Comparison of the effects of independently‐controlled end‐tidal PCO 2 and PO 2 on blood oxygen level–dependent (BOLD) MRI

Purpose To assess the effect of changes in end‐tidal partial pressure of O 2 (P ET O 2 ) on cerebrovascular reactivity (CVR) estimated from changes in blood oxygen level–dependent (BOLD) signal during cyclic changes in end‐tidal partial pressure of CO 2 (P ET CO 2 ). Materials and Methods BOLD response to fixed cyclic step changes in P ET CO 2 (range = 30.4–48.8 mmHg) and P ET O 2 (range = 100.6–444.0 mmHg) was studied in four healthy volunteers. Results The BOLD reactivity to P ET CO 2 and P ET O 2 were 0.283 (0.188–0.379) (median, range) and 0.004 (0.003–0.006)%/mmHg, respectively, in the whole brain; 0.438 (0.382–0.614) vs. 0.006 (0.004–0.009)%/mmHg, respectively, in the gray matter; and 0.075 (0.065–0.093) vs. 0.002 (0.001–0.002)%/mmHg, respectively, in the white matter. Conclusion The BOLD reactivity to P ET O 2 was much smaller than that to P ET CO 2 . However, BOLD reactivity can be significantly distorted by CO 2 ‐induced changes in P ET O 2 . We conclude that P ET O 2 should be carefully controlled during studies that use BOLD reactivity as an indicator of CVR. J. Magn. Reson. Imaging 2007. © 2007 Wiley‐Liss, Inc.