Systemic Vascular Resistance
Systemic vascular resistance (SVR) refers to the resistance to blood flow offered by all the systemic vasculature, excluding the pulmonary vasculature. This is sometimes referred to 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:
SV ≅ MAP ÷ CO
The units for SVR are most commonly expressed as pressure (mmHg) divided by cardiac output (mL/min), or mmHg⋅min⋅mL-1, which is sometimes abbreviated as peripheral resistance units (PRU). Alternatively, SVR may be expressed in centimeter-gram-second (cgs) units as dynes⋅sec⋅cm-5, where 1 mmHg = 1,330 dynes/cm2 and flow (CO) is expressed as cm3/sec. The SVR value in PRU units can be converted to a corresponding value in cgs units by multiplying the PRU value by 80. Although the cgs units are less intuitive, many clinical and experimental studies still express SVR in those units.
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).