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

Purpose: To study hemodynamics in the optic nerve head (ONH) in animals and humans using laser Doppler Flowmetry (LDF) during physiological stimuli su ch as flicker light stimulation, variations of the breathing gas (O-2) conc entration, and systemic mean perfusion pressure (PPm) variations. Methods: LDF is based on the Doppler effect according to which the frequenc y 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. T he laser light scattered from the tissue and the red blood cells (RBCs) is collected by an optical fiber and fed to photodetector. NeXT computer analy sis 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, CO2), flicker light stimulation. and v ariations in the perfusion pressure were studied. Results: The results of these studies demonstrated for the first time a dyn amic coupling of blood flow to function and metabolism in the ONH, mediated by an increase in potassium and nitric oxide release. In addition the resu lts of these studies showed that the ONH blood remains constant during vari ations of the perfusion pressure (PPm) induced by an increase in the intrao cular pressure or an increase of the systemic blood pressure by isometric e xercise. These results confirm an autoregulation in the ONH blood flow in a nimals and humans. Conclusion: LDF is a powerful technique for investigating changes in blood flow in the ONH of anesthetized animals and humans, induced by physiologica l stimuli involving the breathing of various gases, neuronal stimulation, a nd Variations in the perfusion pressure. This highly sensitive and reproduc ible technique opens new avenues in the elucidation of blood flow regulatio n mechanisms in the ONH.