Effective diameter as a determinant of local vascular resistance in presence of vasomotion.

The mean resistance of arterioles exhibiting rhythmic changes in diameter (vasomotion) depends on the mean vascular diameter and the amplitude and shape of the vasomotion pattern. The effective diameter, defined as the diameter of a tube with constant diameter and the same vascular resistance as the vessel showing vasomotion, was calculated using Poiseuille's law. The effective diameter was used to compare the results of model calculations of square wave, sinusoidal, and triangular vasomotion patterns with those obtained from rabbit tenuissimus muscle arterioles. Due to the variability of the actual vasomotion waveforms, approximation of the effective diameter using the mean diameter, the relative vasomotion amplitude, and an assumed waveform led to erroneous results. Therefore, effective diameter should be calculated directly from the actual arteriolar diameter tracings to take into account all irregularities in the vasomotion pattern.