As nurses 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.
Importantly, 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 (that is, a BP cuff around the patients limb) 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 waaay 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 pressuresderived from the area under the curve of the pressure-time.
Why do I need to watch it?
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.
If you do nothing else, just read this….
Mean arterial pressure (MAP) is generally considered to be the organ perfusion pressure in an individual. Because MAP requires an inconvenient calculation, we’ve all been taught…misled perhaps…into focusing on systolic blood pressure (SBP) as a marker of how well-perfused a patient is, and we tend to ignore the diastolic blood pressure (DBP).
It’s important to remember, however, that we spend most of our lives in diastole, not systole. As a result, our organs spend more time being perfused during diastole than systole. The MAP takes this into account: MAP = (SBP + DBP + DBP)/3. DBP is more important than SBP!
So which patient is perfusing his vital organs better, the one with a BP of 110/40 or the one with a BP of 90/60? Do the MAP calculation…90/60 is better than 110/40!
Pay more attention to those diastolic BPs!”
MAP 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.