PURPOSE. It has been hypothesized that carbon monoxide (CO) acts as an important vascular paracrine factor and plays a role in blood flow regulation in several tissues. The present study investigated the effect of inhaled CO on retinal and choroidal blood flow. METHODS. Fifteen healthy male volunteers were studied in a randomized, double-masked, placebo-controlled design with washout periods of at least 1 week between study days. CO in a dose of 500 ppm or placebo (synthetic air without CO) was

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... inhaled for 60 minutes. Ocular hemodynamics were measured at baseline and at 30 and 60 minutes after start of inhalation. Retinal vessel diameters were measured with a retinal vessel analyzer. RBC velocity was assessed using bidirectional laser Doppler velocimetry. Retinal blood flow was calculated based on retinal vessel diameters and RBC velocity. Fundus pulsation amplitude (FPA) was measured using laser interferometry, and submacular choroidal blood flow using laser Doppler flowmetry. RESULTS. Breathing of CO significantly increased carboxyhemoglobine, from 1.2 Ϯ 0.5% to 8.5 Ϯ 0.9% and 9.4 Ϯ 0.6% at the two time points, respectively (P Ͻ 0.01). The diameter of retinal arteries increased by ϩ3.5 Ϯ 3.8% and ϩ4.2 Ϯ 3.9% (P Ͻ 0.01) in response to CO inhalation. In retinal veins, CO also induced an increase in diameter of ϩ4.3 Ϯ 3.0% and ϩ4.8 Ϯ 5.0%, respectively (P Ͻ 0.01). By contrast, placebo did not influence retinal vessel diameter. RBC velocity tended to increase during CO inhalation (ϩ8 Ϯ 22%), but this effect did not reach the level of significance (P ϭ 0.1). Calculated retinal blood flow increased significantly by ϩ12 Ϯ 5% (P Ͻ 0.02). FPA increased after breathing CO by ϩ20 Ϯ 20% and ϩ26 Ϯ 21% at the two time points, respectively (P Ͻ 0.01). Subfoveal choroidal blood flow increased by ϩ14 Ϯ 9% and ϩ15 Ϯ 9% during breathing of CO (P Ͻ 0.01). CONCLUSIONS. This experiment demonstrated that retinal and choroidal blood flow increase during inhalation of CO. Whether this increase is caused by tissue hypoxia or a yet unknown mechanism has to be clarified. (Invest Ophthalmol Vis Sci. 2005;46:4275-4280)