# mean arterial pressure

1. What is mean arterial pressure and why is it more useful than a blood pressure measurement

thanks julie
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2. ### About julie_hills

Joined: Dec '06; Posts: 5

10 Comments

3. The point of measuring blood pressure is generally to assess systemic perfusion. However, systolic and diastolic pressures reflect only one beat of the heart (contraction and relaxation pressures respectively). If you have a hypo- or hyperdynamic left ventricle, the systolic BP can be misleading. If you have peripheral vasoconstriction, the diastolic BP can be misleading.

You work out MAP by ([2xDBP] + SBP)/3 (because diastole takes up 2/3 of the cardiac cycle and systole takes up 1/3 of the cycle). Therefore, MAP gives you an indication of the mean (average) perfusion pressure across the entire cardiac cycle.

Tissue hypoperfusion is the pathophysiologic endpoint of low blood pressure, and mean arterial pressure (MAP), rather than BP, is the physiologic driving force behind blood flow to organs and tissues.
4. The mean arterial pressure is the average pressure in the arteries. It is calculated by adding the systolic to 2X the diastolic, and dividing the total by three. (ex: 120/80 120 + 80 + 80 =280. 280/3= 93.3 map). The disatolic is added twice since the heart spends more time in diastole that systole. I don't know if or why it is more useful than a regular blood pressure.
5. MAP takes cardiac output into account.
It's a good indicator of perfusion. Perfusion can be insufficient (a low MAP) even if BP is not, if cardiac output is too low.
Here's a formula for MAP that gives a true reading v. the SBP/DBP estimation formula:
MAP=(COxSVR)+CVP
where
CO is cardiac output
SVR is systemic vascular resistance
CVP is central venous pressure

Of course, it's a bit harder to come by.
6. Another thing to note is that when YOU take a BP, you directly measure the Korotkoff sounds for Sys/Dia and then calculate the Mean.

When a machine takes a BP, it directly measures the mean and generates a 'pulse envelope' by which it calculates the Sys/Dia.

So, since we are moving to machine measurements of BP, the Mean is a more reliable number because it's a direct measurement, whereas the Sys/Dia are estimates.

~faith,
Timothy.
7. You can also calculate other important pressure values with the MAP....for example, the cerebral perfusion pressure. Now any idiot understands that a pt w a BP of 60/30 is getting less blood to the brain than someone with a pressure of 130/80, but sometimes it is interesting to see the actual numbers.
8. Quote from RunnerRN
You can also calculate other important pressure values with the MAP....for example, the cerebral perfusion pressure. Now any idiot understands that a pt w a BP of 60/30 is getting less blood to the brain than someone with a pressure of 130/80, but sometimes it is interesting to see the actual numbers.

yes but...you should clarify that you need an icp before you can calculate the cpp. So, in settings where you're unable to 'see' the icp, map is useless if you want a cpp...
9. Quote from RunnerRN
Now any idiot understands that a pt w a BP of 60/30 is getting less blood to the brain than someone with a pressure of 130/80, but sometimes it is interesting to see the actual numbers.
and want to add that Also... a person with a bp of 130/80 can have an AWFUL cpp if the icp is high...(in some cases a patient with a normal icp and a bp of 60/30 may have a 'better' (though obviously not good) cpp than a person with bp 130/80 and a terribly high icp). So...at first glance you can say that 'any idiot understands ...' but in fact, sometimes it's hard to tell.
10. Quote from julie_hills
What is mean arterial pressure and why is it more useful than a blood pressure measurement

thanks julie
As a student, I equate the MAP as a measure of relativity. If a person has a baseline B/P that is quite high, and it falls maybe 8-10 points either systolic or diastolic, it may not "seem" to mean much, but in terms of the MAP (particularly if the decrease is in the diastolic pressure), it could mean a significant decrease in the map and perfusion and thus a patient going into shock. This is why it can be more important in many cases than he actual B/P reading. So as a student who is new to this info, am I on the right track?
11. Quote from SMK1
As a student, I equate the MAP as a measure of relativity. If a person has a baseline B/P that is quite high, and it falls maybe 8-10 points either systolic or diastolic, it may not "seem" to mean much, but in terms of the MAP (particularly if the decrease is in the diastolic pressure), it could mean a significant decrease in the map and perfusion and thus a patient going into shock. This is why it can be more important in many cases than he actual B/P reading. So as a student who is new to this info, am I on the right track?
Yes you're on the right track...but it's actually a bit simpler than that. Think of SBP as blood pressure at its highest, and DBP as its lowests...and MAP a constant or 'average' pressure in the vessels (though it's NOT a 'mathematical' average)...

The origional poster asked 'why' is map more 'useful' than a bp reading. This is mostly true when speaking of people in shock-states. Basically, if a patient's MAP is less than 60, blood is not perfusing organs sufficiently...which will eventually result in failing organs. People with systolic bp's in the 80's or 90's (or even 100's) will at first glance have an 'adequate' blood pressure. However, these same people can have MAP's less than 60...

With this in mind, MOST/ALL asymptomatic/healthy people with SBP's of 80's-90's are fine. But when they're 'shocky'and/or sick mAP is a better indicator of organ perfusion. So...your patient on dopamine has a SBP of 95...at first glance is fine...right?...not always...
12. Thanks to all above for the useful info. As a paramedic and new nursing student. I've been looking at the MAP on monitors and wondering why they bother tracking that.

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