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Diagnostic Tests

Why are there two numbers to each blood pressure?

The first (sometimes called the 'top') number is the systolic blood pressure. Systolic blood pressure derives its name from the fact that it is measured during systole, the period of time when the heart is contracting and pumping blood to the lungs and the rest of the body. Because the heart is forcefully pushing blood into blood vessels, you would expect the systolic pressure to be higher, as is the case.

The second (sometimes called the 'bottom') number is the diastolic blood pressure. Diastole refers to the period of time when the heart is relaxing, filling with blood and getting ready to contract again. During diastole, there is, of course, still blood in the pulmonary blood vessels but the pressure is lower since the heart is relaxing, not contracting.

In addition to the systolic and diastolic pressures there is a calculated value called the mean arterial pressure. This is defined as an average of the systolic and diastolic blood pressures (mean arterial pressure = 2/3 diastolic pressure + 1/3 systolic pressure). The normal mean pulmonary arterial (PA) pressure is about 12-15.

Pulmonary artery hypertension (PAH) is defined as a resting mean PA pressure above 25 mmHg. The higher the mean PA pressure the more likely it is that the right side of your heart (including the right ventricle) will have to adapt (by getting larger and stiffer) in order to handle the increased pressure. As discussed in the sections on symptoms of pulmonary arterial hypertension, shortness of breath and chest pain are not directly correlated with how high your pressures are. There is a lot of variability in how people "feel" elevated pressures and how their hearts tolerate the higher pressures. Your doctor will follow your pulmonary arterial pressures over time to monitor progression of the disease, as well as to monitor the effects of medications you are taking to treat the pulmonary arterial hypertension.