Comparison of different hyperoxic paradigms to induce vasoconstriction: Implications for the investigation of retinal vascular reactivity

PURPOSE. To compare the impact of three different techniques used to induce hyperoxia on end-tidal CO2 ( P-ET CO2). The relationship between change in P-ET CO2 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 O-2 only; O-2 plus CO2, using a nonrebreathing circuit ( with CO2 flow continually adjusted to negate drift of P-ET CO2); or air followed by O-2, 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-ET CO2 and then returned to homeostatic levels was used to determine the impact, if any, of perturbation of PETCO2 on retinal hemodynamics. RESULTS. The difference in group mean P-ET CO2 between baseline and elevated O-2 breathing was significantly different (t-test, P = 0.0038) for O-2- only administration with a nonrebreathing system. The sequential rebreathing technique resulted in a significantly lower difference (i.e., before and during hyperoxia) of individual PETCO2 (t-test, P = 0.0317). The P-ET CO2 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 PETCO2. A relatively modest change in PETCO2 resulted in a significant change in retinal hemodynamics. Rigorous control of PETCO2 is necesssary to attain standardized, reproducible hyperoxic stimuli for the assessment of retinal vascular reactivity.