Purpose: To compare the magnitude of vascular reactivity in response to metabolic provocation in retinal arterioles of varying diameter in healthy young subjects.
Methods: Ten healthy young subjects (26.2 +/- 3.5 years [mean +/- SD]) attended for three sessions. Session 1 was used to select two discrete hemodynamic measurement sites along the superior temporal arteriole. Retinal arteriolar blood flow was assessed at relatively narrow and wide sites. At sessions 2 and 3, CO(2) and O(2) were sequentially administered (and alternated across sessions) using manual gas flow control via a modified sequential rebreathing circuit to achieve target hypercapnia and hyperoxia. Blood flow was assessed for each gas phase. Total vascular reactivity capacity (TVRC) was taken as the difference in flow between hypercapnia and hyperoxia.
Results: The baseline diameter for the narrow and wide measurement sites was 92.4 microm (+/-13.6) and 116.7 microm (+/-12.7), respectively (ReANOVA; P < 0.0001). Hyperoxia induced a decrease in blood flow, whereas hypercapnia increased flow (P < 0.0001). TVRC was greater for the wide than for the narrow measurement sites (Delta flow narrow = 3.0 microL/min versus Delta flow wide = 6.6 microL/min; P < 0.0001). In terms of percentage change in flow relative to baseline, TVRC was the same between the wide and narrow sites (Delta narrow = 67% versus Delta wide = 61%; P > 0.05).
Conclusions: In response to metabolic provocation, absolute TVRC was greater for retinal arteriolar measurement sites with wider baseline vessel diameters. However, percentage change in retinal blood flow was the same irrespective of initial arteriolar diameter.