Usually the blood pressure in the pulmonary arteries is much lower than the blood pressure in the aorta and its adjacent vessels. Pulmonary blood pressure is considered high if the average pressure in the pulmonary arteries is higher than 25 mmHg (millimeter of mercury) at rest or 30 mmHg after exertion. The disease, called Pulmonary Arterial Hypertension, refers to a situation when the small pulmonary arteries are damaged.
•    Contraction of the blood vessels
•    Reshaping of the cells that make up the blood vessels and muscles
•    Small blood clots (thrombosis)
•    An increase in pulmonary resistance
•    A decrease in cardiac output
Patients would usually complain of shortness of breath during exertion, fatigue, palpitation and sudden loss of consciousness during exertion. The disease can lead to right-sided heart failure, which can be fatal. 

The disease's classification is based on the site of injury - arterial, venous, or for other reasons (hypoventilation, a chronic thromboembolic disease) - and the clinical course of the disease. 
Pulmonary arterial hypertension can be primary (Idiopathic Pulmonary Arterial Hypertension) and diagnosed with a right-sided heart catheterization, or secondary to other illnesses or conditions, among them chronic pulmonary embolisms, COPD, pulmonary fibrosis or cystic fibrosis, severe sleep apnea, heart valve diseases, cardio-vascular birth defects, toxic substances or appetite suppressors, AIDS, collagen diseases such as lupus, scleroderma, rheumatoid arthritis or vasculitis.

An eco-cardiogram will usually indicate the possibility of pulmonary arterial hypertension by evaluating the systolic pressure in the pulmonary artery and monitoring the heart valves. 
When there's a significant suspicion of pulmonary hypertension, other diseases need to be ruled out as the cause. If indeed the hypertension is secondary to another disease, usually treatment of the base disease will lower the hypertension, improve the patient's condition or at the very least prevent further deterioration. Only after ruling out other diseases will a right-sided heart catheterization is done to confirm the diagnosis, evaluate diagnostic markers and check that the pulmonary arteries respond to medication (vasoreactivity test).
It is highly recommended to refer patients with pulmonary hypertension to a medical center with the proper experience in diagnosing and treating this disease. A specialist will perform tests to locate the cause of the hypertension: HIV test, blood tests for autoimmune diseases, liver functions, expanded pulmonary functions test, a 6 minutes walking test, lung CT with contrast, V/Q lung scan, polysomnography and more. 
If all these tests come back negative, the diagnosis is that this is a primary (idiopathic) pulmonary arterial hypertension. In this case a right-sided heart catheterization will be done to confirm the diagnosis and examine the reactiveness of the pulmonary blood vessels. The catheter is usually inserted through a vain in the neck or thigh and measures the pressure in the heart and in the lung artery. The pressure in the right atrium is measured when the balloon is pulled into a small vessel that can't be passed. Blood flow to and from the heart is also measured. 
The pressure height is significant in determining the course of treatment. Sometimes irregular blood flow indicates a defect in the heart or in a valve that wasn't previously diagnosed. In such a case surgery might improve the situation. Measuring the heart's function helps determine if the patient suffers from heart failure or is about to develop heart failure, and administer the relevant treatment. 

This information was edited from the article "Pulmonary arterial hypertension - Classification, diagnosis and treatment" by Prof. Gabriel Izbicki and Dr. Daniel Ben Dayan, published (in Hebrew) on the website The Medical.