Response of choroidal blood flow in the foveal region to hyperoxia and hyperoxia-hypercapnia

Purpose. Arterial carbon dioxide tension and arterial oxygen tension are im portant determinants of retinal and cerebral blood flow. In the present stu dy the hypothesis that changes in arterial blood gases also influence choro idal blood flow was tested. Methods. The effect of breathing different mixtures of oxygen (O-2) and car bon dioxide (CO2) on choroidal blood flow in the foveal region was investig ated in healthy subjects. The study was performed in a randomized, double-m asked four way cross-over design in 16 subjects. Using a compact laser Dopp ler flowmeter, red blood cell velocity (ChBVel), volume (ChBVol), and flow (ChBF) in the choroidal vasculature were measured during the breathing of v arious mixtures of O-2 and CO2 (hyperoxia-hypercapnia): 100% O-2, 97% O-2 3% CO2, 95% O-2 + 5% CO2 (carbogen) and 92% O-2 + 8% CO2. Arterial oxygen tension (pO(2)) and carbon dioxide tension (pCO(2)) were measured from arte rialized blood samples from the earlobe. Results. Breathing 100% O-2 had no significant effect on ChBVel (-3.7%), Ch BVol (+1.7%) and ChBF (-4.3%). Addition of 3% CO2 to O-2 also produced no s ignificant change on these blood flow parameters. In contrast, carbogen sig nificantly increased ChBVel (10.0 +/- 4.4%, 95% CI, p < 0.001) and ChBF (12 .5 +/- 11.7%, p = 0.002). The effect of 92% O-2 + 8% CO2 was more pronounce d since it significantly increased ChBVel and ChBF by 15.5 +/- 7.5% (p < 0. 001) and 16.2 +/- 11.0% (p < 0.001), respectively. None of the gas mixtures induced a significant change in ChBVol. The increase in ChBF was approxima tely 1.5% per 1 mmHg increase in pCO(2). Conclusions. This study demonstrates that, in healthy subjects, pCO(2) is a n important determinant of foveal choroidal blood flow, whereas pO(2) has l ittle impact on it.