Systemic Vascular Resistance (SVR)

This is Ohm's law with blood in place of current. The pressure drop across the systemic circuit is the mean arterial pressure minus the central venous pressure, the flow is the cardiac output, and the ratio is the resistance. With MAP 90, CVP 5 and an output of 5 L/min the answer is $80 \times 85 / 5 = 1360$, against a usual reference band of roughly 800 to 1200 dyn·s·cm⁻⁵.

The 80 is pure unit conversion and nothing else. Divide mmHg by L/min and you get a resistance in mmHg·min/L, a perfectly reasonable unit that nobody uses; the literature settled on the CGS unit dyn·s·cm⁻⁵, and 80 is the factor between them. It is not a physiological constant, and reporting a resistance without saying which unit it is in makes the number meaningless. Some units drop the 80 entirely and report Wood units instead, where the same patient scores 17.

The physics behind the number is where the real caution lies. Poiseuille's law says resistance scales with the fourth power of vessel radius, so tiny changes in arteriolar tone move it enormously, and SVR is dominated by the arterioles rather than by the large arteries. But the calculation lumps a branching, pulsatile, non-Newtonian system into a single number and assumes steady laminar flow through rigid tubes, none of which is true. Because CVP is usually the smallest and least reliably measured term, and cardiac output carries its own 10 to 20 percent uncertainty, a calculated SVR is best read as a trend across a treatment rather than as a precise quantity.