Effect of carbogen breathing and acetazolamide on optic disc PO2.

PURPOSE: Acetazolamide was previously shown to increase optic disc partial pressure of oxygen (PO(2)). The study was conducted to evaluate optic disc PO(2) variations during normoxia, hyperoxia (100% O(2)), and carbogen breathing (95% O(2), 5% CO(2)), before and after intravenous administration of acetazolamide.

METHODS

PO(2) measurements were obtained at intervascular areas of the optic disc in nine anesthetized minipigs using oxygen-sensitive microelectrodes (10-microm tip diameter) placed at <50 microm from the optic disc. PO(2) 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 PO(2) increased moderately (DeltaPO(2) = 4.81 +/- 1.16 mm Hg (mean +/- SD; 24%; P < 0.001) after a much larger increase in systemic PaO(2). Carbogen breathing induced a significant increase in both systemic PaO(2) and PaCO(2), which resulted in a large increase in optic disc PO(2) (DeltaPO(2) = 13.17 +/- 2.18 mm Hg; 67%; P < 0.001). Acetazolamide induced a slow and progressive increase in both systemic PaCO(2) and optic disc PO(2) (30 minutes DeltaPO(2) = 4.24 +/- 2.45 mm Hg; 24%; P < 0.04). However, it was when carbogen was simultaneously administered that optic disc PO(2) increased most substantially (DeltaPO(2) = 18.91 +/- 5.23 mm Hg; 90%; P < 0.002).

CONCLUSIONS

Carbogen breathing increases optic disc Po(2) significantly in minipigs, more than hyperoxia. The association of acetazolamide injection with carbogen breathing could induce an additional increase in optic disc PO(2) through the effect of higher systemic PaCO(2).