EVect of breathing 100% oxygen on retinal and optic nerve head capillary blood flow in smokers and non-smokers

Aim—The eVect of breathing 100% oxygen on retinal and optic nerve head capillary blood flow in smokers and non-smokers was investigated using scanning laser Doppler flowmetry (SLDF) as a new noninvasive method to visualise and quantify ocular blood flow. Method—10 eyes of 10 young healthy nonsmoking volunteers (mean age 26 (SD 3) years) and nine eyes of nine young healthy smoking volunteers (mean age 26 (4) years) were investigated. All participants were asked not to smoke or consume caVeine containing drinks for at least 4 hours before the measurements. Blood flow measurements were performed before and after 100% oxygen was applied to the subjects through a mask over a period of 5 minutes (6 litres per minute). Juxtapapillary retinal and optic nerve head blood flow were determined in arbitrary units using SLDF representing a combination of laser Doppler flowmetry and a scanning laser system allowing visualisation and quantification of the retinal and optic nerve head blood flow. Blood flow was determined in an area of 100 μm × 100 μm. The level of carboxyhaemoglobin was determined in all subjects. A Wilcoxon matched pairs signed ranks test (nonparametric) was used for statistical evaluation. Results—In the non-smoking group, retinal ‘flow’ was reduced by 33% (p = 0.005), optic nerve head ‘flow’ by 37% (p = 0.005). In the smoking group retinal flow was reduced by 10% (p = 0.01), optic nerve head flow by 13% (p <0.008). The diVerence in reactivity to oxygen breathing between smokers and non-smokers was highly significant (p <0.00001). Increased carboxyhaemoglobin levels were not found in either of the groups. A significant reduction of the mean arterial blood pressure of 6% (5%) (p <0.02) was observed in the non-smoking group after administration of oxygen. Conclusion—These results indicate that hyperoxia leads to a decrease in capillary blood flow of the retina and optic nerve head secondary to vasoconstriction, and that smokers do not respond to oxygen breathing as non-smokers do. The findings might be based on factors such as long term eVects of nicotine on the sympathetic and parasympathetic nervous system. (Br J Ophthalmol 1997;81:365–369) It is well known that vascular tone and blood flow of the retina are significantly influenced by arterial oxygen tension. The vasoconstricting eVect of oxygen on retinal vessels has been described by several authors. 2 There have been studies examining the eVect of breathing pure oxygen in normals, diabetics, smokers, and others. Morgado et al have recently investigated the acute eVect of oxygen breathing on the vascular regulation of retinal blood flow in smokers before and after smoking. The eVects of smoking on the circulation, especially the microcirculation, are based on various mechanisms. An increased intake of carbon monoxide leads to an increased level of carboxyhaemoglobin in the blood of smokers thus reducing the aYnity of the haemoglobin for oxygen—that is, the ability to bind oxygen. Heavy smokers are supposed to have carboxyhaemoglobin levels of up to 10% compared with 0.1% to 0.5% in non-smokers. Nicotine has a direct eVect on the vascular system in smokers. In low concentrations nicotine causes an excitation of sympathetic and parasympathetic ganglia. The eVect on sympathetic ganglia prevails. Direct stimulation of sympathetic ganglia leads to an increase of the heart rate and the total peripheral resistance due to vasoconstriction. Furthermore, it causes a stimulation of the adrenal glands: the release of noradrenaline contributes to the increase of the peripheral resistance and heart rate. The purpose of this study was to identify differences in the ocular vascular reactivity to oxygen between smokers and non-smokers as well as investigating the acute eVect of smoking on the retinal and papillary microcirculation and its eVect on oxygen reactivity. Method and subjects Scanning laser Doppler flowmetry (SLDF) is a combination of a scanning laser system and the well established laser Doppler flowmetry. Measuring blood flow by laser Doppler flowmetry is based on the optical Doppler eVect: light is scattered by tissue and moving particles undergoing a shift in frequency (Df). Scanning laser Doppler flowmetry uses the ability of a laser scanning tomograph to measure the amount of back scattered light at diVerent locations in an area of interest in a very short time. Since it is suYcient, when measuring a British Journal of Ophthalmology 1997;81:365–369 365 Ophthalmology and Eye Hospital, University Erlangen-Nürnberg, Germany M Langhans G Michelson M J M Groh