Physiology, Mean Arterial Pressure

The definition of mean arterial pressure (MAP) is the average arterial pressure throughout one cardiac cycle, systole, and diastole. MAP is influenced by cardiac output and systemic vascular resistance, each of which is influenced by several variables. These will be discussed further under the Mechanism heading of this article.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.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.A common method used to estimate the MAP is the following formula: MAP = 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. 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.