Simplified model of laser Doppler signals during reactive hyperaemia.

Laser Doppler flowmetry (LDF) is a non-invasive method to measure tissue blood flow. During reactive hyperaemia, the LDF signal increases to a peak and then returns to a resting value. A simplified model is developed to explain these variations. The emphasis is on simulating the effects occurring rather than on trying to mimic the anatomical structure of the microcirculation. A single blood vessel is therefore analysed. The increasing value of blood velocity is studied, and vasodilatation as well as vasoconstriction are taken into account. The model parameters are calculated using wavelets. For a 2-min occlusion on a healthy subject, the radius of the vessel is initially 15 microns, increasing to 24.6 microns at the peak, reached 14 s after the release of the occlusion. The model shows that the high value of the LDF signal during the initial phase of reactive hyperaemia is produced by an increasing number of erythrocytes in a cross-section, due to vasodilatation rather than an increase in moving blood cell velocities. Moreover, the rapidity of the vasodilatation and vasoconstriction effects determine the rapidity of the signal variations. The paper aims to give a basic solution to develop a numerical model.