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S.T. Venkataraman, C. Hudson, J.G. Flanagan, L. Rodrigues, A. Mardimae, J.A. Fisher; Retinal Arteriolar and Capillary Vascular Reactivity Assessment in Response to Isoxic Hypercapnia . Invest. Ophthalmol. Vis. Sci. 2006;47(13):490.
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To investigate the impact of isoxic hypercapnia induced using a computer driven gas flow controller, on retinal arteriolar and capillary hemodynamics in young healthy subjects.
Isoxic hypercapnia was induced in 6 young, healthy subjects (mean: 26±2.2 yrs; range 23–29 yrs) using a sequential gas delivery breathing circuit and a custom built, computer–driven gas flow controller. The flow controller was able to induce predetermined (‘target’) end–tidal gas concentrations of O2 and CO2. The ventilation protocol consisted of establishing the partial pressure of CO2 in end–tidal gas (PETCO2) at 38mmHg as a baseline, followed by a 15 minute period of isoxic hypercapnia to achieve a 15% increase in PETCO2. Subjects were then brought to their baseline PETCO2 levels for 10 min. One eye was randomly selected for the study. The study comprised of two randomized sessions, 30 minutes apart, on a single day. Session two was identical in terms of gas provocation but was necessary in order to accommodate the assessment of both arteriolar and capillary hemodynamics. Retinal arteriolar and capillary hemodynamics were assessed using the Canon Laser Blood Flowmeter and the Heidelberg Retina Flowmeter, respectively.
Retinal arteriolar diameter, blood velocity and flow increased significantly during isoxic hypercapnia (reANOVA adj. p<0.05). Retinal capillary blood flow also increased significantly in the optic nerve head (ONH), macula nasal and foveal areas (re ANOVA adj. p<0.05) during this period, however, there was no significant change in the temporal macular blood flow. PETCO2 increased in a square wave manner at session 1 from 38.1mmHg (SD 0.4) to 43.9mmHg (SD1.0) and similarly at session 2 from 38.5mmHg (SD 0.7) to 44.0mmHg (SD 0.6) (p<0.0001). There was a physiologically negligible change in PETO2 during sessions 1 and 2 from 108.8mmHg (SD 2.9) to 112.3mmHg (SD 1.3) and from 108.9mmHg (SD 4.0) to 112.7mmHg (SD 3.0), respectively.
A standardized, square wave 15% increase in PETCO2 resulted in an increase of retinal arteriolar flow of 26.2% (SD 7.6) and of ONH and mean retinal capillary blood flow of 32.6% (SD 18.3) and 18.4% (SD 11.6), respectively. These vascular responses were observed without the confounding effect of change in O2 that normally occurs concurrently with change in PCO2.
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