Effects of hypoxemia on the a- and b-waves of the electroretinogram in the cat retina

Purpose. Slow components of the electroretinogram (ERG) are sensitive to even mild hypoxemia (60 < P(a)o₂ < 100 mm Hg) in the cat eye. However, the electrical responses of the inner retina remain unchanged until P(a)o₂ 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 he Lamb and Pugh a-wave model during normoxia and hypoxemia. During mild hypoxemia (P(a)o₂ 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 (P(a)o₂ 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 P(a)o₂ 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.