Mean Arterial Pressure (MAP)

Blood pressure is not constant through a heartbeat, so the pressure that actually drives flow into the organs is the time-weighted average over one cycle. That true average needs an arterial waveform to compute. The bedside shortcut works because diastole takes roughly twice as long as systole at ordinary heart rates, so weighting the diastolic pressure by two thirds and the systolic by one third gets close. Written out, that weighting is $\mathrm{MAP} = \mathrm{DBP} + (\mathrm{SBP} - \mathrm{DBP})/3$, and for a textbook 120/80 it gives $80 + 40/3 = 93.3$ mmHg.

The classic mistake is averaging the two numbers. A plain average of 120 and 80 is 100 mmHg, seven points high, and the error grows with the pulse pressure. In a patient with 180/60 the plain average says 120 while the correct estimate says 100, and a 20 mmHg error at that end of the range is the difference between a reassuring chart and a worrying one.

The one-third rule is itself an approximation that assumes a normal heart rate. At a tachycardia of 140, diastole shortens far more than systole does, the true weighting moves toward one half, and this estimate reads low. Invasive arterial lines integrate the real waveform and do not use the formula at all, which is why a monitor's MAP and the MAP you compute from a cuff reading can honestly disagree by several mmHg.