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M. Kisilevsky, A. Mardimae, M. Slessarev, J. Han, J. Fisher, C. Hudson; Retinal Arteriolar and Middle Cerebral Artery Responses to Combined Hypercarbic / Hyperoxic Stimuli. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2092.
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The relative effect of simultaneously administered oxygen and carbon dioxide on the retinal and cerebral vessels is unknown. The purpose of this study was to quantify and compare the superior-temporal retinal arteriole (RA) and middle cerebral artery (MCA) responses to a series of hypercarbic and combined hypercarbic/hyperoxic stimuli.
Twelve young healthy volunteers participated in the study. End-tidal pressure of carbon dioxide was raised and maintained at 23% from the baseline during 3 levels of oxygen end-tidal pressure (PETO2 ) i.e. isoxia, 300 and 500 mmHg hyperoxia. Retinal vessel diameter and blood velocity was measured using the Canon Laser Blood Flowmeter; MCA blood velocity was measured using a transcranial Doppler ultrasound system.
Hypercarbia caused a 17% increase in retinal arteriolar blood velocity that resulted in a 21% increase in calculated flow. Hypercarbia/hyperoxia-500mmHg resulted in a decrease of 8% in retinal arteriolar diameter, 16% in velocity and 29% in calculated flow. Hypercarbia/hyperoxia-300mmHg induced a 14% decrease in calculated RA flow. MCA blood velocity increased 45% in response to hypercarbia. The difference in hypercarbia-induced increase in velocity was significantly greater for the MCA than RA (p<0.001). Increase in PETO2 did not change the hypercarbia-induced increase in MCA blood velocity.
Hypercarbia-induced vasodilation in retinal arterioles occurs to a lesser extent than in the MCA. Hyperoxia reverses hypercarbia-induced vasodilation in retinal arterioles in a concentration-dependent manner; while hypercarbia-induced vasodilation in MCA is unaffected by subsequent increase in PETO2. The differences in vascular reactivity of the superior-temporal retinal arteriole and middle cerebral artery were quantified for the first time using precise control of end-tidal concentrations of O2 and CO2.
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