Mean Arterial Pressure (MAP)

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Introduction:

The MAP, or mean arterial pressure, is a measure of the average blood pressure in the arteries. It’s important because it helps to determine the perfusion, or blood flow, to the organs and tissues of the body. To calculate the MAP, you need to know the systolic blood pressure (the top number in a blood pressure reading) and the diastolic blood pressure (the bottom number). The number is calculated in mmHg (or millimeters of mercury), which is the same unit of measurement used for your blood pressure. A normal MAP is going to vary depending on the resource, but generally a MAP of 60-65 is considered normal.

Calculating MAP

To calculate the MAP, you need to know the systolic blood pressure (the top number in a blood pressure reading) and the diastolic blood pressure (the bottom number). The formula for MAP is: MAP = (2 x diastolic blood pressure) + systolic blood pressure / 3

For example, if a patient’s blood pressure is 120/80 mmHg, the MAP would be calculated as:

MAP = (2 x 80) + 120 / 3 = 93.3 mmHg

You can also look at a simpler formula for MAP as: Systolic + Diastolic + Diastolic = Big Number. Big Number / 3 = MAP

120 + 80 + 80 = 280. 280 / 3 = 93.3 mmHg

Why is MAP Important?

The MAP is important because it helps to determine the perfusion, or blood flow, to the organs and tissues of the body. A MAP that is too low can indicate that there is not enough blood flow to the organs, which can lead to organ damage or failure. A MAP that is too high can also be a problem because it can put extra stress on the heart and blood vessels, which can lead to hypertension and other health problems.

So, the MAP is a measure of the average blood pressure in the arteries, it is important because it helps to determine the perfusion, or blood flow, to the organs and tissues of the body and is used to monitor the effectiveness of certain treatments and to detect changes in the patient’s condition.

Do all organs react to MAP the same way?

The ideal MAP pressure for organ perfusion depends on the specific organ, but generally, a MAP of at least 60 mmHg is considered necessary to maintain adequate blood flow to the organs. A MAP less than 60 mmHg is considered low and can indicate poor perfusion and organ hypoperfusion, which can lead to organ damage or failure.

For example:

The brain has a relatively low tolerance for changes in perfusion, a MAP of less than 60 mmHg can lead to cerebral hypoperfusion and can cause confusion, loss of consciousness, and other neurological symptoms.
The kidney also has a relatively low tolerance for changes in perfusion, a MAP of less than 60 mmHg can lead to renal hypoperfusion, which can cause a decrease in urine output and an increase in creatinine levels.
The heart, on the other hand, can tolerate more fluctuation in perfusion, a MAP of at least 70 mmHg is considered necessary to maintain adequate blood flow to the heart and to prevent cardiac ischemia.

The heart requires a higher MAP to function normally?

While the heart can function at a lower MAP, a higher MAP is ideal for it to work without complications. The heart does require a higher MAP than the brain to maintain adequate blood flow; this is because the heart is a muscle that needs a steady blood flow to function correctly and to pump blood to the rest of the body. The heart also has a higher oxygen demand than other organs, so it needs a higher MAP to ensure that it receives an adequate amount of oxygen-rich blood. The heart can also tolerate more fluctuation in perfusion compared to the brain because the heart has a robust blood supply and a network of collaterals (smaller blood vessels) that can take over the blood flow in case of blockages in the main blood vessels. This feature allows the heart to function even with low blood pressure.

On the other hand, the brain is a highly sensitive organ that requires a steady blood flow to function properly. The brain also has a lower tolerance for changes in perfusion, as it has a limited capacity to store oxygen and glucose. A decrease in blood flow to the brain can cause cerebral hypoperfusion and can cause confusion, loss of consciousness, and other neurological symptoms. So, the heart requires a higher MAP than the brain to maintain adequate blood flow and to pump blood to the rest of the body, the heart also has a higher oxygen demand than other organs, and it can tolerate more fluctuation in perfusion compared to the brain because it has a robust blood supply and network of collaterals. On the other hand, the brain is a highly sensitive organ that requires a steady blood flow and has a limited capacity to store oxygen and glucose, making it more susceptible to hypoperfusion.

Scenarios

A Typical Person

A typical person has a blood pressure that is in the ballpark of 120/80, and this can vary based on body type and other health issues, or athletic ability. If we look at a person who has a lower blood pressure of 110/70, are they perfusing?

110 + 70 + 70 = 250

250 / 3 = 83.3

With a MAP of 83, which is clearly above the normally required, this person definitely has normal perfusion.

A Septic Patient

When someone is septic, they are in a form of distributive shock – meaning their blood vessels are dilated. This can be problematic for septic patients, and is why they are typically going to be hypotensive when they get sicker.

If we have a patient with a blood pressure of 88/40, are they perfusing normally?

88 + 40 + 40 = 168

168 / 3 = 56

With a MAP of 56, this patient is definitely going to be sick and likely confused and altered.

Same Patient, After Fluids

We have the same septic patient who we have given fluids to. Their blood pressure has changed to 88/50. The only thing that changed is the diastolic pressure, and that could be because we just have more fluid in the container. Your diastolic number is generally going to be considered more important than your systolic pressure, because your diastolic pressure is used in 2 of the 3 numbers in the formula, so it contributes to a MAP more than a systolic pressure.

88 + 50 + 50 = 188

188 / 3 = 62.6

With this MAP being 62, it is in the ballpark of the 60-65 that we want. This patient is still sick, and that is still low, but they are less sick than when we first got to them.

Head Injury

When patients have a head injury, they can develop into a thing called Cushings Triad. Cushings Triad is defined as: Bradycardia, Hypertension, Irregular Respirations.

If that patient now has a blood pressure of 192/110, this can be problematic.

192 + 110 + 110 = 412

412 / 3 = 137.3

That 137 MAP is really high – and over a long period of time it is definitely not good. Imagine you had a water hose that had more pressure in it than was made to hold, eventually it can burst and cause a leak. The same thing is true for the human body. So while this person is perfusing, too high of a blood pressure can be just as dangerous as one that is too low.

Tips and Tricks

MAP is composed of 3 numbers, and then divide that large number by 3
MAP of 60 or greater is generally considered good
MAP that is too high or low can be dangerous
Brain and Kidneys require a MAP of 60 generally
Your heart likes a MAP of 70 to be safe