Ocular vasodynamic changes in light and darkness in smokers.

PURPOSE To determine whether smokers have a reduced capacity for increased retinal blood flow velocity in darkness. METHODS The peak systolic flow velocities (V(S)) and end diastolic flow velocities (V(D)) were measured by ultrasound (i.e., color Doppler equipment), in light and darkness in the ophthalmic and central retinal arteries in 20 cigarette smokers and 20 matched nonsmokers. The resistive index (RI) was calculated as RI = (V(S) - V(D))/V(S). RESULTS In the ophthalmic artery in nonsmokers, the V(D) was markedly increased in darkness and the RI was correspondingly reduced. After the subject was re-exposed to light, the RI was markedly increased. In smokers the V(S) and V(D) did not change significantly in the different conditions of light and darkness. In the central retinal artery in nonsmokers, the V(S) and V(D) were markedly increased in darkness and decreased after re-exposure to light. In smokers, the corresponding changes were much smaller and not significant. CONCLUSIONS The normal capacity for increased blood flow velocity in the central retinal artery in darkness was markedly reduced in smokers. This finding may explain the reduced dark vision after recent smoking reported in several studies and probably reflects the combined effects of an increased blood viscosity, the vasoconstrictive action of nicotine, and a reduced capacity of the blood to transport oxygen, as the hemoglobin is partly occupied by carbon monoxide.