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.