Easy Ways How to Calculate the MAP (Mean Arterial Pressure)
An understanding of the mean arterial pressure or MAP begins with knowing what the human blood pressure is all about and how it is determined. Most people recognize that blood pressure depends on the forceful contraction of the heart, which beats steadily and in a rhythmic fashion in order to pump blood to all portions of the human body.
There are two basic measurements that are ordinarily done in order to assess this critical function of the heart; these are the measurement of the systolic blood pressure and the diastolic blood pressure. From these two measurements, a rough estimate of the mean arterial pressure can be arrived at. Using a simple sphygmomanometer or blood pressure measuring device, an individual’s blood pressure can be determined and the mean arterial blood pressure can be calculated from this measurement.
The systolic blood pressure is the pressure generated on the walls of the arteries in the human body as the heart is contracting. This is the “upper” number when it comes to measuring the blood pressure. The diastolic blood pressure, on the other hand, is the pressure found against the blood vessel walls during the period of time when the heart is in diastole or is relaxed. This is the “lower” number measured when checking an individual’s blood pressure. The normal blood pressure in an adult is about 120/80 mm Hg.
How to Calculate the MAP ?
The true calculation of the MAP is the cardiac output multiplied by the systemic vascular resistance added to the central venous pressure. In most cases, the central venous pressure is zero or nearly zero, which makes the MAP most dependent on the cardiac output and the systemic vascular resistance.
In reality, the most accurate representation of the MAP is determined by placing an invasive central line in a person and determining the actual cardiac output, central venous pressure, and systemic vascular resistance. This, however, is not very practical and, as it turns out, the MAP can be calculated simply by knowing the systolic blood pressure and the diastolic blood pressure. Machines that determine an individual’s blood pressure reading will often also show the MAP.
The equation with which the MAP can be calculated is the this: MAP = DBP + 1/3(SBP-DBP), where the DBP is the diastolic blood pressure and the SBP is the systolic blood pressure. So for example is the blood pressure is 120/80 mm Hg his MAP would be 93,3. This is how you would calculate it-
MAP = DBP + 1/3(SBP-DBP)
MAP= 80+ 1/3 (120-80)
MAP= 80+ 1/3(40)
MAP = 80+13,3
MAP= 93,3 mm Hg
Another way to do this would be to multiply the diastolic blood pressure by 2 then add the sum to the systolic blood pressure and divide it by 3. It gives you the same answer.
MAP = SBP + 2 (DBP)/ 3
MAP = 120 +2 (80)/ 3
MAP = 120 +160 /3
MAP = 280/3
MAP = 93,3 mm Hg
This will be relatively accurate with heart rates that are within the normal range; however, if the heart rate is very high, the pulse pressure narrows and the MAP becomes closer to the actual mean or arithmetic average of the two extremes in blood pressure determined at the doctor’s office. This is where digital, non-manual blood pressure machines have the advantage, as this complex relationship between heart rate, blood pressure, and MAP can be factored into the equation.
Why is the Mean Arterial Pressure Important?
The mean arterial blood pressure can be described as an average of the systolic and diastolic blood pressure in the blood vessels of the human body. It becomes important because it represents the pressure available for the perfusion of all of the major organs in the body. Science tells us that it is necessary to have a MAP that is a minimum of 60 mm Hg in order to provide blood flow to the brain, kidneys, and coronary arteries—each of which will suffer significantly if there is a lapse in perfusion.
In many ways, the mean arterial blood pressure is a measurement of the perfusion pressure in the body. The normal range for MAP is between 70 mm Hg and 100 mm Hg. As it turns out, numbers that significantly deviate from this normal range will have a marked effect on organs that cannot afford to have poor perfusion, even if for a few minutes. The MAP then becomes a measurement of the consistency of blood flow to the major organs.
So, what if the MAP is too high or too low? A high mean arterial pressure means that there is resistance to the force of the contraction of the heart muscle. High MAP readings put extra stress on the heart so that it must work much harder than is sustainable in order to perfuse the major organs. It does not necessarily mean that these organs are better perfused than would be the case for a normal MAP; it just means that the “resistance” to blood flow is higher than it should be.
