Extension of brain tolerance to hyperbaric O2 by intermittent air breaks is related to the time of CBF increase

Intermittent air breaks during hyperbaric oxygen (HBO2) exposures protect against pulmonary and central nervous system (CNS) toxicity. The mechanisms of this beneficial effect from intermittency are not known. In this study, we examined if release of vasoconstriction during HBO2 exposure indicates a threshold for toxic dose of HBO2 and how it may be related to tolerance by intermittency. Awake rats instrumented for EEG and cerebral blood flow (CBF) measurement were exposed to 100% O2 at 6 ATA (absolute pressure). Air breaks of 3 or 10 min were given at different times after CBF increase. Following the air break, animals were exposed to 100% O2 until seizure and total O2 time was used to calculate benefit/toxicity. The most beneficial schedule was then used to assess the role of the multiple air breaks in extension of HBO2 tolerance. A significant increase in seizure latency was observed in animals with a single 3- or 10 min air break given 5-10 min after CBF increase. No change in seizure latency was observed when air breaks were given beyond (>10 and <5 min) this window. The duration of total O2 time to seizures was doubled with multiple 3 min air breaks, and quadrupled with 10 min air breaks compared with continuous HBO2 exposures. With more time spent on O2, the duration of air breaks was not sufficient for recovery from O2 toxicity and for CBF to return to baseline. Results show that an “optimal window” of HBO2 exposure is required for benefits by intermittent exposure to air.