease severity is based on presenting symptoms defined by the World Health Organization (WHO) functional classification system.


Classification of Pulmonary Arterial Hypertension can be made on the basis of clinical or pathological features along with the degree of functional ability present.

Functional classification

The functional classification is the measure of the restrictions imposed by the disease process on a patient’s functional ability. It is a very important part of the clinical picture of patients with pulmonary arterial hypertension (PAH). This functional classification is strongly predictive of mortality, and is an important feature in the choice of drug therapy prescribed by your PH Physician.

The New York Heart Association (NYHA) classification system was established in 1928 for patients with heart disease, and was based on the severity of clinical symptoms and the disease prognosis. This system has been updated on several occasions since then and the most recent update was in 1994.

In 1998, the World Health Organization (WHO) convened an expert panel in Evian, France, to re-evaluate the diagnostic classification system for patients with pulmonary hypertension. The WHO functional classification system establishes the importance of identifying near collapse (syncope) and complete collapse in this group of patients. Hence, PAH patients who present or experience syncope are assigned to WHO functional class IV .The WHO classification and New York Heart Association ( NYHA) classification are nearly identical, and are sometimes referred to as NYHA,/WHO classification. In everyday clinical use, the two classification systems are used interchangeably when characterising the degree of clinical and functional limitation that the patient with PH is experiencing.

Clinical classification

Historically, pulmonary hypertension was classified into two distinct categories: primary pulmonary hypertension (PPH) and secondary pulmonary hypertension, depending on the absence or presence of identifiable causes or risk factors. In 1998 the WHO created a new classification system for pulmonary hypertension based on the underlying disease mechanism, clinical presentation and suitable therapeutic options. This Evian classification (or sometimes referred to as WHO 1998 classification) includes five distinct groups. Pulmonary arterial hypertension became one of the categories. The term ‘primary pulmonary hypertension’ which was previously used was retained within this category and included subcategories of ‘sporadic PAH’ and ‘familial PAH’. However, in Venice, Italy in 2003, another symposium took place and the Evian classification was updated again. The term ‘primary pulmonary hypertension’ was removed and the subcategory of ‘sporadic PAH’ was replaced by ‘idiopathic PAH’.

Functional Classification System

Adopted from Diagnosis and management of pulmonary arterial hypertension: ACCP Evidence-Based Clinical Practice Guidelines. Chest. 2004; 126:7S-10

New York Heart Association functional classification

Class 1: No symptoms with ordinary physical activity.
Class 2: Symptoms with ordinary activity. Slight limitation of activity.
Class 3: Symptoms with less than ordinary activity. Marked limitation of activity.
Class 4: Symptoms with any activity or even at rest.

World Health Organisation functional assessment classification

Class I: Patients with PH but without resulting limitation of physical activity. Ordinary physical activity does not cause undue dyspnoea or fatigue, chest pain, or near syncope.
Class II: Patients with PH resulting in slight limitation of physical activity. They are comfortable at rest. Ordinary physical activity causes undue dyspnoea or fatigue, chest pain, or near syncope.
Class III: Patients with PH resulting in marked limitation of physical activity. They are comfortable at rest. Less than ordinary activity causes undue dyspnoea or fatigue, chest pain, or near syncope.
Class IV: Patients with PH with inability to carry out any physical activity without symptoms. These patients manifest signs of right-heart failure. Dyspnoea and / or fatigue may even be present at rest. Discomfort is increased by any physical activity.

Diagnostic Classification

Pulmonary arterial hypertension
Associated with

  • Collagen vascular disease
  • Congenital left-to-right shunt
  • Portal hypertension
  • Infection with human immunodeficiency virus
  • Drugs and toxins
  • Other conditions*

Associated with substantial venous or capillary involvement

  • Pulmonary veno-occlusive disease
  • Pulmonary capillary hemangiomatosis
  • Persistent pulmonary hypertension of the newborn

Pulmonary hypertension with left heart disease

  • Left-sided atrial or ventricular heart disease
  • Left-sided valvular heart disease

Pulmonary hypertension associated with lung disease or hypoxemia or both

  • Chronic obstructive pulmonary disease
  • Interstitial lung disease
  • Sleep-disordered breathing
  • Alveolar hypoventilation disorders
  • Chronic exposure to high altitude
  • Developmental abnormalities

Pulmonary hypertension due to chronic thrombotic or embolic disease or both

  • Thromboembolic obstruction of proximal pulmonary arteries
  • Thromboembolic obstruction of distal pulmonary arteries
  • Nonthrombotic pulmonary embolism (tumour, parasites, foreign material)


Sarcoidosis, pulmonary Langerhans’-cell histiocytosis, lymphangiomatosis, and compression of pulmonary vessels (adenopathy, tumour, and fibrosing mediastinitis)

* This classification was adapted from Simonneau et al. 3
† These conditions include thyroid disorders, type 1 glycogen storage disease, Gaucher’s disease, hereditary hemorrhagic telangiectasia, hemoglobinopathies, myeloproliferative disorders, and splenectomy