Effects of hypercapnia on the electroretinogram in sevoflurane and isoflurane anaesthetized dogs

To evaluate the effects of induced hypercapnia on the electroretinogram (ERG) in beagle dogs anaesthetized with isoflurane and sevoflurane. Binocular, full-field flash photopic and scotopic ERGs were obtained from six healthy neutered female beagle dogs. In order to determine Vmax and the photopic negative response (PhNR), photopic ERG luminance–response curves were generated with 17 different light stimuli. Photopic flicker ERGs were obtained at 30-Hz temporal frequency. Scotopic ERGs were recorded after 35 min of dark adaptation. For all animals, this procedure was performed once in four different sessions: isoflurane + end-tidal [CO₂] at 35 mmHg ± 3 mmHg (ISON), isoflurane + end-tidal [CO₂] at 65 mmHg ± 3 mmHg (ISOH), sevoflurane + end-tidal [CO₂] at 35 mmHg ± 3 mmHg (SEVON), isoflurane + end-tidal [CO₂] at 65 mmHg ± 3 mmHg (SEVOH). In photopic conditions, b-wave amplitudes were significantly smaller in hypercapnic groups (ISON = 170.6 ± 12.1 μV; ISOH = 132.6 ± 24.9 μV; SEVON = 170.9 ± 14.4 μV; SEVOH = 130.2 ± 22.8 μV). Similarly, in scotopic conditions, b-wave amplitudes were significantly decreased when CO₂ was increased (ISON = 89.4 ± 14.7 μV; ISOH = 58.2 ± 17.6 μV; SEVON = 93.4 ± 24.1 μV; SEVOH = 56.2 ± 22.2 μV). Flicker peak times were significantly increased in hypercapnic groups (ISON = 25.9 ± 0.4 ms; ISOH = 27.7 ± 1.2 ms; SEVON = 25.9 ± 1.5 ms; SEVOH = 27.2 ± 0.7 ms). Our results clearly indicate that induced hypercapnia significantly alters the genesis of the electroretinogram at level of ON-pathway and suggest that OFF-pathway is unaffected and that ERGs obtained from isoflurane or sevoflurane anaesthetized dogs are almost identical. Control of hypercapnia must be taken into consideration when ERGs are performed under inhaled anaesthesia in dogs.