As we know, the systolic blood pressure is the maximum pressure in a vessel during the cardiac cycle. Diastolic pressure is the minimum pressure in a vessel. The pulse pressure is the difference between the systolic and diastolic pressures.
The mean arterial pressure is not simply an average of systolic and diastolic pressures, because the diastolic pressure contributes more to mean pressure than does systolic pressure (Figure 12). The equation, Pave. = Ps + 2Pd/3 , may be used to estimate mean pressure. Its accuracy in this regard depends on the exact shape of the pulse wave. If the pulse wave is wide, it may be applicable. If the pulse wave is narrow, a different equation such as, P = Ps + 3Pd/4, may be a better representation.
The "true mean" or "area mean" method allows one to calculate the exact mean pressure; however, it is laborious and slow. To do this you draw a line through the heart rate of one entire cardiac cycle, such that the area of the curve above the rectangle equals the sum of the two areas below the rectangle. The line drawn is the mean pressure.
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