Blood pressure calculator with mean arterial pressure

The categories are those of the American hypertension guideline, which splits the values into four steps, from normal to stage 2 hypertension [1]. Hypertension starts at 130/80 mmHg. The step is read separately on each of the two numbers, and the final category is the higher of the two. A reading of 118/82 mmHg therefore lands at 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, and the treatment target comes down below 130/80 mmHg in both. 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 category also enters the estimate of cardiovascular risk, which you can work out 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, compared with systole. The classic approximation adds to the diastolic one third of the difference between the two values.

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

Pulse pressure is the difference between the systolic and the diastolic. 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.