Prolonged elevations of the mean arterial blood pressure cause the heart muscle tissue to hypertrophy in order to continue to push blood through the systemic blood vessels. This can only be sustained for a finite period of time before there is enlargement of the heart, heart failure, and an increased likelihood of a myocardial infarction or heart attack. It also means that the pressure reaching the delicate arteries in the brain will be too high, leading to an increased chance of having a stroke due to high blood pressure.
A low mean arterial blood pressure can be just as dangerous as a high mean arterial blood pressure. When the MAP drops to levels below 60 mm Hg, there is diminished perfusion of the vital organs—some of which do not tolerate a prolonged reduction in blood flow. The brain does not tolerate low mean arterial pressures for longer than a few minutes and the end result will be brain tissue hypoperfusion and stroke. In addition, a low MAP will cause death to cells of the kidneys and other major organs, leading to shock and eventual death.
What affects the MAP?
There are several factors that determine the MAP, with the two main factors being the cardiac output and the resistance to blood flow. The cardiac output is the amount of blood per minute that is pumped out of the left ventricle of the heart. This number averages 5.25 liters per minute.
Anything decreasing the cardiac output will diminish the mean arterial pressure. This includes any cause of hypovolemia, including severe dehydration and trauma with significant blood loss. Heart failure or ventricular failure will decrease the ability of the heart to push blood through the body, reducing the cardiac output and the mean arterial pressure.
Because the cardiac output is the heart rate multiplied by the stroke volume, a rise in heart rate from an increased sympathetic tone in the body that is not accompanied by reductions in systemic vascular resistance can lead to an elevated MAP. Conversely, a reduction in heart rate by itself will reduce the cardiac output. Emotional stress alone will raise catecholamines in the body, increasing the heart rate and constricting the blood flow.
On the other side of the equation leading to the mean arterial pressure is the systemic vascular resistance. A reduction or elevation of the resistance to blood flow will affect the MAP. Septic shock causes low MAP values because severe infections decrease vascular resistance in the body. Infectious organisms give off toxins that the body responds to by releasing cytokines in an effort to fight off the infection. The end result is dilation of the blood vessels and a reduction in blood pressure numbers, including the mean arterial pressure.
On the other hand, there are things that tend to increase peripheral vascular resistance and will increase the MAP. Drugs that cause vasoconstriction will lead to an increase in the MAP by restricting blood flow through the vessels. Increased blood viscosity will adversely affect the flow of blood and will increase the vascular resistance. Anything that results in inelasticity of the blood vessels, such as underlying vascular disease, will potentially increase the peripheral vascular resistance, which will reduce the flow of blood through the vessels and will reduce vital organ perfusion.
Fluid retention from kidney failure will increase the amount of fluid the heart must pump through the system and may increase the MAP. Kidney failure itself activates the renin-angiotensin-aldosterone system. This leads to chemical changes in the body that act to cause vasoconstriction and an increase in mean arterial pressure by increasing the vascular resistance.
The total vessel length will increase the force necessary to push blood through the body. This means that having an increased body mass, such as is seen in obesity, will increase the length of the blood vessels in the body, increasing the vascular resistance. If this occurs in an individual who also has arteriosclerosis, which also increases the body’s vascular resistance, the end result will be an increased mean arterial pressure and an increased potential for its negative effects on the end organs.
Because the MAP is the product of the cardiac output and the systemic vascular resistance, any situation that balances these two factors will not change the MAP. It is only an imbalance of these two factors that results in an increase or decrease in this mean value.
How is the MAP regulated?
The mean arterial pressure is normally regulated by the body by affecting the cardiac output and systemic vascular resistance together. In particular, when one of these is affected, the systems of the body will tend to affect the other so that the MAP will be preserved. When an individual suffers blood loss, for example, the body responds to the potential for a decreased cardiac output by increasing the heart rate (which raises cardiac output) and by constricting the blood vessels (which increases the peripheral vascular resistance). This is the body’s attempt to preserve organ perfusion.
