Resistance or conductance for expression of arterial vascular tone.

The traditional expression of vascular tone is resistance (pressure gradient/flow). It is shown that common manipulations of resistance data, such as arithmetic averaging or linear regression, can lead to substantial errors in situations where blood flow changes to a much greater extent than pressure gradient (the usual in vivo situation). These errors do not occur when conductance is used. Calculations of the extent of vascular escape from vasoconstrictor stimuli and analysis of dose-response data by classical pharmacodynamic methods can be used where such data are expressed as conductance but not when such data are expressed as resistance. Thus, vascular conductance best reflects vascular tone in situations where changes in tone lead primarily to changes in flow. Resistance is the appropriate index for a constant flow preparation, where changes in vascular tone lead primarily to changes in perfusion pressure gradient.