We all diligently watch our patients blood pressure, recording it in our observation charts…but do we pay enough attention to the mean arterial pressure (MAP)? That innocent little number placed in brackets or hiding off to one side of the monitor screen.
What the heck is that number? Is it important? Should I record it?
MAP is defined as the average arterial blood pressure during a single cardiac cycle.
The reason that it is so important is that it reflects the haemodynamic perfusion pressure of the vital organs.
how is it calculated?
The simple way to calculate the patients MAP is to use the following formula:
MAP = [ (2 x diastolic) + systolic ] divided by 3.
The reason that the diastolic value is multiplied by 2, is that the diastolic portion of the cardiac cycle is twice as long as the systolic. Or you could say, it takes twice as long for the ventricles to fill with blood as it takes for them to pump it out….. at a normal resting heart-rate.
In a bradycardic or tachycardic patient this relationship between systolic and diastolic values changes, and the formula is not as accurate. When using non-invasive BP monitoring (BP cuff around the patients arm) the monitor uses this formula to determine the MAP, so it is less accurate in the unstable patient.
During invasive monitoring of BP (using an arterial line) a complex formula is used that is way beyond my understanding to attain a much more accurate and real time value.
OK, if you must know… it is obtained via Fourier analysis of the arterial waveform, or as the time-weighted integral of the instantaneous pressures derived from the area under the curve of the pressure-time.
do I need to watch it?
Definitely. I guess a rough analogy would be that the MAP is the oil gauge for your patients motor.
A MAP of at least 60 is necessary to perfuse the coronary arteries, brain, and kidneys. Normal range is around 70 – 110 mmHg.
It is a vital sign to monitor anytime the patient has a potential problem with perfusion of his organs. Some examples (and there are many more) might include:
- a patient with septic shock on vasopressors.
- head injured patients.
- Cardiac patients on vasodilator (GTN) infusion.
- Patient with a dissecting abdominal aneurysm who needs to have his BP controlled within a narrow range so as not to cause increased bleeding.
In a head injured patient, the brain is at risk of ischaemi injury due to insufficient blood flow if the MAP falls below 50. On the other hand, a MAP above 160 reflects excess cerebral blood flow and may result in raised intercrainial pressures.
is it true that women are worse than men at reading a MAP?
Now, if you think I am brave enough to tackle that question you must be crazy!