Systemic vascular resistance (SVR) is the resistance of the circuit (the arterial system) into which the heart is pumping the blood. The situation is analogous to an electrical circuit, for which Ohm's Law states that
V / I = R
where V is potential difference (volts), I is current (amps) and R is resistance (ohms).
In the case of the circulation
SVR = (MAP - RAP) ÷ CO
where MAP is mean arterial pressure, RAP is right atrial pressure and CO is cardiac output.
In this equation, RAP is subtracted from MAP to give the pressure across the vascular bed between the left ventricle and right atrium, although this may not be logical!
In a normal individual, if
systolic arterial pressure = 125 mm Hg
diastolic arterial pressure = 90 mm Hg
RAP = 5 mm Hg
then at a normal resting cardiac output (for an average adult) of 5 l/min
SVR = (MAP - RAP) / CO = 90 / 5 = 18 Woods units
Instead of the arbitrary Woods units it is more common for vascular resistance to be measured in dyn s cm-5. These units are derived as follows:
The pressure exerted by a column of mercury 1 mm high is given by:
(where 13.6 is the density of mercury and 981 is the acceleration due to gravity; 1 dyne is the force needed to accelerate a mass of 1 g by 1 cm per second per second, i.e.
Therefore, dividing pressure by flow gives units of
To convert from Woods units to dyn s cm-5 :
multiply by 1333 / (1000 / 60) = 80
In the example above, 18 Woods units is equivalent to
Also see indexed values