The electroretinogram and choroidal PO2 in the cat during elevated intraocular pressure

The effects of elevated intraocular pressure (IOP) on the dark-adapted standing potential (SP), b- and c-waves of the electroretinogram (ERG) and choroidal PO2 of the intact cat eye were studied. During pressure elevation the SP increased, c-wave amplitude increased and the b-wave decreased, with the SP and c-wave being considerably more sensitive to pressure elevation. Inspiration of 100% O2 reversed the effects of elevated IOP on the c-wave and SP. ERG changes were found to be more consistent when related to perfusion pressure (PP = MAP - IOP) rather than absolute IOP. Initial increases in c-wave amplitudes and SP were observed at PPs as high as 105 mm Hg. The SP was most sensitive to IOP increases; the PP at which a change was first detected averaged 84.0 +/- 16.3 mm Hg (n = 7 cats). The PP at which the earliest c-wave change occurred was 78.7 +/- 16.7 mm Hg (n = 12 cats). The PP at which a b-wave change was first observed was 51.5 +/- 14.9 mm Hg (n = 11 cats). Choroidal PO2 was measured with an oxygen microelectrode and decreased with increasing IOP. A positive correlation was found (r = 0.78) between an increase in c-wave amplitude and decrease in choroidal PO2. From our results we can hypothesize that increased IOP led to decreased choroidal blood flow and outer retinal hypoxia measured as decreased choroidal PO2, and that this hypoxia was responsible for the c-wave and standing potential changes we observed.