Relative magnitude of vascular reactivity in the major arterioles of the retina☆,☆☆
Introduction
The magnitude of change of retinal blood flow as a result of the perturbation of inspired oxygen (O2), and/or carbon dioxide (CO2), represents a measure of retinal vascular reactivity, sometimes termed metabolic autoregulation. Our group has shown that retinal vascular reactivity to standardized inhaled gas stimuli, that is isocapnic O2 or normoxic CO2, is reduced in type 2 diabetic patients and in previously untreated primary open angle glaucoma (POAG) patients, respectively, when compared to age-matched controls undertaking the identical procedure (Gilmore et al., 2007, Gilmore et al., 2008, Venkataraman et al., 2010). Clearly, the extent of vasoconstrictive or vasodilatory reserve of the retinal vessels is compromised in patients with diabetes and glaucoma, respectively. Also, short-term treatment with dorzolamide eye drops in patients with previously untreated POAG results in the recovery of retinal vascular reactivity (Venkataraman et al., 2010). Furthermore, the use of standardized inhaled gas stimuli in sequence, or in combination as a mix of O2 and CO2, has proven to be a useful tool to investigate basic aspects of blood flow physiology in the retina and the brain (Tayyari et al., 2009, Kisilevsky et al., 2008).
Several studies (Gilmore et al., 2007, Gilmore et al., 2008, Venkataraman et al., 2010, Tayyari et al., 2009, Kisilevsky et al., 2008) have utilized a single retinal arteriole to assess vascular reactivity and then have made the assumption that the vasoactive response of this isolated measurement site was representative of all the similarly sized retinal vessels. Blood flow, assessed using scanning laser Doppler flowmetry (SLDF) of the retinal capillaries, has been found to show no significant difference between the superior and inferior macular areas (Kimura et al., 2003, Harris et al., 2003). Earlier work from the same research group using SLDF, however, has reported that blood flow of the retinal capillaries in the inferior macular area was greater than in the superior macular area (Chung et al., 1999). The same study found that capillary vascular reactivity in the inferior macular area was less responsive to hypercapnia in terms of the vasodilatory response but more responsive to hyperoxia in terms of the vasoconstrictive response (Chung et al., 1999). A recent paper from our group demonstrated that absolute vascular reactivity was greater for retinal arteriolar measurement sites closer to the optic nerve head (ONH; i.e. vessels of wider diameter) but that the percent change in vascular reactivity was the same irrespective of position from the ONH i.e. irrespective of arteriolar diameter (Tayyari et al., 2009). To the best of our knowledge, the relative magnitude of vascular reactivity to inhaled gas stimuli in the superior and inferior retinal arterioles has not been systematically investigated, although since the retinal circulation is a closed system, any changes in blood flow measured in the retinal arterioles should be the same as in the capillaries distal to those measurement sites (Chung et al., 1999). Definition of this relationship is important in order to draw conclusions about the vasoactive response of the whole of the retinal vasculature from a test performed at a single measurement site. The aim of the study was, therefore, to compare the magnitude of retinal vascular reactivity in response to inhaled gas provocation at equivalent measurement sites in the superior and inferior temporal arterioles of the retina. We hypothesized that the two measurement sites would manifest an equivalent magnitude of vascular reactivity to inhaled gas stimuli.
Section snippets
Sample
The sample was comprised of 17 young, healthy volunteers recruited from office employees, and students at the Department of Ophthalmology and Vision Sciences, University of Toronto. Inclusion criteria consisted of a best corrected visual acuity of 0.0 log MAR or better, and refractive error within ± 6.00 DS and/or ± 2.50 DC. Exclusion criteria consisted of a history of ocular disease including lenticular opacity, or intraocular or refractive surgery (to avoid light scatter that may interfere with
Results
Seventeen healthy volunteers (11 white non-Hispanic and 6 Asian; mean age 24.4±4.7 years, range19–34 years; 7 men and 10 women) participated in this study. The IOP, systolic blood pressure (SBP), and diastolic blood pressure (DBP) did not change (p > 0.05) between baseline and the following assessments (Table 1).
During the hyperoxic phase, the diameter, blood velocity and blood flow decreased in the STA (p = 0.004, p < 0.001, p < 0.001, respectively) and the ITA (p = 0.003, p < 0.001, p < 0.001, respectively;
Discussion
It is important to comprehensively understand the basic vascular reactivity properties of healthy human retinal arterioles because retinal vascular regulation is compromised in a number of ocular diseases such as glaucoma or diabetic retinopathy (Gilmore et al., 2007, Gilmore et al., 2008, Venkataraman et al., 2010, Wang et al., 2009, Yazdanfar et al., 2003, Feke and Pasquale, 2008, Harris et al., 2008, Weinreb, 2008, Flammer and Mozaffarieh, 2007, Sehi et al., 2005). It is also imperative to
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Cited by (0)
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Support: Canadian Institutes of Health Research (CIHR) Operating Grant (CH).
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Financial interest: Drs. Fisher and Hudson are shareholders in Thornhill Research Inc.