Mean arterial pressure

Not to be confused with mean airway pressure.

Mean arterial pressure
Representation of the arterial pressure waveform over one cardiac cycle. The notch in the curve is associated with closing of the aortic valve.
MeSH	D062186

In medicine, the mean arterial pressure (MAP) is an average calculated blood pressure in an individual during a single cardiac cycle. Although methods of estimating MAP vary, a common calculation is to take one-third of the pulse pressure (the difference between the systolic and diastolic pressures), and add that amount to the diastolic pressure. A normal MAP is about 90 mmHg.

Mean arterial pressure = diastolic blood pressure + ⁠(systolic blood pressure - diastolic blood pressure)/3⁠

MAP is altered by cardiac output and systemic vascular resistance. It is used clinically to estimate the risk of cardiovascular diseases, where a MAP of 90 mmHg or less is low risk, and a MAP of greater than 96 mmHg represents "stage one hypertension" with increased risk.

Testing

Arterial line

Mean arterial pressure can be measured directly or estimate from systolic and diastolic blood pressure by using a formula. The least invasive method is the use of a blood pressure cuff which gives the values to calculate an estimate of the mean pressure. A similar method is to use a oscillometric blood pressure device that works by a cuff only method where a microprocessor determines the systolic and diastolic blood pressure. Invasively, an arterial catheter with a transducer is placed and the mean pressure is determined by the subsequent waveform.

Estimating MAP

Mean arterial pressure in relation to systolic and diastolic pressure in blood vessels

While MAP can only be measured directly by invasive monitoring, it can be estimated by using a formula in which the lower (diastolic) blood pressure is doubled and added to the higher (systolic) blood pressure and that composite sum then is divided by 3 to estimate MAP.

Thus, a common way to estimate mean arterial pressure is to take one-third of the pulse pressure added to the diastolic pressure:

$MAP\approx DP+1/3(SP-DP)$

where:

• DP = diastolic pressure
• SP = systolic pressure
• MAP = mean arterial pressure

Systolic pressure minus diastolic pressure equals the pulse pressure which may be substituted in.

Blood pressure cuff

Another way to find the MAP is to use the systemic vascular resistance equated ($R$), which is represented mathematically by the formula

$R=\Delta P/Q$

where $\Delta P$ is the change in pressure across the systemic circulation from its beginning to its end and $Q$ is the flow through the vasculature (equal to cardiac output).

In other words:

$SVR=(MAP-CVP)/CO$

Therefore, MAP can be determined by rearranging the equation to:

$MAP=(CO\cdot SVR)+CVP$

where:

• $CO$ is cardiac output
• $SVR$ is systemic vascular resistance
• $CVP$ is central venous pressure and usually is small enough to be neglected in this formula.

This is only valid at normal resting heart rates during which $MAP$ can be approximated using the measured systolic ($SP$) and diastolic ($DP$) blood pressures:

Elevated heart rate

At high heart rates $MAP$ is more closely approximated by the arithmetic mean of systolic and diastolic pressures because of the change in shape of the arterial pressure pulse.

For a more accurate formula of $MAP$ for elevated heart rates use:

$MAP\simeq DP+0.01\times \exp(4.14-40.74/HR)\times PP$

Where

• HR = heart rate.
• DP = diastolic pressure
• MAP = mean arterial pressure
• PP = pulse pressure which is systolic minus diastolic pressure

Most accurate

The version of the MAP equation multiplying 0.412 by pulse pressure and adding diastolic blood is indicated to correlate better than other versions of the equation with left ventricular hypertrophy, carotid wall thickness and aortic stiffness. It is expressed:

$MAP=DBP+(0.412\times PP)$

where:

• DBP = diastolic pressure
• MAP = mean arterial pressure
• PP = pulse pressure

Young patients

For young patients with congenital heart disease a slight alteration to the factor used found to be more precise. This was written as:

$MAP=DBP+(0.475\times PP)$

where:

• DBP = diastolic pressure
• MAP = mean arterial pressure
• PP = pulse pressure

This added precision means cerebral blood flow can be more accurately maintained in uncontrolled hypertension.

Neonates

For neonates, because of their altered physiology, a different formula has been proposed for a more precise reading:

$MAP=DBP+(0.466\times PP)$

where:

• DBP = diastolic pressure
• MAP = mean arterial pressure
• PP = pulse pressure

It has also been suggested that when getting readings from a neonates radial arterial line, mean arterial pressure can be approximated by averaging the systolic and diastolic pressure.

Other formula versions

Other formulas used to estimate mean arterial pressure are:

$MAP=DBP+(0.33PP)+5$

or

$MAP=DBP+[0.33+(0.0012\times HR)]\times PP$

or

$MAP=DAP+PP/3$

or

$MAP=DAP+(PP/3)+5mmHg$

• MAP = mean arterial pressure
• PP = pulse pressure
• DAP = diastolic aortic pressure
• DPB = diastolic blood pressure

Clinical significance

Thresholds for 24 hr. mean arterial pressure (MAP)

24 hr. MAP category	24 hr. MAP
Normal	< 90 mmHg
Elevated blood pressure	90 to < 92 mmHg
Stage 1 hypertension	92 to < 96 mmHg
Stage 2 hypertension	> 96 mmHg

Mean arterial pressure is a major determinant of the perfusion pressure seen by organs in the body. MAP levels greater than 90 mmHg increase the risk stepwise of having higher risk of cardiovascular diseases, such as stroke, and mortality.

Hypotension

When assessing hypotension, the context of the baseline blood pressure needs to be considered. Acute decreases in mean arterial pressure of around 25% put people at increased risk for organ damage and potential mortality. Even one minute at a MAP of 50 mmHg, or accumulative effects over short periods, increases the risk of mortality by 5%, and can result in organ failure or complications.

In people hospitalized with shock, a MAP of 65 mmHg lasting for more than two hours was associated with higher mortality. In people with sepsis, the vasopressor dosage may be titrated on the basis of estimated MAP.

MAP may be used like systolic blood pressure in monitoring and treating target blood pressure. Both are used as targets for assessing sepsis, major trauma, stroke, and intracranial bleeding.

Hypertension

In younger people, elevated MAP is used more commonly than pulse pressure in the prediction of stroke. However in older people, MAP is less predictive of stroke and a better predictor of cardiovascular disease.

See also

• Blood pressure
• Hypertension
• Hypotension
• Systemic vascular resistance
• Pulse pressure
• Mean systemic pressure