Mean Arterial Pressure Calculator
The human heart is a pulsatile pump in that its function is characterized by alternating periods of contraction and relaxation. During the contraction phase (systole), blood is ejected from both the left and right ventricles and pumped into the systemic circulation and pulmonary circulation, respectively. During the relaxation phase of the heart (diastole), the ventricles are filled with blood in preparation for the next contraction phase. While the left and right ventricles contract nearly simultaneously, each pumps blood into a different artery. Contraction of the left ventricle leads to the opening of aortic semilunar valve in order to eject blood into the aorta and, hence, the systemic circulation. Contraction of the right ventricle leads to the opening of pulmonary semilunar valve in order to eject blood into the pulmonary trunk (which branches out into the right pulmonary artery and left pulmonary artery) and, hence, the pulmonary circulation. To prevent back flow of blood into the ventricles, the semilunar valves close during ventricular diastole. The cycles of ventricular contraction and relaxation lead to maximum (systolic) and minimum (diastolic) levels of blood pressure in the major arteries in which blood is initially pumped. Typical systolic and diastolic arterial blood pressure values are 120/80 mm Hg (millimeters of mercury) for the systemic circulation and 30/10 mm Hg for the pulmonary circulation. The difference between the systolic pressure and diastolic pressure is referred to as the pulse pressure. The repeating cycles of cardiac contraction and relaxation are described separately under the Cardiac Cycle.
Table 1. Normal and abnormal arterial blood pressure values
||60 – 79
||90 – 119
||80 – 89
||120 – 139
|Stage 1 Hypertension
||90 – 99
||140 – 159
|Stage 2 Hypertension
While systolic and diastolic arterial blood pressure values are important measurements and have tremendous diagnostic value (see Table 1), it is the mean arterial pressure (MAP) that drives blood through the vasculature from the arteries to arterioles, capillaries, venules, veins, and back to the heart. It is also the mean arterial pressure that is carefully regulated by short-term and long-term regulatory and compensatory mechanisms. The mean arterial pressure is a time-weighted average of pressure values in large systemic arteries during the cardiac cycle. The mean arterial pressure is a function of (1) the rate at which the heart pumps blood into the large arteries, (2) the rate of blood flow out of the large arteries to enter smaller arteries and arterioles, and (3) arterial wall compliance. If the ventricles spent an equal length of time in systole and diastole, the mean arterial pressure could simply be estimated as the mathematical average of systolic and diastolic pressure values. In reality, however, the ventricles spend approximately one-third (1/3) of their time in systole, and two-thirds (2/3) in diastole (see Fig. 1). Therefore, a simple average of the systolic and diastolic pressure values is not an adequate estimate of the mean arterial pressure. Instead, a simple approximation equation is typically used to estimate the mean arterial pressure, where mean arterial pressure = diastolic pressure + (1/3) × pulse pressure (see below).
Figure 1. Normal blood pressure fluctuations in the aorta and other large systemic arteries.
Contraction and relaxation of the ventricles in the heart lead to maximum (systolic) and minimum (diastolic) blood pressure values in the aorta as well as other arteries. Typical systolic and diastolic pressure values of 120/80 mm Hg are shown. The mean arterial pressure is shown to be approximately 93 mm Hg. Following ventricular systole, pressure in the left ventricle rapidly drops to below that in the aorta, thus, causing the aortic semilunar value to close. Closure of the aortic semilunar valve causes the appearance of the dicrotic notch. A typical resting cycle of systole plus diastole lasts approximately 1 second.
Mean arterial pressure equation
- Mean Arterial Pressure is an approximation for the time-weighted average of blood pressure values in large system arteries during the cardiac cycle.
- Diastolic Blood Pressure is the minimum blood pressure measured in large systemic arteries. The lowest value occurs just before the start of every ventricular systole.
- Pulse Pressure is the difference between systolic blood pressure and diastolic blood pressure. Therefore, Pulse Pressure = Systolic Pressure - Diastolic Pressure.
Mean arterial pressure calculator
Each calculator cell shown below corresponds to a term in the formula presented above. Enter appropriate values in all cells except the one you wish to calculate. Therefore, at least two cells must have values, and no more than one cell may be blank. The value of the blank cell will be calculated based on the other values entered. After a calculation is performed, the calculated cell will be highlighted and subsequent calculations will calculate the value of the highlighted cell (with no requirement to have a blank cell). However, a blank cell has priority over a highlighted cell.
Please note that pulse pressure is automatically calculated from the difference between systolic pressure and diastolic pressure. The calculated value of pulse pressure cannot be changed (i.e., it is a read-only value).
Interpretation of mean arterial pressure
The mean arterial pressure represents the average arterial pressure throughout the cardiac cycle, and is the force that drives blood through the vasculature. Because resistance to blood flow is very low in large arteries, there is very little difference between the mean arterial pressure in the aorta and large systemic arteries. It is for this reason that the brachial artery in the upper arms can be used to measure blood pressure. In humans, a mean arterial pressure of at least 60 mm Hg is required to keep blood flowing through the vasculature. It must be kept in mind that there are situations in which vastly different systolic and diastolic pressure values lead to the same estimate of mean arterial pressure. For example, the calculated mean arterial pressure is the same whether the systolic/diastolic pressure values are 120/80 mm Hg or 160/60 mm Hg. While this may be an extreme example, there are pathophysiological situations in which a reduction in arterial compliance leads to an elevated systolic pressure and reduced diastolic pressure, yielding a calculated mean arterial pressure that is close to normal. An example is arteriosclerosis. In these cases, pulse pressure may be a better diagnostic indicator.
Posted: Saturday, March 19, 2011
Last updated: Sunday, June 26, 2011