Systemic Vascular Resistance
Systemic vascular resistance (SVR) refers to the resistance to blood flow offered by all of the systemic vasculature, excluding the pulmonary vasculature. This is sometimes referred as total peripheral resistance (TPR). SVR is therefore determined by factors that influence vascular resistance in individual vascular beds. Mechanisms that cause vasoconstriction increase SVR, and those mechanisms that cause vasodilation decrease SVR. Although SVR is primarily determined by changes in blood vessel diameters, changes in blood viscosity also affect SVR.
SVR can be calculated if cardiac output (CO), mean arterial pressure (MAP), and central venous pressure (CVP) are known.
SVR = (MAP - CVP) ÷ CO
Because CVP is normally near 0 mmHg, the calculation is sometimes simplified to:
SVR = MAP ÷ CO
It is very important to note that SVR can be calculated from MAP and CO, but it is not determined by either of these variables. A more accurate way to view this relationship is that at a given CO, if the MAP is very high, it is because SVR is high. Mathematically, SVR is the dependent variable in the above equations; however, physiologically, SVR and CO are normally the independent variables and MAP is the dependent variable (see Mean Arterial Pressure).