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G. Garhofer, H. Resch, G. Weigert, L. Schmetterer; Inhaled Carbon Monoxide Increases Retinal and Choroidal Blood Flow in Healthy Volunteers . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3907.
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Purpose: Beside this known toxic effects, there is increasing evidence that carbon monoxide (CO) acts as an important vascular paracrine factor and may play a role in blood flow regulation in several tissues. Thus, the current study seeks to investigate the effect of inhaled CO on retinal and choroidal blood flow. Methods: 10 healthy male volunteers were studied in a randomized, double–blind, placebo–controlled design with washout periods of at least one week between study days. CO in a dose of 500ppm or placebo (room air) was inhaled for 60 minutes. Ocular haemodynamics were measured at baseline and 30 as well as 60 minutes after start of inhalation. Retinal vessel diameters were measured with a Retinal Vessel Analyzer. Red blood cell velocity was assessed using a bi–directional laser Doppler flowmeter. Retina blood flow was calculated based on vessel diameter measurement and red blood cell speed. Pulsatile choroidal blood flow was measured using laser interferometry. Results: Breathing of CO increased carboxyhaemoglobine (HbCO) significantly from 1.2±0.5% to 8.5±0.9 and 9.4±0.6%, at the two time points respectively (p<0.01). Retinal arteries increased by +3.5±3.8% and +4.2±3.9% (p<0.01). In retinal veins, CO again induced an increases in diameters of +4.3±3.0% and +4.8±5.0%, respectively (p<0.01). Placebo did not influence retinal vessel diameters. Red blood cell velocity tended to increase by +8±22% but this effect did not reach level of significance (p=0.1). However, calculated retinal blood flow increased significantly by +12±25% (p<0.02). Choroidal blood flow increased after breathing CO by +20.2±20.0% and +26±21%, respectively (p<0.01). Conclusions: Our experiment demonstrates that retinal and choroidal blood flow increases during inhalation of CO. If this increase is caused by tissue hypoxia or a yet unknown mechanism has yet to be clarified.
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