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1 From the Departments of Biomedical Engineering, and 2 Neurobiology and Physiology, and the 3 Institute for Neuroscience, Northwestern University, Evanston, Illinois.
PURPOSE. Slow components of the electroretinogram (ERG) are sensitive to even mild hypoxemia (60 < PaO2 < 100 mm Hg) in the cat eye. However, the electrical responses of the inner retina remain unchanged until PaO2 is below 40 mm Hg. In this study, the effects of hypoxemia on photoreceptors, on which both slow ERG components and inner retinal activity depend, were examined by recording the a-wave of the ERG.
METHODS. The ERG of dark-adapted, anesthetized cats was recorded between an Ag-AgCl electrode in the vitreous humor and a reference electrode near the eye. Responses to bright flashes of diffuse white light were recorded at 3-minute intervals during hypoxemic episodes lasting 15 minutes to 2 hours.
RESULTS. The cat a-wave was well described by the Lamb and Pugh a-wave model during normoxia and hypoxemia. During mild hypoxemia (PaO2 of 50–60 mm Hg), small changes in a-wave amplitude were detected but did not become greater during severe hypoxemia. The mean decrease in the a-wave amplitude during severe hypoxemia (PaO2 of 20–30 mm Hg) was 8.9% from the mean amplitude during air breathing. The effects of hypoxemia were more severe on the b-wave amplitude. The mean decrease in the b-wave was 35% at PaO2 of 20–30 mm Hg.
CONCLUSIONS. The a-wave is more resistant to severe hypoxemia than the b-wave. This implies that photoreceptor transduction works almost normally during hypoxemia and that failure of inner retinal PO2 regulation causes the decrease in the b-wave. Previously observed changes in the amplitudes of slow ERG components during hypoxemia may result from changes in the ionic environment, rather than a failure of photoreceptor energy metabolism.
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