حاسبة متوسط الضغط الشرياني - MAP

Mean arterial pressure (MAP) is the average pressure driving blood through the circulation across one cardiac cycle. It matters more than systolic or diastolic pressure alone for one specific question: whether organs are being perfused. Because the heart spends about twice as long in diastole as in systole at rest, MAP is not the midpoint between the two readings but a weighted average that sits closer to the diastolic value.

المعادلة

MAP = DBP + (SBP - DBP) / 3
equivalently  MAP = (SBP + 2 x DBP) / 3

• SBP = systolic blood pressure in mmHg
• DBP = diastolic blood pressure in mmHg
• (SBP - DBP) is the pulse pressure

A standard physiological approximation that weights diastole twice as heavily as systole, reflecting their relative durations at a normal resting heart rate. Invasive arterial monitoring computes MAP by integrating the pressure waveform directly rather than using this formula.

How the calculation works

Arterial pressure is not constant through the cardiac cycle: it peaks during ejection and falls through diastole. The true mean is the area under the pressure curve divided by the cycle duration. At a resting heart rate, diastole occupies roughly two thirds of each cycle, so the simple approximation of one third systolic plus two thirds diastolic tracks the integrated value closely enough for bedside use.

The approximation degrades as heart rate rises. Tachycardia shortens diastole disproportionately, so the two-thirds weighting no longer holds and the formula increasingly underestimates the true mean. This is one reason intensive care units rely on the MAP computed by an arterial line, which integrates the actual waveform, rather than on a value derived from a cuff reading.

How to read your result

MAP is used clinically as a perfusion threshold rather than as a target to optimise in healthy people. The commonly cited floor of 65 mmHg comes from sepsis resuscitation guidance and applies to critically ill adults; it is not a target for someone taking their blood pressure at home.

• The Surviving Sepsis Campaign recommends an initial MAP target of 65 mmHg in adults with septic shock requiring vasopressors.
• A higher floor is often chosen for patients with chronic hypertension, whose autoregulation has adapted to higher pressures, or after certain neurosurgical procedures.

Why MAP rather than systolic pressure

Systolic pressure is the number most people know, but it is the most variable part of the waveform and the most sensitive to arterial stiffness. An older adult with stiff arteries can have a high systolic reading and a low diastolic reading, giving a wide pulse pressure and a MAP that is unremarkable. Judging perfusion from systolic pressure alone in that person can be misleading in either direction.

MAP also behaves more predictably as a driving pressure. Organ blood flow depends on the difference between mean arterial pressure and the pressure downstream, so MAP is the term that appears in the physiology. That said, MAP is a global average and cannot describe regional perfusion: a patient can have an adequate MAP while a specific vascular bed remains underperfused because of vasoconstriction, thrombosis or raised local tissue pressure.

حدود الاستخدام

• The one-third-two-thirds weighting assumes a normal resting heart rate. It becomes progressively less accurate in tachycardia, where diastole shortens disproportionately.
• A cuff-derived MAP is an approximation of an approximation. Automated oscillometric devices estimate systolic and diastolic values from the oscillation envelope, and errors in those inputs propagate.
• MAP describes average driving pressure across the whole circulation. Adequate MAP does not guarantee adequate perfusion of any particular organ, which also depends on local vascular resistance and venous pressure.
• The 65 mmHg threshold comes from critical care research in septic shock. It is not a home monitoring target and should not be used to judge whether ordinary blood pressure readings are acceptable.
• MAP does not replace systolic and diastolic values for diagnosing or staging hypertension, which guidelines define using those readings directly.
• Cuff readings taken during pain, anxiety, movement or with the wrong cuff size can be wrong by 10 mmHg or more, and any MAP computed from them inherits that error.

الأسئلة الشائعة

What is a normal mean arterial pressure?

Roughly 70 to 100 mmHg in a healthy resting adult. Values below about 60 mmHg suggest perfusion of vital organs may be inadequate, and persistently elevated values above 100 mmHg are consistent with hypertension.

Why is MAP not just the average of systolic and diastolic?

Because the heart spends about twice as long in diastole as in systole at rest. Averaging the two equally would overweight the brief systolic peak. The standard formula weights diastole twice as heavily, which is why MAP sits closer to the diastolic reading.

Why is 65 mmHg the number used in intensive care?

It is the initial MAP target recommended by the Surviving Sepsis Campaign for adults in septic shock on vasopressors, based on trials comparing higher and lower targets. It is a resuscitation floor for critically ill patients, not a general health target.

Can I calculate MAP from a home blood pressure monitor?

You can apply the formula to any systolic and diastolic pair, but the result inherits the accuracy of the cuff reading, and the formula assumes a normal resting heart rate. It is useful for understanding your numbers, not for making treatment decisions.

Does a normal MAP mean my blood pressure is fine?

Not necessarily. Someone with stiff arteries can have a markedly high systolic and low diastolic pressure that average to a normal-looking MAP. Hypertension is diagnosed and staged from systolic and diastolic values, not from MAP.

المراجع

1. DeMers D, Wachs D. Physiology, Mean Arterial Pressure. StatPearls, NCBI Bookshelf.
2. Evans L et al. Surviving Sepsis Campaign: International Guidelines for Management of Sepsis and Septic Shock 2021. Critical Care Medicine, 2021.
3. Asfar P et al. High versus low blood-pressure target in patients with septic shock. New England Journal of Medicine, 2014.
4. Whelton PK et al. 2017 ACC/AHA/AAPA/ABC/ACPM/AGS/APhA/ASH/ASPC/NMA/PCNA Guideline for the Prevention, Detection, Evaluation, and Management of High Blood Pressure in Adults.