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Comparison of Different Hyperoxic Paradigms to Induce Vasoconstriction: Implications for the Investigation of Retinal Vascular Reactivity

¹From the Multi-disciplinary Laboratory for the Research of Sight-Threatening Diabetic Retinopathy, Department of Ophthalmology and Vision Science, University of Toronto and School of Optometry, University of Waterloo, Ontario, Canada; and the ²Department of Anesthesiology, Toronto General Hospital, Toronto, Ontario, Canada.

PURPOSE. To compare the impact of three different techniques used to induce hyperoxia on end-tidal CO₂ (P_ETCO₂). The relationship between change in P_ETCO₂ and retinal hemodynamics was also assessed to determine the clinical research relevance of this parameter.

METHODS. The sample comprised 10 normal subjects (mean age, 25 years; range, 21–49 years). Each subject attended for three sessions. At each session, subjects initially breathed air followed by O₂ only; O₂ plus CO₂, using a nonrebreathing circuit (with CO₂ flow continually adjusted to negate drift of P_ETCO₂); or air followed by O₂, using a sequential rebreathing circuit. In addition, using a separate sample of eight normal subjects (mean age, 26.5 years; range, 24–36 years), a methodology that initially raised P_ETCO₂ and then returned to homeostatic levels was used to determine the impact, if any, of perturbation of P_ETCO₂ on retinal hemodynamics.

RESULTS. The difference in group mean P_ETCO₂ between baseline and elevated O₂ breathing was significantly different (t-test, P = 0.0038) for O₂-only administration with a nonrebreathing system. The sequential rebreathing technique resulted in a significantly lower difference (i.e., before and during hyperoxia) of individual P_ETCO₂ (t-test, P = 0.0317). The P_ETCO₂ perturbation resulted in a significant (P < 0.005) change of retinal arteriolar diameter, blood velocity, and blood flow.

CONCLUSIONS. The sequential rebreathing technique resulted in a reduced variability of P_ETCO₂. A relatively modest change in P_ETCO₂ resulted in a significant change in retinal hemodynamics. Rigorous control of P_ETCO₂ is necesssary to attain standardized, reproducible hyperoxic stimuli for the assessment of retinal vascular reactivity.

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