Simplified model of laser Doppler signals during reactive hyperaemia

Laser Doppler flowmetry (LDF) is a non-invasive method to measure tissue bl ood flow. During reactive hyperaemia, the LDF signal increases to a peak an d then returns to a resting value. A simplified model is developed to expla in these variations. The emphasis is on simulating the effects occurring ra ther than on trying to mimic the anatomical structure of the microcirculati on. A single blood vessel is therefore analysed. The increasing value of bl ood velocity is studied, and vasodilatation as well as vasoconstriction are taken into account. The model parameters are calculated using wavelets. Fo r a 2-min occlusion on a healthy subject, the radius of the vessel is initi ally 15 mu m, increasing to 24.6 mu m at the peak, reached 14s after the re lease of the occlusion. The model shows that the high value of the LDF sign al during the initial phase of reactive hyperaemia is produced by an increa sing number of erythrocytes in a cross-section, due to vasodilatation rathe r than an increase in moving blood cell velocities. Moreover, the rapidity of the vasodilatation and vasoconstriction effects determine the rapidity o f the signal variations. The paper aims to give a basic solution to develop a numerical model.