Cerebrovascular effects of 73% and 100% O2 delivered in the laboratory: Steady hyperoxia or oscillations with normoxiaCerebrovascular effects of 73 and 100 O2 delivered in the laboratory: Constant gas or oscillations with normoxia

Fluctuations in inspired oxygen partial pressure (PO2) are normal in current-generation tactical jets that deliver oxygen concentrations specific for cabin altitude. Onboard oxygen generating systems (OBOGS) provide oscillating concentrations within the acceptable band. In a previous study, our laboratory simulated fluctuations in oxygen partial pressure (PO2) of the magnitude expected from an F-35 OBOGS. We measured average blood flow velocity in the middle cerebral artery (MCAv) and related variables (transcranial Doppler; TCD) and frontal cortex blood oxygenation (near infrared spectroscopy; NIRS) as well as a number of other variables, and found no significant effects of the PO2 fluctuations. This IRB-approved study, an extension of that work, examined effects on TCD and NIRS measures of the maximum PO2 fluctuations possible while avoiding even mild hypoxia, namely, between PO2 with 100 O2 and that with air, with 30 s spent breathing each gas. Because naval aviators often fly at a cabin altitude of 8,000 ft while breathing the maximum oxygen fraction supplied by the OBOGS, measurements at the equivalent PO2, 73 O2 in our laboratory, were included. Effects of steady air and steady hyperoxia also were assessed. Values with disturbances of carbon dioxide (CO2) balance generated by hyperventilation or breath-holding under all of the O2 conditions were also measured. Ten participants of the first study returned for a single experimental session. No important effect of O2 condition was seen during normocapnia. In contrast, hypocapnia (hyperventilation) dramatically reduced, and hypercapnia (breath-holding) unequivocally increased, MCAv, and the O2 conditions modulated those effects. We conclude that square-wave fluctuations in inspired PO2 with period 1 minute have little effect on circulation to or oxygenation of the brain. However, hyper- and hypoventilation are potent stimuli of changes in cerebral artery velocity, and the changes are functions of PO2.