The effect of liquid breathing on respondent conditioning activity in small animals in hyperbaria

Introduction. Without special preventive measures, the consequences of hyperbaric exposure to the body of warm-blooded animals and humans, as well as the subsequent transition to normal conditions, can be deadly due to the occurrence of specific diseases (decompression sickness), the toxic effects of oxygen, nitrogen and helium., as well as injuries, for example, barotrauma of the lungs and other air-bearing cavities of the body. The use of special liquids for breathing provides a fundamentally different approach to the exploration of hydrospace, devoid of etiopathogenetic prerequisites for the development of these conditions. However, in order to perform tasks in hyperbaric conditions, it is necessary that the technology allows for conscious activity. It has been established that with liquid breathing (LB) under normobaria conditions, conditioned reflex activity (CRA) in small laboratory animals is preserved, but there is no data on its safety in conditions of hyperbaria.

The study aims to assess the preservation of respondent conditioning activity in small laboratory animals with independent LB in immersion in the respiratory fluid (RF) under conditions of hyperbaria.

Materials and methods. The authors have conducted a study on male Syrian hamsters 4 months old, 120–140 g weight. For the study, a labyrinth was used, which allows to study CRA in small laboratory animals in a liquid environment.

The labyrinth was placed in a hyperbaric setup with liquid medium to get required external pressure. The study had 2 stages. Animals were trained a conditioned response to drowning avoiding while air breathing at the 1^st stage. The animal was placed on the lower level, then the labyrinth was immersed in the tank filled with water. The speed of immersion was to provide the animal's head above the water level. The risk of drowning made the animal to search for a passway to the higher level. The researchers have trained 3 times a day for 10 days under atmospheric pressure conditions. At the 2^nd stage, the authors studied the effect of liquid respiration on the state of conditioned reflex activity of animals in normal (group 1) and hyperbaria (group 2) conditions. In the case of hyperbaric exposure, an aquarium filled with oxygenated perfluorohexane enriched with oxygen was placed in a hyperbaric stand.

The labyrinth with a fixed at the lower-level animal was totally immersed in aquarium. The specialists closed the bayonet shutter of the stand and performed compression to a preset pressure, after which the animal was removed from fixation. Animal was observed with video translation. The time counting of the labyrinth passage began since fixation had been released. CRA status was valued by the mean finishing time or by the labyrinth level at which animal had stopped.

Results. At atmospheric pressure, 100% of the animals completed the labyrinth, while the average passage time was 45±12 s. The labyrinth trial with 1,2 MPa gauge pressure value was successfully completed by the 60% of animals, mean trial time was 76±25 s, the least 40% of animals had stopped from 3^rd to at 5^th level. It was also noteworthy that in the process of LB in animals of all groups there was a significant decrease in rectal temperature. The temperature difference before and after the study in groups I and II was 6.0 and 8.1°C. The decrease in body temperature directly depended on the duration of immersion of animals in the RF, which for groups I and II was 155±17 and 246±29 s, respectively.

Conclusion. With independent liquid breathing in immersion in the respiratory fluid in hyperbaria, there is a decrease in CRA, which can be caused both by the direct influence of excessive environmental pressure and by other factors, including developing hypothermia.

Ethics. The study was conducted in accordance with the Ethical principles of the Declaration of Helsinki. The Clinical Study Protocol was reviewed at a meeting by the local Ethics Committee of Izmerov Research Institute of Occupational Health (Protocol No. 8 of November 23, 2016).

Contributions:
Bonitenko E.Yu. — study concept and design, writing;
Kotskiy M.A. — study concept and design, data collection and processing, writing;
Tonshin A.A. — study concept and design, writing;
Makarov A.F. — data collection and processing, editing;
Kovtun A.L. — editing;
Bala A.M. — editing;
Muravskaya M.P. — data collection and processing;
Tkachuk Yu.V. — data collection and processing;
Bondarenko A.V. — data collection and processing.

Funding. The study was funded by The Advanced Research Foundation, Moscow, Russian Federation.

Conflict of interest. The authors declare no conflict of interest.

Received: 10.04.2026 / Accepted: 27.04.2026 / Published: 02.06.2026

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