Pulmonary Arterial Hypertension (PAH) means that you have high blood pressure in the arteries that go from your heart to your lungs. It is different to ordinary high blood pressure, and it cannot be measured with a regular blood pressure monitor. With PAH, the tiny arteries in your lungs become narrow and it is harder for the blood to flow through them so this raises the blood pressure in the lungs. Your heart has to work harder to pump blood through the tiny arteries which may even be partially blocked, and the heart muscle may weaken over time, and can eventually lead to fatal heart failure.