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 blood pressure in an individual during a single cardiac cycle. MAP is altered by cardiac output and systemic vascular resistance.

Testing

Arterial line

Mean arterial pressure can be measured directly or determined by using a formula. The least invasive method is the use of an blood pressure cuff which gives the values to calculate 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.

Calculation

While MAP can only be measured directly by invasive monitoring. The MAP 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.

Normal heart rate

The most common formula for calculating mean arterial pressure is:

$MAP=DP+1/3(SP-DP)$

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, Mean arterial pressure 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 calculate mean arterial pressure are:

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

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. It is believed that a MAP that is greater than 70 mmHg is enough to sustain the organs of the average person. MAP is normally between 65 and 110 mmHg.

Hypotension

When assessing hypotension, the context of the patient’s baseline blood pressure needs to be considered. Acute decreases in mean arterial pressure of around 25% put patients at increased risk for end-organ damage and potential morbidity and mortality. Even 1 minute at a mean arterial pressure of 50 mmHg, or accumulative effects over short periods, increases the risk of mortality by 5% and can result in organ failure or complications.

MAP may be used like systolic blood pressure in monitoring and treating target blood pressure. Both have been shown advantageous targets for sepsis, major trauma, stroke, intracranial bleed. In patients with sepsis, the vasopressor dosage may be titrated on the basis of estimated MAP.

Hypertension

In younger patients, elevated mean arterial pressure has been shown to be more important than pulse pressure in the prediction of stroke. However in older patients, MAP has been found to be less predictive of stroke and a better predictor of cardiovascular disease.

See also

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

External links

• Mean Arterial Pressure Calculator
• More Information on usage of the Mean Arterial Pressure