Dopamine, Neurochemical Processes, and Oxygen Toxicity at Pressure

Does hyperbaric oxygen cause narcosis or hyperexcitability? A quantitative EEG analysis

Divers breathe higher partial pressures of oxygen at depth than at the surface. The literature and diving community are divided on whether or not oxygen is narcotic. Conversely, hyperbaric oxygen may induce dose-dependent cerebral hyperexcitability. This study evaluated whether hyperbaric oxygen causes similar narcotic effects to nitrogen, and investigated oxygen's hyperexcitability effect. Twelve human participants breathed "normobaric" air and 100% oxygen, and "hyperbaric" 100% oxygen at 142 and 284 kPa, while psychometric performance, electroencephalography (EEG), and task load perception were measured. EEG was analyzed with functional connectivity and temporal complexity algorithms. The spatial functional connectivity, estimated using mutual information, was summarized with the global efficiency network measure. Temporal complexity was calculated with a "default-mode-network (DMN) complexity" algorithm. Hyperbaric oxygen-breathing caused no change in EEG global efficiency or in the psychometric test. However, oxygen caused a significant reduction of DMN complexity and a reduction in task load perception. Hyperbaric oxygen did not cause the same changes in EEG global efficiency seen with hyperbaric air, which likely related to a narcotic effect of nitrogen. Hyperbaric oxygen seemed to disturb the time evolution of EEG patterns that could be taken as evidence of early oxygen-induced cortical hyperexcitability. These findings suggest that hyperbaric oxygen is not narcotic and will help inform divers' decisions on suitable gas mixtures.

Variation of Cognitive Function During a Short Stay at Hypobaric Hypoxia Chamber (Altitude: 3842 M)

Objective

To observe the effects of a fast-acute ascent to high altitude on brain cognitive function and transcranial doppler parameters in order to understand the physiological countermeasures of hypoxia.

Methods

17 high-altitude-naïve male subjects (mean age was 26.3 ± 8.1 years) participated in the study. We measured Critical Flicker Fusion Frequency (CFFF), blood oxygen saturation, Psychology Experiment Building (PEBL) including three tests (Modified Math Processing Task, Perceptual Vigilance Task, and Time Estimation Task), as well as Cerebral Blood Flow index (CBFi), mean cerebral artery Systolic and diastolic velocities, Cerebral Pulsatility index (CPi), and heart Rate. All were measured at sea level, at least 1 h after arrival at the hypobaric hypoxia equivalent of 3842 m and 1 h after return to sea level.

Results

Under acute exposure to hypobaric hypoxic conditions, significant decrease in CFFF [42.1 ± 1 vs. 43.5 ± 1.7 Hz at sea level (asl), p < 0.01], CBFi (611 ± 51 vs. 665 ± 71 asl, p < 0.01) and blood oxygen saturation (83 ± 4% vs. 98 ± 1% asl, p < 0.001) as compared to pre-ascent values were observed. Physiological countermeasures to hypoxia could be involved as there was no significant change in neuropsychometric tests, Systolic and Diastolic velocities and CPi. A significant increase in Heart Rate (81 ± 15 bpm vs. 66 ± 15 bpm asl, p < 0.001) was observed. All parameters returned to their basal values 1 h after regaining sea level.

Conclusion

Hypoxia results in a decrease in CFFF, CBFi and oxygen saturation and in an increase in heart rate. As it decreased, Cerebral Blood Flow index does not seem to be the physiological measurement of choice to hypoxia explaining the maintenance of cognitive performance after acute exposure to hypobaric hypoxia and requires further investigation. Cerebral oxygen delivery and extraction could be one of the underlying mechanisms.

Early detection of diving-related cognitive impairment of different nitrogen-oxygen gas mixtures using critical flicker fusion frequency

INTRODUCTION

Cognitive impairment related to inert gas narcosis (IGN) is a threat to diving safety and operations at depth that might be reduced by using enriched air nitrox (EANx) mixtures. Using critical flicker fusion frequency (CFFF), a possible early detection of cognitive abilities/cerebral arousal impairment when breathing different oxygen (O2) fractions was investigated.

METHODS

Eight male volunteers performed, in random order, two dry chamber dives breathing either air or EANx40 (40% O₂-60% nitrogen) for 20 minutes (min) at 0.4 MPa. Cognition and arousal were assessed before the dive; upon arrival at 0.4 MPa; after 15 min exposure at 0.4 MPa; on surfacing and 30 min post-dive using behavioural computer-based testing psychology experiment building language (PEBL) and by CFFF while continuously recording brain oxygenation with near-infrared spectroscopy.

RESULTS

In both breathing conditions, CFFF and PEBL demonstrated a significant inverse correlation (Pearson r of -0.90, P < 0.0001), improved cognitive abilities/cerebral arousal occurred upon arrival at 0.4 MPa followed by a progressive deterioration. Initial brain activation was associated with a significant increase in oxyhaemoglobin (HbO2) and a simultaneous decrease of deoxyhaemoglobin (HHb). The magnitude of the changes was significantly greater under EANx (P = 0.038).

CONCLUSIONS

Since changes were not related to haemodynamic variables, HbO₂ and HHb values indicate a significant, O₂-dependent activation in the prefrontal cortex. Owing to the correlation with some tests from the PEBL, CFFF could be a convenient measure of cognitive performance/ability in extreme environments, likely under the direct influence of oxygen partial pressure, a potent modulator of IGN symptoms.

Assessment of the interaction of hyperbaric N2, CO2, and O2 on psychomotor performance in divers

Diving narcosis results from the complex interaction of gases, activities, and environmental conditions. We hypothesized that these interactions could be separated into their component parts. Where previous studies have tested single cognitive tasks sequentially, we varied inspired partial pressures of CO₂, N₂, and O₂ in immersed, exercising subjects while assessing multitasking performance with the Multi-Attribute Task Battery II (MATB-II) flight simulator. Cognitive performance was tested under 20 conditions of gas partial pressure and exercise in 42 male subjects meeting U.S. Navy age and fitness profiles. Inspired nitrogen (N₂) and oxygen (O₂) partial pressures were 0, 4.5, and 5.6 ATA and 0.21, 1.0, and 1.22 ATA, respectively, at rest and during 100-W immersed exercise with and without 0.075-ATA CO₂ Linear regression modeled the association of gas partial pressure with task performance while controlling for exercise, hypercapnic ventilatory response, dive training, video game frequency, and age. Subjects served as their own controls. Impairment of memory, attention, and planning, but not motor tasks, was associated with N₂ partial pressures >4.5 ATA. Sea level O₂ at 0.925 ATA partially rescued motor and memory reaction time impaired by 0.075-ATA CO₂; however, at hyperbaric pressures an unexpectedly strong interaction between CO₂, N₂, and exercise caused incapacitating narcosis with amnesia, which was augmented by O₂ Perception of narcosis was not correlated with actual scores. The relative contributions of factors associated with diving narcosis will be useful to predict the effects of gas mixtures and exercise conditions on the cognitive performance of divers. The O₂ effects are consistent with O₂ narcosis or enhanced O₂ toxicity.