[Studies of the hemodynamics of the optic head nerve using laser Doppler flowmetry]

To study hemodynamics in the optic nerve head (ONH) in animals and humans using laser Doppler flowmetry (LDF) during physiological stimuli such as flicker light stimulation, variations of the breathing gas (O(2)) concentration, and systemic mean perfusion pressure (PP(m)) variations.

LDF is based on the Doppler effect according to which the frequency of the light scattered by a moving particle (i.e., red blood cells in the vessels) is shifted in frequency compared to that of the incident light. The laser light scattered from the tissue and the red blood cells (RBCs) is collected by an optical fiber and fed to photodetector. NeXT computer analysis provides relative measurements of the Vel(ONH) which expresses the mean velocity of the RBCs, the Vol(ONH) representing the number of RBCs and the F(ONH) an expression of the flux of RBCs in the volume sampled by the laser. Modifications of the LDF parameters during physiological stimuli such as modifications of breathing gas (O(2), CO(2)), flicker light stimulation, and variations in the perfusion pressure were studied.

The results of these studies demonstrated for the first time a dynamic coupling of blood flow to function and metabolism in the ONH, mediated by an increase in potassium and nitric oxide release. In addition the results of these studies showed that the ONH blood remains constant during variations of the perfusion pressure (PP(m)) induced by an increase in the intraocular pressure or an increase of the systemic blood pressure by isometric exercise. These results confirm an autoregulation in the ONH blood flow in animals and humans.

LDF is a powerful technique for investigating changes in blood flow in the ONH of anesthetized animals and humans, induced by physiological stimuli involving the breathing of various gases, neuronal stimulation, and variations in the perfusion pressure. This highly sensitive and reproducible technique opens new avenues in the elucidation of blood flow regulation mechanisms in the ONH.