Mean Arterial Pressure

Definition

The Mean Arterial Pressure refers to the average pressure of the blood circulating through a person’s arteries, during the cardiac cycle. The value of the mean arterial pressure is normally derived from the systolic blood pressure and diastolic blood pressure of the patient.

The mean arterial pressure is often used for the indication of the blood flow, being considered a more faithful and accurate measurement than the systolic blood pressure. This is because the mean arterial pressure takes into account that 2/3 of the cardiac cycle is represented by the diastole. The mean arterial pressure can be calculated using the formulas presented below, in all patients for which the systolic and diastolic blood pressure values can be obtained.

Normal Value of Mean Arterial Pressure

The normal value of the mean arterial pressure is between 70 and 110 mmHg.

Formula

The mathematical formula for the calculation of the mean arterial pressure is:

Mean arterial pressure = (cardiac output x systemic vascular resistance) + central venous pressure

Note: given the small values of the central venous pressure, this can be neglected in calculating the mean arterial pressure. However, both changes in the cardiac output and the systemic vascular resistance can affect the values of the mean arterial pressure.

In real life medical practice, the mean arterial pressure is not determined by knowing the values of the cardiac output and the ones of the systemic vascular resistance. Instead, the arterial pressure is measured, either through direct or indirect measurements. The most faithful measurement of the mean arterial pressure is obtained with the help of specialized devices, these presenting a reduced risk for errors. It is important to remember that the true value of the mean arterial pressure can be measured using invasive techniques; this is why, if there is no need for the value of the systemic vascular resistance to be identified, in practice the values of the systolic and diastolic blood pressure are going to be measured.

Given the fact that the mean arterial pressure is calculated with the help of the cardiac output and the systemic vascular resistance, it should be mentioned that the variations of the latter can influence the relationship between the mean arterial pressure and cardiac output negatively, making the measurement unreliable. For example, if a patient is in cardiogenic shock, he/she might have a poor cardiac output but a high systemic vascular resistance – this will result in an acceptable value for the mean arterial pressure, even though the patient’s cardiac output is obviously too reduced in order to provide the necessary perfusion with oxygen to the vital organs and tissues.

Calculation

In patients who have normal heart rates, the mean arterial pressure can be calculated with the help of the systolic and diastolic pressures. It is recommended that the mean arterial pressure is calculated when the clinical scenario of the patient indicates the need for such measurements, rather than using the values of the systolic blood pressure. The mean arterial pressure should also be calculated in patients who are suffering from different acute medical problems, with a high concern being presented for the adequate perfusion of the internal organs with oxygen (risk of hypoxia or ischemia).

Mean arterial pressure = diastolic pressure + 1/3 (systolic pressure – diastolic pressure)

Other variants for calculation include:

Mean arterial pressure = 2/3 (diastolic pressure) + 1/3 (systolic pressure)

Mean arterial pressure = (2 x diastolic pressure) + systolic pressure / 3

Mean arterial pressure = diastolic pressure + 1/3 (pulse pressure)

The calculation of the mean arterial pressure for patients who have high heart rates is done through the arithmetic media, given the fact that there will be a change in the shape of the arterial pressure pulse.

Mean arterial pressure = (diastolic pressure + systolic pressure) / 2

Clinical Significance

The clinical significance of the mean arterial pressure lies in the fact that it represents the perfusion pressure of the different internal organs. As it was already mentioned the normal values of the mean arterial pressure vary between 70 and 110 mmHg but, in general, it is believed that even a pressure of 60 mmHg could provide the internal organs with the necessary supply of blood (adequate tissue perfusion – oxygenation).

The measuring of the mean arterial pressure offers vital information in patients who are suffering from generalized infections (sepsis) or those who have gone through trauma. It can also be useful for assessing the vital state of those who have suffered a stroke, a hemorrhage at the level of the brain or hypertension due to a medical emergency.

The decrease of the mean arterial pressure under the value of 60 mmHg, especially when it occurs for a longer period of time, signifies that the vital organs do not receive the adequate supply of blood and thus no oxygen. In the medical literature, this condition is known as hypoxia or ischemia, leading to damage and negative consequences within the affected organs.

Regardless of the condition the patient is suffering from, it can represent an objective of the treatment (short-term or long-term) to bring the values of the mean arterial pressure within normal limits. The values of the mean arterial pressure guide the treatment that the doctor is going to administer to the patient – for example, if a patient is suffering from sepsis (generalized infection), the doctor will recommend vasopressors based on the values of the mean arterial pressure.

In conclusion, the mean arterial pressure can deliver useful information about the state of perfusion of the vital organs, signifying the need for immediate medical intervention. The mean arterial pressure can be calculated through mathematical formulas but it is important to remember that it can be negatively influenced by the changes in the cardiac output or the systemic vascular resistance. At the same time, the true value of the mean arterial pressure can be calculated only through invasive methods, so the measuring of the systolic and diastolic blood pressure is often preferred in the general clinical setting.