Determination of retinal blood vessel diameters and arteriovenous ratios in systemic hypertension:: comparison of different calculation formulae

Background Generalized arteriolar narrowing is one of the retinal changes influenced by systemic hypertension. The ratio of retinal arteriolar to venular diameters is often used as a marker of generalized arteriolar narrowing. There are several ways to determine the retinal arteriovenous ratio (A/V ratio). However, no comparison of retinal vascular measurements and A/V ratios determined by different formulae has been presented. Methods Eighty-seven men participating in the Tampere Ambulatory Hypertension Study returned for a 10-year follow-up examination in which fundus photographs were taken of both eyes. The diameters of retinal arterioles and venules were measured 1 and 2 disc diameters from the optic disc edge. The A/V ratio was determined using mean arteriole and venule width, the sum of widths of arterioles and venules, the sum of squares of widths of arterioles and venules, the central retinal artery equivalent (CRAE) and the central retinal venous equivalent (CRVE). The repeatability of measurements and A/V ratios was determined. Comparison was made between A/V ratios determined by different calculation formulas. Results In general, determination of A/V ratios yielded lower deviation than that of diameters of arterioles and venules separately. Calculation of A/V ratios using different formulas gave different ratio levels. According to linear regression analysis, the A/V ratio calculated using the sum of squares of widths of arterioles and venules correlated best with CRAE/CRVE (R-2 0.92) and A/V ratios calculated using the mean arteriole and venule widths or the sum of widths of arterioles and venules resulted in clearly lower associations (R-2 0.38-0.40 and R-2 0.41-0.48, respectively). Of all A/V ratios, CRAE/CRVE had the best repeatability. No statistically significant differences were found between measurements from right and left eyes. Conclusions The high repeatability of CRAE/CRVE and sum of squares of widths of arterioles/ sum of squares of widths of venules, as well as the good association of the two formulae, favor the use of these in evaluation of retinal vascular changes in systemic diseases. Our results also showed that if only one eye can be examined, it seems to suffice if fundus vascular alterations associated with systemic hypertension are evaluated.