Iris blood flow response to acute decreases in ocular perfusion pressure: a laser Doppler flowmetry study in humans

The relationship between blood flow and ocular perfusion pressure in the iris vascular bed of the human eye has not been established yet. Consequently, it is not known whether the iris circulation has some autoregulatory capacity. The aim of the present study was to investigate this relationship in the particular case where the perfusion pressure was decreased by increasing the intraocular pressure. Using laser Doppler flowmetry, mean velocity, volume and flow of blood through the iris were measured in normal subjects during acute decreases of the mean ocular perfusion pressure induced by raising the intraocular pressure with a scleral suction cup. Two experimental paradigms were applied. In the first, the suction pressure was rapidly increased from baseline, in steps of 50-100 mmHg, to a level where the intraocular pressure was above the mean ophthalmic artery blood pressure. In the second, the suction pressure was increased from baseline in four successive steps of 50 mmHg each. The suction pressure was kept constant for 2 min at each step. With the first paradigm (nine eyes), a 72% decrease in perfusion pressure induced a 63% decrease of iris blood flow due mainly to a drop in blood velocity. Immediately after suction release, blood flow increased by 62% above baseline and then returned to its baseline value within 2 min. With the second paradigm (six eyes), a 28% decrease in perfusion pressure resulted in a 30% flow reduction, which was due to significant decreases (P<0. 001) of both blood volume and velocity. Combining the results of both paradigms, we observed a significant linear correlation between iris blood flow and perfusion pressure (R =0.964, P<0.001). These results demonstrate that a decrease of the perfusion pressure due to an increase of the intraocular pressure induces a decrease of the iris blood flow. No evidence of an autoregulatory process in the iris vascular bed could be demonstrated. A reactive hyperemia was observed in response to a sudden increase in perfusion pressure occurring after a period of decreased blood flow.