Provocative intraocular pressure challenge preferentially decreases venous oxygen saturation despite no reduction in blood flow.

Abstract

Ocular disease can both alter the retina's oxygen requirements, and decrease its ability to cope with changes in metabolic demand. We examined the influence of a moderate intraocular pressure (IOP) elevation on three outcome measures: arterial and venous oxygen saturation, blood flow, and the pattern electroretinogram (PERG). We increased IOP to ˜30mmHg in 23 healthy participants (22-39years) using a mechanical probe applied to the eyelid, thereby lowering ocular perfusion pressure (OPP) by ~30%. The Oxymap retinal oximeter was used to measure oxygen saturation for arteries and veins. Blood flow, volume and velocity were measured using the Heidelberg retinal flowmeter and steady-state PERG waveforms (8.34Hz) were recorded bilaterally (200 sweeps). For each outcome measure, data was obtained three times: at baseline, 1min into sustained IOP elevation, and 1min after the probe was removed. During IOP elevation, changes in oxygen saturation of retinal arteries failed to reach statistical significance [F(1,30)=3.69, p=0.05], whereas venous oxygen saturation was significantly reduced [F(1,21)=27.43, p<0.01]. Blood flow increased slightly [F(2,40)=6.28, p<0.0001], PERG amplitude significantly reduced [F(2,44)=24.24, p<0.0001] and PERG phase was significantly delayed [F(2,44)=17.00, p<0.0001]. Contralateral eyes were unchanged. OPP reduction correlated little with PERG amplitude, PERG phase or venous oxygen saturation. Mild, acute IOP elevation increases arterio-venous oxygen saturation differences primarily through lowering venous oxygen saturation, suggesting increased oxygen consumption by healthy neurons when physiologically stressed.