Mean Arterial Pressure
MAP = (CO × SVR) + CVP
Because CVP is usually at or near 0 mmHg, this relationship is often simplified to:
MAP approx = CO × SVR
Therefore, changes in either CO or SVR will affect MAP. If CO and SVR change reciprocally and proportionately, then MAP will not change. For example, if CO doubles and SVR decreases by one-half, MAP does not change (if CVP = 0). It is important to note that variables found in equation 1 are all interdependent. This means that changing one variable changes all of the others.
In practice, MAP is not determined by knowing the CO and SVR, but rather by direct or indirect measurements of arterial pressure. From the aortic pressure trace over time, the shape of the pressure trace yields a mean pressure value (geometric mean) that is less than the arithmetic average of the systolic and diastolic pressures as shown to the right.
At normal resting heart rates, MAP can be approximated by the following equation:
For example, if systolic pressure is 120 mmHg and diastolic pressure is 80 mmHg, then the mean arterial pressure is approximately 93 mmHg using this calculation. At high heart rates, however, MAP is closer to the arithmetic average of systolic and diastolic pressure (therefore, almost 100 mmHg in this example) because of the change in shape of the arterial pressure pulse (it becomes narrower). To determine mean arterial pressure with absolute accuracy, analog electronic circuitry or digital techniques are used. In normal clinical practice, however, systolic and diastolic pressures are measured, not MAP. That measurement is only measured when SVR needs to be calculated.
To learn what factors alter cardiac output and systemic vascular resistance, and therefore alter arterial pressure, CLICK HERE