Mean arterial pressure (MAP) = Diastolic pressure + 1/3 Pulse pressure
Pulse pressure = Systolic pressure - Diastolic pressure
Mean arterial pressure is generally regarded as a measure of cardiac output and peripheral resistance. It is considered to be perfusion pressure seen by organs in the body. A MAP of at least 60 is necessary to perfuse the coronary arteries, brain, and kidneys. Normal range is around 70 – 110 mmHg. It's slightly less than the value half way between systolic and diastolic pressure because systole is shorter than diastole, so you put the diastole twice.
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.
