A 5-day Dry Immersion Study on 20 Healthy Male Volunteers
Condition: Weightlessness
Study type: Interventional

Summary: 
      Dry immersion (DI) is a ground-based model of prolonged conditions of simulated microgravity.
      Dry immersion involves immersing the subject in water covered with an elastic waterproof
      fabric. As a result, the immersed subject, who is freely suspended in the water mass, remains
      dry. Within a relatively short duration, the model can faithfully reproduce most
      physiological effects of actual microgravity, including centralization of body fluids,
      support unloading, and hypokinesia.

      The main objective of the present study is to investigate the physiological effects of 5 days
      of dry immersion in 20 healthy male subjects, and to obtain DI-in-Men Reference Dataset. A
      set of measurements will assess the changes in the cardiovascular, neuro-ophthalmological,
      hematological, metabolic, sensorimotor, immune, muscle and bone systems.
    

Detailed description: 
      Space flights have shown the possibilities and limitations of human adaptation to space. For
      the last 50 years, results have shown that the space environment and microgravity in
      particular, cause changes that may affect the performance of astronauts. These physiological
      changes are now better known: prolonged exposure to weightlessness can lead to significant
      loss of bone and muscle mass, strength, cardiovascular and sensory-motor deconditioning,
      immune, hormonal and metabolic changes .

      Moreover, recently a new suite of physiological adaptations and consequences of space flight
      has been acknowledged. Indeed, after long flights, some astronauts present persistent
      ophthalmologic changes, mostly a hyperopic shift, an increase in optic nerve sheath diameter
      and occasionally a papillary oedema now defined by National Aeronautics and Space
      Administration (NASA) as Spaceflight-Associated Neuro-ocular Syndrome (SANS). Some of these
      vision changes remain unresolved for years post-flight. This phenomenon has most likely
      existed since the beginning of human space flight but is just recently being recognized as a
      major consequence of adaptation to microgravity.

      Overall, spaceflight induces physiological multi-system deconditioning which may impact
      astronauts' efficiency and create difficulties upon their return to normal gravity.
      Understanding the underlying mechanisms of these processes and developing efficient
      countermeasures to prevent, limit or reverse this deconditioning remain important challenges
      and major priorities for manned space programs.

      The space agencies are actively engaged in studying the physiological adaptation to space
      environment through studies on board the International Space Station (ISS) but also on the
      ground. Indeed, considering the limited number of flight opportunities, the difficulties
      related to the performance of in-flight experiments (operational constraints for astronauts,
      limited capabilities of in-flight biomedical devices), ground-based experiments simulating
      the effects of weightlessness are used to better understand the mechanisms of physiological
      adaptation, design and validate the countermeasures. Different methods are used to simulate
      microgravity on Earth. However, two approaches, -6° head-down bed rest (HDBR) and dry
      immersion (DI) have provided possibilities for long-term exposures with findings closest to
      those seen with a weightless state. They produce changes in body composition (including body
      fluid redistribution), cardiovascular and skeletal muscle characteristics that resemble the
      effects of microgravity.

      The common physiological denominator is the combination of a cephalad shift of body fluids
      and reduced physical activity. Being similar in their effects on the human body, these
      models, however, differ in their specifics and acting factors. The HDBR, as the name implies,
      implicates a long (from several weeks to a year) stay in the supine position, the head tilted
      down by -6° from the horizontal plane. HDBR is the most frequently used ground-based
      simulation for gravitational unloading of the human body in western countries.

      Unlike bed rest, dry immersion provides a unique opportunity to study the physiological
      effects of the lack of a supporting structure for the body. Dry immersion means immersing the
      subject into thermoneutral water, while covered with a special elastic free floating
      waterproof fabric. The subject, surrounded by the tarp and "freely suspended" in the water
      mass, remains dry. During horizontal immersion, pressure forces are distributed nearly
      equally around the entire surface of the body (only the head and neck are not entirely
      supported by water). The absence of mechanical support of specific anatomic zones during
      immersion creates a state akin to weightlessness called "supportlessness". Physiological
      changes under DI develop more rapidly and are more profound than under HDBR . This advanced
      ground-based model is extremely suited to test countermeasures for microgravity-induced
      deconditioning and physical inactivity-related pathologies.

      In 2015, DI facility has been installed at the Space Clinic in Toulouse (France), and a first
      in Europe three-day dry immersion study was carried out in 12 healthy male volunteers. That
      study demonstrated an important headward fluid transfer with a significant dilatation of the
      jugular veins, an increase in venous blood velocities and intracranial pressure, as well as
      ophthalmological changes consistent with a presumable increase in intracranial pressure at
      the head at over 20 mmHg (normal values 7 to 15 mmHg), which confirms that dry immersion is a
      good model to simulate the effects of fluid transfer. In 2019, a second dry-immersion
      experiment, this time lasting 5 days, was conducted in 18 healthy male volunteers. The goal
      was to quantify the effect of venoconstrictive thigh cuffs, used for 10 hours/day as a
      countermeasure, on the time course and extent of DI-induced alterations in body fluids,
      cephalic circulation, fundus, and brain. These two studies have sparked the interest in the
      dry immersion model.

      Indeed, the European Space Agency (ESA) has decided to include this model in the research
      programs it promotes on the effects of weightlessness. As a first step, ESA decided to carry
      out a standardization work like the one done on the bedrest model. ESA tasked a group of
      European experts to design a first study and the tests that would need to be carried out to
      better understand and validate the model.

      Few studies conducted to date have investigated gender differences in the astronaut
      population.

      The small number of female astronauts may be part of the reason why scientific data are
      lacking to draw valid conclusions about possible gender differences. However, if women
      currently constitute only about 12% of astronauts (only 72 women out of 596 astronauts as of
      December 2021), they are and will be more and more represented in crews. They now constitute
      30% of American crews and NASA (US space agency) has announced gender parity for crews on
      future lunar missions. It is therefore essential to study the physiological changes induced
      by weightlessness in both sexes, and to develop efficient sex-specific countermeasures. The
      expert group has therefore concluded that two different studies with the same design should
      be carried out, one in women and one in men, to obtain a comparable dataset. An immersion
      period of 5 days was determined to induce the physiological changes they wish to study. A
      battery of tests has thus been defined by this expert group based on standard tests carried
      out in bedrest studies (Bedrest Standard Measurements), supplemented by additional tests to
      further investigate the model and to acquire a better understanding of the time course of the
      physiological changes in both sexes.

      This work has started in 2021 with a first study, conducted in women during a 5-day immersion
      period (awaiting publication). The next step is to realize a similar set of measurements in
      men, to standardize the dry immersion model taking into account sex differences.

      This study falls within this context and will be the first ESA dry immersion study carried
      out in men. Its objective is to obtain a standardized dataset for DI in men, which will serve
      as a basis for the development and evaluation of countermeasures to support future space
      missions. The main physiological systems will be explored before, during and after the 5 days
      of immersion through a battery of specific tests and measurements. The results will be
      analyzed by scientists specializing in each field in order to better understand the dry
      immersion model, to compare its effects with those of the bedrest model and those of
      spaceflight. Comparison with results obtained in the female population will be a major part
      of the analysis. The clinical (adverse effects, comfort of subjects) and operational aspects
      are also part of the secondary objectives of the study.
    

Intervention name(s): Dry immersion
Intervention type(s): Other
Intervention description(s): Subjects are immersed up to the neck for 5 days in a specially designed bath filled with tap water.
Intervention model: Single Group Assignment
Masking: None (Open Label)
Primary purpose: Basic Science
Keywords: Microgravity simulation; Models; Weightlessness