Regional and total vascular tone have traditionally been assessed by using calculated resistance as the index. However, in most situations, regional flow or cardiac output changes over a much wider range than does blood pressure. With flow in the denominator (resistance), the index is nonlinearly related to the changing parameter, thus rendering even simple arithmetic means inaccurate. Vascular conductance uses flow in the numerator and results in an index that is linearly related to flow, thus enabling a variety of relationships to be demonstrated that are concealed or distorted when studied by the use of vascular resistance.