Over time, the pulmonary arteries become increasingly narrow, as the walls of the blood vessels become thicker and less flexible. The narrowing of the pulmonary arteries restricts blood flow to the lungs, which makes it harder for the heart to pump blood through the arteries. Trying to maintain the blood flow, the heart has to pump harder, increasing the pressure in the pulmonary arteries.
What does this mean?
When the right side of the heart has to work continuously harder to pump blood through the pulmonary arteries, it can gradually become weaker. In some cases, this can lead to right heart failure.
Drug and toxin induced PAH is where certain drugs and toxins can bring about PAH.[^5]:
Early diagnosis of PAH is important as it is linked with better health outcomes, however, PAH is often difficult to diagnose. A key problem is that the early symptoms - breathlessness, dizziness and fatigue - are not very specific and can be easily confused with many other, more common conditions like asthma or even lack of fitness or depression. Due to its complexity, PAH is usually diagnosed at specialist centres where healthcare professionals (HCPs) conduct a series of tests to rule out other possible conditions, and confirm diagnosis. 
Tests available to suspect and confirm a PAH diagnosis include:
Right heart catheterisation and vasoreactivity
An invasive procedure that enables direct and accurate measurement of the condition of the right heart and pulmonary vessels.15 This is the definitive test for diagnosing PAH15
There are several specific medicines to treat PAH. All listed medicines relax and widen the blood vessels in the lungs as well as reduce and prevent the overgrowth of cells in the walls of the vessels. To help people with PAH get the best quality of life possible, PAH specialists may recommend one or more treatments including oral, inhaled and intravenous medications, at a time. In severe cases, lung transplantation may be needed.
Other supportive treatments that may be recommended for PAH include:
People living with PAH are advised to speak to their HCP to identify the best treatment option for them. While there is no cure for PAH yet, treatments can help people manage and slow progression of the condition.
A progressive condition is a disease that, in most cases, worsens over time.
In PAH the term ‘functional class’ is commonly used to assess the severity of the disease and can be used to evaluate the progression of the condition and to determine treatments. PAH is categorised into four functional classes:
Class I – Patients with pulmonary hypertension 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 pulmonary hypertension 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 pulmonary hypertension 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 pulmonary hypertension 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.
Table from ESC/ERS Guidelines for the diagnosis and treatment of pulmonary hypertension: The Joint Task Force for the Diagnosis and Treatment of Pulmonary Hypertension
In current management of PAH, the risk of disease progression in patients is assessed with a comprehensive set of clinical tests, imaging and biomarkers, rather than just WHO functional class. It is important to perform this risk assessment regularly over the course of your disease to identify early signs of disease progression and escalate treatment as appropriate.