Hyperoxic hyperventilation (HHV) precedes onset of CNS oxygen toxicity in awake rats: real time monitoring of cardiorespiration and neural activity in awake rats breathing hyperbaric oxygen (HBO 2 )

CO 2 chemosensitive neurons in the dorsocaudal medulla are stimulated by hyperoxia. We hypothesized that a predictable pattern of HHV precedes onset of first electrical discharge (FED) in the EEG of awake rats breathing HBO 2 . Sprague‐Dawley rats were implanted with a radio‐transmitter for recording diaphragmatic EMG, ECG, EEG, body temperature, and activity. After ≥1 week recovery, a rat was placed inside a plexiglas chamber located inside a hyperbaric chamber. The rat chamber (O 2 ) & outer chamber (air) each were pressurized to 4–6 atmospheres (ATA) until FED. Oxidative preconditioning (2.5 ATA × 2.5 hr) was also tested 24 hr prior to diving to 4–6 ATA to determine if it increased latency to FED. 14 rats underwent 119 dives over 4 months with at least 1 week between each dive. The latency to FED at 4 ATA O 2 in naïve rats was 19.3±8.2 min and at 6 ATA O 2 it was 4.0±1.8 min. Oxidative preconditioning did not affect the latency to FED but it significantly decreased the HHV response. Hyperoxia stimulated ventilation in all rats, particularly in the minutes preceding FED. The HHV response was proportional to the hypercapnic ventilatory response. Our findings indicate that HHV precedes onset of CNS oxygen toxicity and that HHV response exhibits O 2 plasticity. Our initial findings suggest that HHV may be useful for predicting the onset of seizure in HBO 2 in awake rats (ONR).