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Effect of Carbogen Breathing and Acetazolamide on Optic Disc PO₂

¹From the Department of Ophthalmology, University Hospitals of Geneva, Geneva, Switzerland; and the ²Jules Gonin Eye Hospital, University of Lausanne, Lausanne, Switzerland.

PURPOSE. Acetazolamide was previously shown to increase optic disc partial pressure of oxygen (PO₂). The study was conducted to evaluate optic disc PO₂ variations during normoxia, hyperoxia (100% O₂), and carbogen breathing (95% O₂, 5% CO₂), before and after intravenous administration of acetazolamide.

METHODS. PO₂ measurements were obtained at intervascular areas of the optic disc in nine anesthetized minipigs using oxygen-sensitive microelectrodes (10-µm tip diameter) placed at <50 µm from the optic disc. PO₂ was measured continuously during 10 minutes under normoxia, hyperoxia, or carbogen breathing. Oxygen measurements were repeated under these conditions after intravenous injection of acetazolamide (500-mg bolus).

RESULTS. In hyperoxia, optic disc PO₂ increased moderately (PO₂ = 4.81 ± 1.16 mm Hg (mean ± SD; 24%; P < 0.001) after a much larger increase in systemic PaO₂. Carbogen breathing induced a significant increase in both systemic PaO₂ and PaCO₂, which resulted in a large increase in optic disc PO₂ (PO₂ = 13.17 ± 2.18 mm Hg; 67%; P < 0.001). Acetazolamide induced a slow and progressive increase in both systemic PaCO₂ and optic disc PO₂ (30 minutes PO₂ = 4.24 ± 2.45 mm Hg; 24%; P < 0.04). However, it was when carbogen was simultaneously administered that optic disc PO₂ increased most substantially (PO₂ = 18.91 ± 5.23 mm Hg; 90%; P < 0.002).

CONCLUSIONS. Carbogen breathing increases optic disc PO₂ significantly in minipigs, more than hyperoxia. The association of acetazolamide injection with carbogen breathing could induce an additional increase in optic disc PO₂ through the effect of higher systemic PaCO₂.

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