Blood flow in the human optic nerve head during isometric exercise.

Investigating blood flow autoregulation in the optic nerve is important to understand the physiopathology of various ocular diseases such as glaucoma. This investigation requires that one establishes the relationship between optic nerve blood flow and perfusion pressure. Previous work has documented the effect of lowering the perfusion pressure on optic nerve blood flow. The purpose of the present study was to investigate the effect of elevated perfusion pressure on blood flow in this tissue. Laser Doppler flowmetry was applied to measure relative mean velocity, volume and flux of red blood cells in the tissue of the optic nerve head. These parameters were measured in 13 subjects during isometric exercise consisting of squatting. In the range of perfusion pressures from 56+/-4 to 80+/-5 mmHg (30+/-8%), there was no significant variation of mean velocity, volume and flux of red blood cells, but vascular resistance increased by about 50%. Intraocular pressure was increased significantly above baseline at the end of squatting and decreased during recovery. The results suggest that the maintenance of constant blood flow is achieved by an increase in vascular resistance taking place either at the arterioles feeding or at the veins draining the blood from the ONH or at the ophthalmic artery and/or vessels between this artery and the site of LDF measurements. Combining the results of this study with those of a previous one where perfusion pressure was decreased by increasing the intraocular pressure, we show the entire relationship between perfusion pressure and optic nerve blood flow in man. Copyright 1998 Academic Press.