Mean Arterial Pressure Calculator

What mean arterial pressure actually is

Mean arterial pressure, MAP, is estimated from a cuff reading as MAP = diastolic + (systolic − diastolic) / 3, which weights the diastolic pressure twice as heavily as the systolic. It is the average pressure in the arteries across one whole cardiac cycle, and it is the pressure that drives blood into tissue. That makes it the more useful number of the two you can read off a cuff: perfusion of the kidney, brain and gut depends on the mean, not on the peak. A systolic pressure of 160 with a diastolic of 60 and one of 110 with a diastolic of 85 both give a MAP in the low nineties, and both perfuse similarly, despite looking like very different readings.

The average is not the midpoint, because the heart does not spend equal time at each pressure. At a resting rate, diastole lasts about twice as long as systole, so the pressure sits closer to the diastolic value for most of the cycle. That is the whole content of the formula:

• MAP = diastolic + (systolic − diastolic) / 3
• which rearranges to MAP = (systolic + 2 × diastolic) / 3

The second form makes the weighting explicit. Diastole gets two of the three shares. A useful consequence for checking your own arithmetic: adding 3 mmHg to the diastolic pressure moves MAP by 2, while adding 3 to the systolic moves it by only 1.

Where the one third rule fails

The weighting assumes a normal heart rate, and it is the first thing to go in tachycardia. As the rate climbs, diastole shortens far more than systole does, because systole is largely fixed by the mechanics of contraction while diastole is simply whatever time is left over. At 150 beats per minute diastole no longer occupies two thirds of the cycle, so weighting it as though it does underestimates the true mean.

True mean arterial pressure is the time integral of the arterial pressure waveform divided by the cycle length, which is exactly what an arterial line computes and displays. When an arterial trace is available its MAP is the number to trust, and it will not generally equal the value this formula gives from the same systolic and diastolic figures. The formula is for a cuff reading at an ordinary rate, which is the situation it was built for.

Two further situations where the calculated value misleads. In severe aortic regurgitation the diastolic pressure falls away as blood runs back into the ventricle, so the waveform is nothing like the assumed shape. And an automated oscillometric cuff does not measure systolic and diastolic pressure directly at all: it estimates the mean from the point of maximum oscillation and derives the other two, so on many monitors the displayed MAP is the measured quantity and the systolic and diastolic are the estimates, which is the reverse of what most people assume.

Pulse pressure carries separate information

Pulse pressure is systolic minus diastolic, normally around 40 mmHg. It is not a component of MAP so much as an independent reading of what the heart and the aorta are doing, and two patients with the same MAP can have very different pulse pressures and very different problems.

• Wide. A stiff aorta, which is why pulse pressure rises steadily with age and why isolated systolic hypertension is the common pattern in the elderly. Also aortic regurgitation, thyrotoxicosis, anaemia and any other state with a large stroke volume or a fast diastolic run off.
• Narrow. A small stroke volume reaching the aorta: cardiogenic shock, severe aortic stenosis, tamponade, or significant hypovolaemia. A narrowing pulse pressure in someone bleeding is an earlier sign than a falling systolic, because compensatory vasoconstriction holds the diastolic pressure up while the stroke volume is already falling.

The 65 mmHg threshold, and what it is not

A MAP of 65 mmHg is the initial resuscitation target in the sepsis guidelines. It was chosen as roughly the pressure below which autoregulation begins to fail in the kidney and brain of most adults, which is the point at which blood flow starts to follow pressure instead of being held constant.

It is a threshold for acting, not a definition of normality, and it is not universal. Someone with long standing untreated hypertension has their autoregulatory range shifted upwards and may need a higher mean to perfuse the same organs, which is why chasing a single number in that patient can cause the injury it is meant to prevent. A young, fit adult can be perfectly well perfused below it. Read it as a floor worth investigating rather than a target to hit.

Common mistakes

• Averaging systolic and diastolic. The midpoint of 120 and 80 is 100, but MAP is 93.3. The error is small at normal pressures and grows with pulse pressure: at 180 over 60 the midpoint gives 120 against a true MAP of 100.
• Using the formula on an arterial trace that shows its own MAP. The monitor has integrated the actual waveform. Recomputing from the systolic and diastolic throws that away.
• Treating MAP as interchangeable with systolic pressure for stroke risk. Systolic pressure and pulse pressure predict cardiovascular events in older adults better than the mean does. MAP is the perfusion number, not the risk number.
• Comparing a cuff MAP against an arterial line MAP and expecting agreement. They are measuring by different principles and routinely differ by 5 to 10 mmHg, more if the cuff is the wrong size or the limb is not at heart level.

Converting units first? Use the pressure conversion table.