Blood pressure calculator with mean arterial pressure

The categories are those of the American hypertension guideline, which divides readings into four categories, from normal to stage 2 hypertension [1]. Hypertension starts at 130/80 mmHg. Each of the two numbers is classified separately, and the final category is the higher of the two. A reading of 118/82 mmHg therefore falls into stage 1, even though the systolic looks reassuring.

The European guideline keeps the older diagnostic threshold of 140/90 mmHg [2]. The difference is largely one of naming, because both guidelines call for a change in lifestyle at the values between the two thresholds. In both, the treatment target is below 130/80 mmHg. In patients with diabetes the target stays the same, if treatment is well tolerated. Between 120 and 129 mmHg systolic, with the diastolic below 80, the reading is called elevated. The badge under the reading has four labels: Normal, Elevated, Stage 1 hypertension and Stage 2 hypertension. The systolic value is also used to estimate cardiovascular risk, which you can do with the PREVENT calculator.

Mean arterial pressure is the average pressure in the arteries across one cardiac cycle. It is not the arithmetic mean of the two numbers, because the heart spends more time in diastole than in systole. The standard approximation adds one third of the difference between the two values to the diastolic pressure.

The figure matters most in the hospital, where it shows whether the perfusion pressure of the organs is sufficient. At home it mostly provides context. The approximation assumes a heart at rest, and at high heart rates it overestimates, because diastole shortens.

Pulse pressure is the difference between systolic and diastolic blood pressure. In young people it sits around 40 mmHg. With age the large arteries stiffen, the systolic climbs, the diastolic stays put or even falls, and the gap widens.

When the systolic reaches 130 mmHg or more and the diastolic stays below 80 mmHg, the pattern is called isolated systolic hypertension, and the calculator flags it separately [1]. It is the most common form of hypertension after 65 and it is treated, even though the second number looks normal.

The European measurement recommendations call for five minutes of rest, sitting with the back supported, feet on the floor and the arm supported at heart level [3]. The cuff goes on bare skin, not over clothing, and its size has to match the arm. A cuff that is too small gives falsely high readings.

Two or three readings are taken a minute apart, and the first is discarded. No coffee, smoking or exercise in the previous half hour. The diagnosis is not made on a single reading, but on the average of several days.