Tuesday, August 12, 2008

Mean Arterial Pressure (MAP)

Mean Arterial Pressure (MAP)


Mean arterial pressure is a term to describe a notional average blood pressure in an individual. It is defined as the average arterial pressure during a single cardiac cycle.

The blood pressure in our body is always changing and it is part of the human physiology. It is very rare that someone's blood pressure is always the same. Because we are always active and we are always doing different things like eating, sitting, standing, walking, breathing, moving around and doing so many other things throughout a day.


As blood is pumped out of the left ventricle into the arteries, pressure is generated. The mean arterial pressure (MAP) is determined by the Cardiac Output (CO), Systemic Vascular Resistance (SVR), and Central Venous Pressure (CVP) according to the following relationship, which is based upon the relationship between Flow, Pressure and Resistance :

MAP = (CO × SVR) + CVP

Because CVP is usually at or near 0 mmHg, this relationship is often simplified to:

MAP Approx = CO × SVR

Therefore, changes in either CO or SVR will affect MAP. If CO and SVR change reciprocally and proportionately, then MAP will not change. For example, if CO doubles and SVR decreases by one-half, MAP does not change (if CVP = 0). It is important to note that variables found in are all interdependent. This means that changing one variable changes all of the others.

In practice, MAP is not determined by knowing the CO and SVR, but rather by direct or indirect measurements of arterial pressure. From the aortic pressure trace over time, the shape of the pressure trace yields a mean pressure value (geometric mean) that is less than the arithmetic average of the systolic and diastolic pressures as shown to the right.

Why Measuring The MAP?

There are different reasons that mean arterial pressure is taken and recorded. Not all are related to health. The mean arterial pressure is known as part of a couple biological processes that do not show any health problems. When an arterial blood flow goes through the body it is going somewhere that it has in mind. Usually the blood is pumped through the arteries and left in the beds of capillaries that will run across the surface of different organs and give them the nutritional substances that are needed to operate properly.

Perfusion Pressure is Thought to Actually be The Mean Arterial Pressure.
MAP = Perfusion Pressure

For the mean arterial pressure to allow an organ to operate the way that it should, it must be at around 60 mmHg. This is enough for an organ of the average size person as long as it is remains at this spot. If the value falls below the average, it means that there is not enough blood pumping into the organ and this will cause the organ to become ischemic. The result will be tissue damage to the organ. The mean arterial pressure should be checked and calculated on a regular basis. Some health officials check the value of a person's arterial pressure when they check a person's blood pressure. It is just one of the many calculations that have to be made about blood pressure.

Simplyfied Mean Arterial Pressure (MAP) Equation

MAP = [(2 x Diastolic)+Systolic] / 3

  • MAP around 70 - 110 : Normal
  • MAP of about 60 : Necessary to Perfuse Coronary Arteries, Brain, Kidneys
  • MAP less than 60 : Low Perfusion - Will Cause Ischemic Organ and Tissue Damage
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