Comparison of retinal arteriolar and venular variability in healthy subjects

Study of retinal autoregulation is important because vascular dysfunction is a precursor of many retinal diseases. Previous research has focused on venular blood flow because the minimal venular pulsatility was thought to provide more reproducible results. This study compared the variability of arteriolar and venular blood flow measurements in response to isocapnic hyperoxia, a provocation known to constrict blood vessels and reduce blood velocity. Data was collected using a non-invasive laser Doppler instrument that permitted the simultaneous measurement of retinal blood velocity and vessel diameter, allowing the derivation of blood flow. Measurements were collected from 20 young subjects before, during and after exposure to hyperoxia. Isocapnia was maintained throughout hyperoxia using a previously validated sequential re-breathing circuit. Arteriolar and venular diameters decreased during hyperoxia by 8.7% (p=0.0001) and 14.2% (p=0.0001), respectively. Hyperoxia caused significant decreases in arteriolar and venular blood velocity (31.2%, p=0.0001 and 18.0%, p=0.0001, respectively) and flow (43.2%, p=0.0001 and 40.0%, p=0.0002, respectively). The coefficients of variation for intra-individual measurements of diameter, velocity and flow were comparable in magnitude between the two vessel types. Measures of arteriolar pulsatility, such as Pulsatility ratio, Resistivity ratio and Pulsatility index, increased significantly during hyperoxia, indicating increased downstream vascular resistance. We conclude that retinal arterioles and venules provide equally reproducible results for autoregulation studies and that arteriolar pulsatility profiles provide additional useful information regarding vascular resistance.