Mean Arterial Pressure (MAP)
Average arterial pressure throughout one cardiac cycle, systole, and diastole.
Surrogate indicator of blood flow and believed to be a better indicator of tissue perfusion.
To perfuse vital organs requires the maintenance of a minimum MAP of 60 mmHg.
MAP = [Cardiac Output (CO) x Systemic Vascular Resistance (SVR)] + Central Venous Pressure (CVP)
MAP = (CO × SVR) + CVP
Because CVP is usually at or near 0 mmHg, this relationship is often simplified to:
MAP ≈ CO × SVR.
Cardiac output (CO) = Heart Rate (HR) X Stroke Volume (SV).
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.
Systemic vascular resistance is determined primarily by the radius of the blood vessels.
Decreasing the radius of the vessels increases vascular resistance.
Increasing the radius of the vessels would have the opposite effect.
Blood viscosity can also affect systemic vascular resistance.
An increase in hematocrit will increase blood viscosity and increase systemic vascular resistance.
Viscosity, however, is considered only to play a minor role in systemic vascular resistance.
Common formula:
MAP = Diastolic blood pressure + 1/3 (Systolic Blood pressure – Diastolic Blood Pressure)
= DBP + 1/3(SBP – DBP) or
MAP = DBP + 1/3(Pulse Pressure)
MAP = [Systolic Blood Pressure + (2 x Diastolic Blood Pressure)]
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Example, if blood pressure is 82 mm Hg/50 mm Hg,
MAP = SBP + 2 (DBP) = 82 +2 (50) = 182 = 60.67 mmHg; or
3 3 3
MAP = 1/3 (SBP – DBP) + DBP = 1/3 (82-50) + 50 = 10.67 + 50 = 60.67 mmHg
In sepsis, vasopressors are often titrated based on the MAP.
In the guidelines of the Surviving Sepsis Campaign, it is recommended that MAP be maintained ≥ 65 mm Hg.
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