Lessons · Medicine foundations · pulse pressure and mean arterial pressure
Two numbers on a cuff, and the two figures you take from them
Pulse pressure is the systolic minus the diastolic; mean arterial pressure is the diastolic plus one third of that pulse pressure.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
Somebody asks for the mean arterial pressure on a ward round and everyone in the room does it in their head while you are still looking at the screen. It is one subtraction, one division by three and one addition, and once it is in your hands you never lose it.
How to think about it
Subtract to get the pulse pressure, on its own line. Divide that by three. Add it to the diastolic, not to the systolic. The answer always lands nearer the lower number, and that is the sense check: the heart spends more of each beat filling than emptying, so the average pressure sits closer to the resting one.
Worked example
A cuff reads 120 / 80 mmHgSystolic first, diastolic second. Both in millimetres of mercury.
Pulse pressure = 120 − 80 = 40 mmHgThe gap between the two. A figure in its own right.
40 / 3 = 13.3 mmHgOne third of the pulse pressure.
MAP = 80 + 13.3 = 93.3 mmHgAdded to the diastolic. Nearer 80 than 120, as it must be.
A reading of 150 / 90: pulse pressure 60 mmHg, MAP = 90 + 20 = 110 mmHgThe same three steps on a second reading, where the thirds come out whole.
Your turn
A cuff reads 106 / 64 mmHg. Write the line that gives the mean arterial pressure.
MAP = 64 + (106 − 64) / 3 = mmHg
Solve one, graded on the server
The trap
Averaging the two numbers. The halfway point of 120 and 80 is 100, which is nearly seven millimetres of mercury above the real mean, and it is wrong in the same direction every time.