The study showed that retinal blood flow at rest was lower and the diameter smaller in the macular than in peripheral venules (
P > 0.01 for both comparisons) but were comparable in the macular and peripheral arterioles. However, the smaller diameter and lower blood flow of macular venules were counterbalanced by a higher number of these vessels so that the total cross-sectional area of the arterioles as compared to the venules were similar for vessels to the macular area and the retinal periphery. The counts included vessels that were slightly smaller than those in which oxygen saturation and blood flow were measured, which ensured a sufficiently large number for a meaningful statistical analysis to be performed. Ideally, the blood flow and oxygen saturation should have been measured in all these branches, but this could not be achieved because of the limitations in the number of vascular cross sections that could be contained within the Doppler OCT scans and the limitations in the resolution of both oximetry and Doppler OCT.
19,24 On the fundus photographs, it could be observed that the asymmetric branching pattern with a higher number of primary venular than arteriolar branches to the macular area were compensated by more extensive branching of the arterioles than the venules closer to the fovea. This ensured the normal alternating pattern of pre-capillary arterioles supplying and post-capillary venules draining the capillary bed.
25 The increase in the arterial blood pressure induced by isometric exercise was within the physiological range and it can therefore be expected that the observed changes in diameter, blood flow, and oxygen saturation had reflected responses that might be observed during normal life. The effect of the intervention on retinal blood flow might potentially be modified by changes in the IOP, but the observed increase in this parameter during exercise was within the known variation of repeated measurements
26 and can therefore be considered to have been too low to have affected the results. Although the increase in the arterial blood pressure contracted all the studied arterioles, the contraction was insufficient to maintain the blood flow constant in the peripapillary and peripheral arterioles where a significant increase in the blood flow could be observed. The increased peripapillary and peripheral blood flow during exercise may have resulted in an underestimation of the measured oxygen saturation,
12 which argues that the observation of an increase in venous oxygen saturation in these vessels had been true. In a previous study where isometric exercise for 30 seconds increased the arterial blood pressure by approximately 10 mm Hg, an increased oxygen saturation was only observed in the peripapillary and peripheral venules.
1 However, in the present study, where 2 minutes of exercise increased the arterial blood pressure by approximately 18 mm Hg, the oxygen saturation also increased significantly in the macular venules. Therefore, it is possible that the blood pressure necessary to induce a significant autoregulatory response with derived effects on blood flow and oxygen saturation may differ among peripheral and macular arterioles.
27 Such regional differences is supported by previous findings that spontaneous diameter oscillations and ischemic conditioning in the retina in healthy persons may differ among the peripheral and macular vessels.
28–30 The background for these differences, as well as the higher oxygen saturation in the macular than in the peripheral venules at rest
11 is unknown, but may be related to the shorter arterio-venous distance through the microcirculation in macular than in the peripheral circulation.
25,31 The observations are also in accordance with previous findings that diameter regulation is affected differently in arterioles supplying the retinal periphery and the macular area in patients with diabetic retinopathy,
32 but these studies were not supplemented with measurements of retinal blood flow. This should be the subject of a future study.