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Purpose: To study the effect of dorzolamide on the preretinal oxygen tension (RPO(2)) in retinal areas affected by experimental branch retinal vein occlusion (BRVO) in pigs., Methods: Experimental BRVO was induced by diathermy close to the optic disc. RPO(2) was measured with an oxygen-sensitive electrode 0.5 mm above the BRVO-affected area, which was compared to the retinal areas not affected by BRVO. In one group of five pigs, RPO(2) was measured at baseline, 1 and 3 hours after BRVO, and after intravenous injection of 500 mg dorzolamide. In a second group of five pigs, RPO(2) was measured 1 week after the BRVO, both before and after intravenous injection of 500 mg dorzolamide., Results: The average baseline RPO(2) was 2.64 +/- 0.09 kPa (mean +/- SD). In the BRVO-affected areas, RPO(2) decreased significantly (by 0.67 +/- 0.29 and 0.94 +/- 0.13 kPa) at 1 hour and 3 hours after BRVO induction. In the non-BRVO areas RPO(2) increased significantly (by 0.51 +/- 0.14 kPa) 1 hour after BRVO induction, but subsequently decreased and reached baseline 3 hours after BRVO induction. One week after BRVO induction, RPO(2) was 0.67 +/- 0.29 kPa lower in affected areas when compared with the non-BRVO areas. In the BRVO-affected areas, dorzolamide increased RPO(2) significantly (by 0.36 +/- 0.21 kPa at 3 to 4 hours and by 0.67 +/- 0.40 kPa) 1 week after BRVO induction., Conclusions: Retinal hypoxia induced by experimental BRVO remained significant 1 week after BRVO. Dorzolamide increased retinal oxygen tension in the BRVO-affected areas both at 4 hours and 1 week after experimental BRVO in pigs.