Carbon dioxide-induced retinopathy in the neonatal rat

Purpose. Hypercarbia has been suggested as a risk factor for retinopathy of prematurity (ROP). We investigated the effect of hypercarbia on the retinal vasculature of the neonatal rat to determine whether hypercarbia alone could induce preretinal neovascularization analogous to ROP. Methods. In a preliminary blood gas study, 8-11-day-old rats were exposed to specific levels of inspired O₂ and either 0.2% CO₂ or 10% CO₂. Arterial blood gases were obtained, and a level of inspired O₂ was determined that, when combined with inspired 10% CO₂ would produce a PaO₂ equivalent to room air (pure hypercarbia). In the formal retinopathy study, 300 newborn rats raised in 12 expanded litters (n = 25 each) were exposed for 7 days to either room air, 10% CO₂ in 21% O₂ and nitrogen (high-inspired CO₂ group) or 10% CO₂ in 12.5% O₂ and nitrogen (pure-hypercarbia group). Each type of exposure was followed by recovery in room air for 5 days. Animals were sacrificed on day 13 and retinae were analyzed, using fluores cein perfusion and ADPase staining techniques. Results. Neovascularization occurred in 19% of rats in the high-inspired CO₂ group, and 14% of rats in the pure-hypercarbia group, compared to 0% of rats exposed to room air alone (p = 0.001, Chi square). Conclusions. In the neonatal rat model, exposure to hypercarbia alone, followed by room air recovery, results in preretinal neovascularization similar to that seen in oxygen-induced retinopathy. Our results support the suggestion that hypercarbia may be a risk factor for retinopathy of prematurity.