Figure 30-12 Arterial waveform. Systolic pressure is defined as the instantaneous maximal pressure; diastolic, the instantaneous minimal pressure; and mean, the average pressure over a cycle. dP/dT refers to the upstroke of the arterial pressure, that is, the rate of pressure generation. Mean pressure is estimated as diastolic plus one third the pulse pressure (systolic — diastolic) when only systolic and diastolic pressure is known.