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

Purpose. Arterial carbon dioxide tension and arterial oxygen tension are important determinants of retinal and cerebral blood flow. In the present study the hypothesis that changes in arterial blood gases also influence choroidal blood flow was tested. Methods. The effect of breathing different mixtures of oxygen (O₂) and carbon dioxide (CO₂) on choroidal blood flow in the foveal region was investigated in healthy subjects. The study was performed in a randomized, double-masked four way cross-over design in 16 subjects. Using a compact laser Doppler flowmeter, red blood cell velocity (ChBVel), volume (ChBVol), and flow (ChBF) in the choroidal vasculature were measured during the breathing of various mixtures of O₂ and CO₂ (hyperoxia-hypercapnia): 100% O₂, 97% O₂ + 3% CO₂, 95% O₂ + 5% CO₂ (carbogen) and 92% O₂ + 8% CO₂. Arterial oxygen tension (pO₂) and carbon dioxide tension (pCO₂) were measured from arterialized blood samples from the earlobe. Results. Breathing 100% O₂ had no significant effect on ChBVel (-3.7%), ChBVol (+1.7%) and ChBF (-4.3%). Addition of 3% CO₂ to O₂ also produced no significant change on these blood flow parameters. In contrast, carbogen significantly increased ChBVel (10.0 ± 4.4%, 95% CI, p < 0.001) and ChBF (12.5 ± 11.7%, p = 0.002). The effect of 92% O₂ + 8% CO₂ was more pronounced 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 approximately 1.5% per 1 mmHg increase in pCO₂. Conclusions. This study demonstrates that, in healthy subjects, pCO₂ is an important determinant of foveal choroidal blood flow, whereas pO₂ has little impact on it.