Mean arterial pressure Physiology

Mean arterial pressure (MAP) is the average arterial pressure throughout one cardiac cycle, systole, and diastole.
Is a time-weighted average of systolic blood pressure and diastolic blood pressure. Systole occupies about one third of the cardiac cycle under resting conditions and diastole occupies about two thirds, giving rise to the following estimate of MAP:

A common method used to estimate the MAP is the following formula:

Mean arterial pressure = DP + 1/3(SP – DP) or MAP = DP + 1/3(PP)

Where DP is the diastolic blood pressure, SP is the systolic blood pressure, and PP is the pulse pressure. Thus, mean arterial pressure is closer to diastolic pressure than to systolic pressure.

This method is often more conducive to measuring MAP in most clinical settings as it offers a quick means of calculation if the blood pressure is known.
Mean arterial pressure is proportional to cardiac output and vascular resistance.
Factors that increase cardiac output or vascular resistance increase MAP.

It is influenced by cardiac output and systemic vascular resistance, each of which is under the influence of several variables.
Mean arterial BP = Cardiac output * Peripheral resistance

Cardiac output is calculated as the product of heart rate and stroke volume. The determination of stroke volume is by ventricular inotropy and preload.

Preload is affected by blood volume and the compliance of veins. Increasing the blood volume increases the preload, increasing the stroke volume, and therefore increasing cardiac output.

Afterload also affects the stroke volume in that an increase in afterload will decrease stroke volume.

Heart rate is affected by the chronotropy, dromotropy, and lusitropy of the myocardium.

To perfuse vital organs requires the maintenance of a minimum MAP of 60 mmHg. If MAP drops below this point for an extended period, end-organ manifestations such as ischemia and myocardial infarction can occur. If the MAP drops significantly, blood will not be able to perfuse cerebral tissues, there will be a loss of consciousness, and neuronal death will occur.

Organ Systems Involved

The cardiovascular system determines the MAP through cardiac output and systemic vascular resistance. Cardiac output is regulated on the level of intravascular volume, preload, afterload, myocardial contractility, heart rate, and conduction velocity. Systemic vascular resistance regulation is via vasoconstriction and dilation.

The renal system affects MAP via the renin-angiotensin-aldosterone system; this is a cascade that ends in the release of aldosterone, which increases sodium reabsorption in the distal convoluted tubules of the kidneys and ultimately increases plasma volume.

The autonomic nervous system plays a role in regulating MAP via baroreceptors located in the carotid sinus and aortic arch. The autonomic nervous system can affect both cardiac output and systemic vascular resistance to maintain MAP in the ideal range.

MAP is not the simple average of systolic and diastolic pressures. Why not?

Systemic arterial pressure is not a single value because arterial pressure varies over the course of each cardiac cycle. Its highest value is systolic pressure, which is measured just after the blood is ejected from the left ventricle into the aorta (i.e., systole).
Its lowest value is diastolic pressure, which is measured as the blood flows from the arteries into the veins and back to the heart (i.e., diastole).
Mean arterial pressure cannot be calculated as the simple average of systolic and diastolic pressures because averaging does not take into account the fact that a greater fraction of each cardiac cycle is spent in diastole (approximately two-thirds) than in systole (approximately one-third).

The use of a sphygmomanometer is the standard way to measure both the systolic and diastolic blood pressures. Once these values are known, a MAP value can easily be determined. An oscillometric blood pressure device can also be used to measure MAP.

A high MAP is anything over 100 mmHg, which indicates that there’s a lot of pressure in the arteries. This can eventually lead to blood clots or damage to the heart muscle, which has to work a lot harder.

Many things that cause very high blood pressure can also cause a high MAP, including:

Anything under 60 mmHg is usually considered a low MAP. It indicates that your blood may not be reaching your major organs. Without blood and nutrients, the tissue of these organs begins to die, leading to permanent organ damage.

A low MAP is considered to be a possible sign of:

Share this Article