Mean arterial pressure (MAP)

What is this for?

Mean arterial pressure is the average pressure driving perfusion through a cardiac cycle. Because roughly two thirds of the cycle is spent in diastole, diastolic pressure is weighted twice as heavily as systolic. MAP is the resuscitation target in sepsis and the pressure that determines organ perfusion.

How to use it

1. Enter the systolic and diastolic pressures from the same reading.
2. Compare the MAP against the target for the clinical situation — 65 mmHg is the usual starting point in sepsis.
3. Note the pulse pressure as well; it carries information the MAP alone does not.

Worked example

A septic patient has a blood pressure of 110/60 mmHg. Calculate the MAP and compare it to the resuscitation target.

Answer: [(2 × 60) + 110] ÷ 3 = 230 ÷ 3 = 76.7 mmHg — above the 65 mmHg target.

Clinical pearls & pitfalls

• Diastolic pressure is weighted twice because the heart spends roughly two thirds of the cardiac cycle in diastole. This is why a patient can have a reassuring systolic and still be inadequately perfused.
• The mental shortcut is diastolic plus one third of the pulse pressure. It gives the identical answer and is far quicker at the bedside.
• A MAP of 65 mmHg is the default sepsis target, but Surviving Sepsis recognises that patients with chronic hypertension may need a higher MAP to perfuse a right-shifted autoregulatory curve.
• The formula assumes a normal heart rate. At tachycardic rates diastole shortens disproportionately and the standard weighting overestimates the true mean; an arterial line measures MAP directly and should be preferred when available.

Assumptions & limitations

• A calculated approximation. An indwelling arterial line integrates the actual pressure waveform and is more accurate, particularly in shock and at extremes of heart rate.
• Assumes a normal heart rate and rhythm. In marked tachycardia, bradycardia, or atrial fibrillation the weighting is less accurate.
• Non-invasive cuff readings are themselves unreliable in hypotension, obesity, and with an incorrectly sized cuff — the calculation cannot compensate for a bad input.
• The target MAP is patient-specific. A single threshold applied to every patient is not appropriate.

References

• Evans L, et al. Surviving Sepsis Campaign: international guidelines for management of sepsis and septic shock 2021. Crit Care Med. 2021;49(11):e1063-e1143.
• Asfar P, et al. High versus low blood-pressure target in patients with septic shock. N Engl J Med. 2014;370(17):1583-1593.
• Magder S. The meaning of blood pressure. Crit Care. 2018;22(1):257.

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