What is Mean Arterial Pressure?
As blood flows from the heart to the arteries, the pumping effect exerts a force against the walls of these vessels, and this pressure is expressed as a ratio of the maximum (systolic pressure) over the minimum (diastolic pressure). Arterial pressure, also known as blood pressure, is a familiar vital sign read as 120/80, for example, measured in mm Hg (symbol for mercury). An individual’s blood pressure (BP) is often affected by factors such as age, heart disease, emotions, activities, body position, and many others.
The mean arterial pressure (MAP) on the other hand, expresses a relationship between the systolic and diastolic pressures (the mean, or approximately, the average). Its significance lies in the fact that it reflects the perfusion pressure, or the force that enables the blood in circulation to supply the vital organs of the body with oxygen and important nutrients.
MAP is affected by factors such as:
- Volume of blood pumped by the heart per minute (cardiac output)
- Heart rate (beats per minute)
- Blood pressure
- Resistance to blood flow in the vessels
An increase or decrease in any of these factors can proportionately affect mean arterial pressure and bring corresponding consequences to the perfusion of major organs like the brain and kidneys.
What MAP Meanings Do Circulatory Changes Bring?
Changes in the factors affecting blood circulation mentioned above can bring about different implications to MAP and how organs are perfused.
Mean arterial pressure meanings are significant to doctors, nurses and health workers who are involved in monitoring a patient's physical status and his response to therapy especially in critical situations.
Normal MAP is at 70-110 mm Hg, where adequate blood supply reaches the vital organs to deliver enough oxygen and nutrients. A significant decrease in MAP therefore results in a deprivation of this supply and can cause organ damage and death, when the condition is prolonged.
An increase in heart rate, as in exercise and emotional states can bring the MAP closer to the arithmetic mean of the systolic and diastolic pressures; vascular resistance usually decreases proportionately. Small and temporary changes do not result in significant consequences because compensatory changes in the circulatory system for adaptation occur. This is often referred to as autoregulation.
However, sudden and prolonged changes like a significant blood loss during trauma can dramatically decrease systolic pressure and bring down MAP to a degree that blood supply to the organs is compromised. If not stopped or replaced, blood loss can result in hypotension (decreased blood pressure) which can result in death.
Other causes of decreases in blood pressure like a massive heart attack can bring about a significant decrease in cardiac output and MAP, preventing perfusion of the brain and kidneys, also leading to organ damage. Certain drugs can also affect blood pressure, heart rate and cardiac output, so that monitoring responses to therapy should be done to ensure that MAP is adequately maintained.
In summary, changes in the circulation can affect mean arterial pressure which could mean perfusion pressure compromise, or adverse effects in the delivery of oxygen and food to the vital organs.
CV Physiology, "Mean Arterial Pressure" accessed 1/1/11
Critical Care, "Clinical review: Interpretation of arterial pressure wave in shock states" accessed 1/2/11