Independent cerebral vasoconstrictive effects of hyperoxia and accompanying arterial hypocapnia at 1 ATA

Abstract

Breathing 100% O2 at 1 atmosphere absolute (ATA) is known to be associated with a decrease in cerebral blood flow (CBF). It is also accompanied by a fall in arterial Pco2 leading to uncertainty as to whether the cerebral vasoconstriction is totally or only in part caused by arterial hypocapnia. We tested the hypothesis that the increase in arterial Po2 while O2 was breathed at 1.0 ATA decreases CBF independently of a concurrent fall in arterial Pco2. CBF was measured in seven healthy men aged 21-62 yr by using noninvasive continuous arterial spin-labeled-perfusion MRI. The tracer in this technique, magnetically labeled protons in blood, has a half-life of seconds, allowing repetitive measurements over short time frames without contamination. CBF and arterial blood gases were measured while breathing air, 100% O2, and 4 and 6% CO2 in air and O2 backgrounds. Arterial Po2 increased from 91.7 +/- 6.8 Torr in air to 576.7 +/- 18.9 Torr in O2. Arterial Pco2 fell from 43.3 +/- 1.8 Torr in air to 40.2 +/- 3.3 Torr in O2. CBF-arterial Pco2 response curves for the air and hyperoxic runs were nearly parallel and separated by a distance representing a 28.7-32.6% decrement in CBF. Regression analysis confirmed the independent cerebral vasoconstrictive effect of increased arterial Po2. The present results also demonstrate that the magnitude of this effect at 1.0 ATA is greater than previously measured.

Hypercapnia-Induced Cerebral Hyperperfusion: An Underrecognized Clinical Entity

“…Although hypoxia can increase CBF, experimental hyperoxia showed a small trend toward CBF decrease. 52,53 There was no statistically significant group difference in measures of ABG percent oxygen saturation in this study.…”

Effect of Hypoxia and Hyperoxia on Cerebral Blood Flow, Blood Oxygenation, and Oxidative Metabolism

“…We therefore added CO 2 to partially offset this side effect. Similar strategies have been used in previous reports in the literature (see also Discussion) (Floyd et al, 2003).…”

“…Under hyperoxia or hypoxia conditions, the relationship between CMRO 2 /CBF and CO 2 may be different (i.e., there could be an interaction term between O 2 and CO 2 effects). There is some evidence suggesting that the interaction term is negligible for CBF (Floyd et al, 2003). However, future studies are needed to examine the interaction effect on CMRO 2 .…”

Cerebral pressure autoregulation in traumatic brain injury

“…Floyd et al 16 have shown that the decrease in CBF while breathing 100% O 2 is attributable to the combination of arterial hyperoxia and hypocapnia and that hyperoxia causes a cerebral vasoconstriction independent of the vasoconstriction associated with arterial hypocapnia. The decrease in ICP and flow velocity is probably due to the vasoconstricting effects of hyperoxia, and a small decrease in ETCO 2 is caused by breathing 100% O 2 .…”