During exercise in the healthy individual, the MAP will be regulated even though there are vast changes in the cardiac output. There are baroreceptor reflexes that respond to stretch of the arterial walls and the heart itself that serve to keep the blood pressure within normal range. There is dilation of the blood vessels in the muscles, which drives down the systemic arterial resistance, and there is dilation of the vessels in the skin after prolong exercise in order to relief the body of heat but also to change the vascular resistance.
It is only when the normal mechanisms in place to maintain the mean arterial pressure begin to fail that the MAP begins to deteriorate or raise to abnormally high levels. An example of this is sepsis. While the body attempts to fight off an infection, there is a failure to maintain the adequacy of the systemic vascular resistance (the blood vessels dilate) and kidney function falters. Failure of the kidneys to be adequately perfused damages their ability to counteract the changes that happen in sepsis so that the MAP only spirals further in a downward direction.
Advancing age diminishes the elasticity of the blood vessels, leading to a reduction in the ability to change the systemic vascular resistance in order to counteract changes in cardiac output. The heart will also respond to a lesser degree to changes in vascular resistance and the MAP can be adversely affected. Again, when there is a lack of balance between the cardiac output and the systemic vascular resistance, the MAP can fall out of the normal range.
How is an Abnormal MAP treated?
An abnormality in the MAP is generally a sign of some underlying problem affecting either the cardiac output or the systemic vascular resistance. This means that the treatment depends on which of these factors is not normal. The ultimate goal is to avoid end organ damage caused either by a MAP that is too low or a MAP that is too high.
A low MAP is dangerous because of underperfusion of the major organs. In order to bring up the pressure, IV fluids or blood transfusions can be given in order to increase the preload to the heart so it has more volume to eject. This will raise the cardiac output, which may increase the MAP. Vasopressors, which are medications that help to constrict the blood flow, can be given to increase the peripheral vascular resistance. In addition, many of these medications increase the heart rate as well, which will also serve to raise the cardiac output.
Treating a MAP that is too high is also important in order to reduce the risk of stroke and secondary heart failure. There are medications, like oral or IV nitroglycerin, which will relax the blood vessels so as to reduce the systemic vascular resistance quickly. The higher the MAP and the longer it is left unchecked, the greater is the chance for heart failure by a heart that cannot exert a force great enough to overcome the high arterial pressures.
Perfusion of the organs is the goal of treating deviations of the MAP. This becomes important when there is a situation in which the heart is already damaged from vascular disease, when head injuries can be made worse if the blood pressure is too high, in septic shock (which is a gross imbalance of compensatory mechanisms), and in abdominal aortic aneurysms, when a high MAP can cause rupture of the damaged aorta.
Websites that Address the MAP
Here are some websites that may help you further understand what the MAP is all about:
- Cardiovascular Physiology Concepts—this is a website that scientifically explains the MAP in detail as well as leads to related pages that aid in understanding the complex relationship between the MAP and other cardiovascular variables.
- Mean Arterial Pressure (MAP)—this site will calculate the MAP and explain what’s normal and what’s abnormal. The calculation will be done for you.
- Blood Pressure—this discusses blood pressure in general and how the MAP is determined from a clinical perspective.
- Factors Regulating Arterial Blood Pressure—this gets into those things that affect blood pressure and, in particular, the mean arterial pressure.
- What is the significance of mean arterial pressure (MAP)?—this website will explain the MAP as well as what things regulate the MAP.
- Factors regulating arterial pressure, cardiac output, and peripheral resistance—this is a website that offers a scientific explanation of those things that influence the MAP.
- Cardiovascular responses during static exercise—this is a research article that explains what happens to the MAP during exercise.
- Exercise Physiology Laboratory—this is a thorough explanation of what happens to cardiac output, peripheral resistance, and blood pressure during normal exercise.
- Analysis of the evidence for the lower limit of systolic and mean arterial pressure in children—this is a research article that explains what is normal with regard to the MAP in children and how to calculate it.
- Low Blood Pressure Treatment—this website is a simple article that helps understand how low arterial pressure is treated.