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Editors Selection IGR 20-1

Blood Flow: Vascular reactivity

Comment by Alon Harris on:

22384 Retinal arteriolar and capillary vascular reactivity in response to isoxic hypercapnia, Venkataraman ST; Hudson C; Fisher JA et al., Experimental Eye Research, 2008; 87: 535-542

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Venkataraman et al. (1490) present an investigation describing the magnitude of vascular reactivity of retinal arterioles in terms of percentage change to that of the retinal capillaries using a novel computer control of inspired gases to provoke isoxic hypercapnia. The researchers attempt to overcome several difficulties in assessing vascular reactivity including unpredictable increases in ventilation during hypercapnia which may result in concomitant changes in the partial pressure of oxygen and corresponding effects on vessel response. Their findings of a comparable magnitude of retinal capillary and arteriolar vascular reactivity contribute to the understanding of ocular vascular autoregulation which has increasing implications in glaucoma, diabetic retinopathy and age related macular degeneration. Strengthening the study are the standardizations of computer-controlled gas blending which may result in consistent repeatable independent changes in P_ETCO₂ and in P_ETO₂. Producing consistent changes in arterial blood partial pressure of carbon dioxide or oxygen is vital during investigations of vasoreactivity and vascular autoregulatory research. For instance, previous research has shown the need to maintain isocapnia during hyperoxia to eliminate confounding stimulus to vasoactive tissues.1 Additionally, the limited invasiveness of the methodologies presented reduces subject discomfort. One limitation of the current study is the small sample size (n = 10) examined. Confirmation of these findings requires a much larger sample with appropriate statistical justification. Another factor is the inclusion of only young healthy individuals (mean age 25) which limits the application of these results to elderly individuals and glaucoma patients who likely have faulty autoregulation. Future research should include a comprehensive assessment of ocular vascular beds expanding beyond the retinal circulation including the choroid, retrobulbar blood vessels and laminar region of the optic nerve head.

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