Cardiovascular System
Mean arterial pressure and organ perfusion
Mean arterial pressure is the average pressure in the arteries over one whole cardiac cycle, and it is the pressure that drives blood to the organs.
Remember
Mean arterial pressure can be estimated as the diastolic pressure plus one third of the pulse pressure (the systolic pressure minus the diastolic pressure), and a mean arterial pressure below about 70 mmHg can impair organ perfusion.
Tissues need a continuous driving pressure, not just a peak at each beat. That driving pressure is the mean arterial pressure (MAP): the average pressure in the arteries across one whole cardiac cycle, which is the average force driving blood into the vessels that serve the organs.
It is not the simple midpoint of the two numbers in a blood pressure reading. Diastole (when the ventricles relax) lasts longer than systole (when they contract) at a normal heart rate, so the average sits closer to the diastolic value.
To estimate it, add one third of the pulse pressure to the diastolic pressure. The pulse pressure is the gap between the systolic and diastolic pressures. For 120/80 mmHg the pulse pressure is 40 mmHg, so MAP is about 93 mmHg. For 120/60 mmHg the pulse pressure is 60 mmHg, a third of it is 20, and MAP is 80 mmHg.
Normal MAP lies roughly between 70 and 110 mmHg. Below about 70 mmHg, perfusion (blood flow through the tissues) can be impaired, depending on the clinical context. A reading of 80/50 mmHg after a haemorrhage gives about 60 mmHg. A reading of 90/40 mmHg in septic shock (sepsis in which the blood pressure stays low despite adequate fluids) gives about 57 mmHg, even though a systolic pressure of 90 mmHg may look less alarming.
MAP depends on cardiac output and on vascular resistance (the opposition to flow, set mainly by how narrow the small arteries are). A weak heart or widespread vasodilation can therefore lower it.
Key facts
- Estimate
- diastolic pressure plus one third of the pulse pressure
- Pulse pressure
- the systolic pressure minus the diastolic pressure
- Worked example
- 120/80 mmHg gives a MAP of about 93 mmHg, and 120/60 mmHg gives 80 mmHg
- Warning level
- below about 70 mmHg, organ perfusion can be impaired, depending on the clinical context
- Normal range
- roughly 70 to 110 mmHg
- Determinants
- cardiac output and vascular resistance both influence MAP, not heart rate alone
Exam traps
- Systolic pressure on its own does not show that flow is adequate, because it is only the peak of each cycle, while the pressure that drives blood into the tissues is the average over the whole cycle.
- Averaging 120 and 60 to get 90 mmHg is a common mistake, because diastole lasts longer than systole and so the true mean lies closer to the diastolic pressure.
- MAP is not weighted towards systolic pressure, because diastole occupies more of the resting cardiac cycle, so it is estimated as the diastolic pressure plus one third of the pulse pressure.
- At 90/40 mmHg the pulse pressure of 50 mmHg does not offset the low diastolic pressure, because the pulse pressure contributes only one third of its value (about 17 mmHg) to the MAP, so the MAP stays low at about 57 mmHg.
- MAP is not set mainly by heart rate, because heart rate contributes through cardiac output and resistance matters just as much.
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