A potential early physiological marker for CNS oxygen toxicity: hyperoxic hyperpnea precedes seizure in unanesthetized rats breathing hyperbaric oxygen

Hyperbaric oxygen (HBO2) stimulates presumptive central CO2-chemoreceptor neurons, increases minute ventilation (V̇min), decreases heart rate (HR) and, if breathed sufficiently long, produces central nervous system oxygen toxicity (CNS-OT; i.e., seizures). The risk of seizures when breathing HBO2 is variable between individuals and its onset is difficult to predict. We have tested the hypothesis that a predictable pattern of cardiorespiration precedes an impending seizure when breathing HBO2. To test this hypothesis, 27 adult male Sprague-Dawley rats were implanted with radiotelemetry transmitters to assess diaphragmatic/abdominal electromyogram, electrocardiogram, and electroencephalogram. Seven days after surgery, each rat was placed in a sealed, continuously ventilated animal chamber inside a hyperbaric chamber. Both chambers were pressurized in parallel using poikilocapnic 100% O2 (animal chamber) and air (hyperbaric chamber) to 4, 5, or 6 atmospheres absolute (ATA). Breathing 1 ATA O2 initially decreased V̇min and HR (Phase 1 of the compound hyperoxic ventilatory response). With continued exposure to normobaric hyperoxia, however, V̇min began increasing toward the end of exposure in one-third of the animals tested. Breathing HBO2 induced an early transient increase in V̇min (Phase 2) and HR during the chamber pressurization, followed by a second significant increase of V̇min ≤8 min prior to seizure (Phase 3). HR, which subsequently decreased during sustained hyperoxia, showed no additional changes prior to seizure. We conclude that hyperoxic hyperpnea (Phase 3 of the compound hyperoxic ventilatory response) is a predictor of an impending seizure while breathing poikilocapnic HBO2 at rest in unanesthetized rats. breathing an o2-enriched gas mixture (e.g., Nitrox) at hyperbaric pressure increases the risk of central nervous system (CNS) oxygen toxicity (CNS-OT) (20). Likewise, breathing pure O2 above 2–3 atmospheres absolute (ATA, where 1 ATA = 760 mmHg) for an extended period (tens of minutes) increases the risk for CNS-OT (2). CNS-OT is manifested as an unpredictable onset of tonic-clonic spasms combined with loss of consciousness. Seizures typically end after the inspired level of oxygen is reduced and thus are not life-threatening per se. The conditions under which CNS-OT can occur, however, make seizures potentially harmful and even life-threatening (e.g., during hyperbaric oxygen therapy for healing a problematic wound or while submerged beneath 30–50 feet of sea water and breathing pure oxygen from a rebreathing apparatus). Currently, the risk of developing CNS-OT is the limiting factor in using hyperbaric oxygen (HBO2) in hyperbaric medicine for wound healing (33); submarine medicine (DISSUB or disabled submarine escape) (7, 40); and closed-circuit technical diving using a rebreathing apparatus for recreational diving, professional diving (oil companies), and military diving in the U.S. Special Operations Forces, which include the Navy SEALs and U.S. Marine Force Reconnaissance units (25). Previous findings from our laboratory showed that neurons in the caudal solitary complex (cSC) of the dorsal medulla oblongata of rats are among the first neurons under in vitro conditions to be stimulated by normobaric hyperoxia (41), hyperbaric hyperoxia (12, 13, 44), and other chemical oxidants (13, 44, 45). CO2-sensitive neurons in the cSC are believed to function in central CO2 chemoreception and drive ventilation because focal mild acidification of the cSC induces hyperventilation during wakefulness and sleep in rats (47). In addition, protracted exposure to hyperoxia stimulates ventilation in humans and animal models, a paradoxical ventilatory response known as hyperoxic hyperventilation; reviewed by Dean et al. (13). Numerous studies have shown that hyperoxia induces hypocapnia secondarily to hyperventilation (4, 10, 30, 31, 35, 51, 54). The degree to which hypocapnic alkalosis occurs subsequent to hyperoxic ventilation, however, has recently been challenged and is currently being debated (21, 22, 28–31). Regardless, a stimulant effect of 100% oxygen on ventilation—whether it is classified as hyperventilation leading to respiratory alkalosis or hyperpnea without pH change—was recognized in 1947 by Dripps and Comroe (18). It was subsequently shown that carotid body denervation does not abolish hyperoxic hyperventilation or the ensuing decrease in end-tidal Pco2, indicating that the paradoxical hyperventilatory response originates centrally (9, 23, 42). As expected, hyperventilation is greatest during isocapnic hyperoxia, but is reported to be blunted in magnitude and sometimes missed during poikilocapnic hyperoxia because O2-induced hyperventilation secondarily lowers arterial Pco2, and thus the magnitude of O2-induced hyperventilation (3, 4, 51, 54). The majority of foregoing observations, when considered together, suggest that certain neurons involved in the control of respiration, including CO2-chemosensitive neurons of the cSC, are highly sensitive to oxidative stimuli such as HBO2. Presumably, this is due to the presence of redox- and nitrosative-sensitive enzymes and intermediates (reactive oxygen and nitrogen species) that are located throughout the caudal medulla oblongata, which are activated during protracted exposure to hyperoxic gas mixtures (11). Accordingly, we hypothesize in this study that hyperoxia stimulates ventilation prior to onset of CNS-OT when breathing HBO2, suggesting that it may be a possible physiological indicator or physiomarker of an impending O2-induced seizure. Our hypothesis is consistent with several anecdotal reports by divers that their diaphragms begin spasmodic contractions prior to onset of HBO2-induced seizures (60). To test this hypothesis, we implanted radiotelemetry modules into rats to continuously monitor cardiorespiration and electroencephalographic activity during exposure to hyperbaric hyperoxia in unanesthetized and unrestrained male Sprague-Dawley rats. These experiments used poikilocapnic hyperoxia to mimic conditions under which CO2 retention does not occur and, presumably, under which end-tidal Pco2 decreases secondarily to O2-induced stimulation of breathing (3, 4, 13, 51, 54). For purposes of this study, however, we will refer to a significant increase in ventilation as hyperoxic hyperpnea, because we did not measure end-tidal Pco2 or blood gases during exposure to HBO2. Consequently, we could not determine whether increased depth of breathing, or rate during HBO2, or a combination of these, caused respiratory alkalosis. Our findings show that hyperoxia stimulates ventilation ≤8 min prior to onset of behavioral seizures in an unanesthetized rat. Thus hyperoxic hyperpnea may be a useful physiomarker of an impending CNS-OT seizure in a resting mammal breathing HBO2