Effect of a decreased ocular perfusion pressure on iris blood flow measured with laser Doppler flowmetry

Purpose To determine whether iris blood flow (IBF) is regulated in response to an acute decrease in mean ocular perfusion pressure (PPm=MOAP-IOP, MOAP =mean ophthalmic arterial pressure) induced by increasing the intraocular p ressure (IOP). Methods Iris blood flow was measured using a slit lamp incorporating a lase r Doppler flowmetry (LDF) module. The study was conducted on 12 normal volu nteers (14 to 59 years old). IOP was raised using a scleral suction cup. In Exp. #1, the suction pressure was successively raised in steps of 50 to 10 0 mm Hg, each lasting about 10 sec, until IOP reached the MOAP level. In Ex p. #2, the suction was raised to 200 mm Hg in 4 successive steps of 2 min d uration. Results In Exp. #1, no significant change of IBF was observed for small dec reases of PPm (<23%); greater decreases of PPm resulted in a linear IBF dec rease (p < 0.01). In Exp. #2, such a IBF versus PP, decrease was also obser ved (p<0.001). Immediately after release of suction, a significant, transie nt IBF increase of 79% above baseline level was observed. Conclusion These results suggest that some IBF regulation occurs for small PP, decreases (<23%); no IBF compensatory mechanism appears to operate for further decreases of PP, (>23%).