A Comparison Study Between 10 Days of Dry Immersion Versus 10 Days of Head-down Bedrest on 20 Healthy Male Volunteers

Standardization of the Dry Immersion Model Used as a Ground-based Model to Mimic Weightlessness. Part 3: a Comparison Study Between 10 Days of Dry Immersion vs. 10 Days of Head-down Bedrest on 20 Healthy Male Volunteers (VIVALDI 3)

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. Different methods are used to simulate weightlessness on Earth, including cellular models, animal models using hind-limb unloading, or on humans with unilateral lower limb suspension. 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 Head-down Bedrest (HDBR) model has been widely used for this purpose and is considered one of the references for reproducing the physiological effects of weightlessness on Earth. During HDBR, subjects are lying down with an angle of -6° between the feet and head, on their side, their back or their front, but must keep one shoulder in contact with the mattress. All daily activities and tests are performed in this position.

One of the advantages of the HDBR model is that it has now been used in a great number of studies internationally, and its effects have long been described and compared with those of microgravity and spaceflight. Long-term bedrest is the gold-standard method for studying the effects of weightlessness and to test countermeasures.

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 objective of the present study is to compare the physiological adaptations to10 days of dry immersion versus 10 days of head-down bedrest in 20 healthy male subjects. A set of measurements will assess the changes in the cardiovascular, neuro-ophthalmological, hematological, metabolic, sensorimotor, immune, muscle and bone systems as a result of both models. The most likely outcome of this study will not be to show a clear superiority of one model over the other. Rather, we expect to show differences in kinetics and intensity of adaptations, that should vary from one system to another. This will help future researchers choose the best model depending on the system they are investigating and the rapidity or intensity of the effect they are exploring. The two models, instead of competing with one another, are probably complementary.

• Healthy male volunteer (see below the description of medical tests and laboratory analysis performed at the selection visit),
• Age 20 to 40,
• No overweight nor excessive thinness with BMI (weight kg/ height m2) between 20 and 26,
• Height between 165cm and 180 cm,
• Certified as healthy by a comprehensive clinical assessment (detailed medical history and complete physical examination): in particular, free from any chronic disease or any acute infectious disease or cardiovascular, neurological, ENT (especially orthostatic hypotension and vestibular disorders), orthopaedic or musculoskeletal disorders,
• Fitness level assessment: 35 ml/min./kg < VO2max < 55 ml/min./kg,
• Non active smokers,
• No alcohol, or drug addiction, and no medical treatment,
• Covered by a Health Insurance System,
• Having signed the informed consent,
• Free from any engagement during the study.

Exclusion Criteria:

• Any history or presence of clinically relevant cardiovascular, neurological or ENT (especially orthostatic hypotension and vestibular disorders), any chronic disease; any acute infectious disease. Particularly:
• Symptomatic orthostatic hypotension whatever the decrease in blood pressure, or asymptomatic postural hypotension defined by a decrease in SBP equal to or greater than 20 mmHg within 3 minutes when changing from the supine to the standing position,
• Cardiac rhythm disorders,
• Hypertension,
• Chronic back pains,
• Vertebral fracture, scoliosis or herniated disc,
• Glaucoma,
• Self-reported hearing problems,
• History of migraines,
• History of hiatus hernia or gastro-esophageal reflux,
• History of thyroid dysfunction, renal stones, diabetes,
• History of head trauma,
• History of genetic muscle and bone diseases of any kind,
• Past records of thrombophlebitis, family history of thrombosis or positive response in thrombosis screening procedure (anti thrombin III, S-protein, C-protein, factor V Leiden mutation and the mutation 20210 of the prothrombin gene),
• Signs of venous insufficiency, varicose veins, or telangiectasia
• Bone mineral density: T-score ≤ -1.5,
• Poor tolerance to blood sampling,
• Having given whole blood (more than 7ml/kg) in a period of 8 weeks or less before the start of the experiment, or having given whole blood more than 5 times in the past 12 months,
• Significant history of allergy, especially no dermatological allergy,
• History of food allergy,
• Significant anomaly detected in the biological analysis,
• Positive reaction to any of the following tests: HVA IgM (hepatitis A), HBs antigen (hepatitis B), anti-HVC antibodies (hepatitis C), anti-HIV1+2 antibodies,
• Vegetarian or vegan,
• Refusal to give permission to contact his general practitioner,
• Subject who, in the judgment of the investigator, is likely to be non-compliant during the study, or unable to cooperate because of a language problem or poor mental development,
• Subject already participating or in the exclusion period of a clinical research,
• Subject who has received more than 6000 Euros within 12 months for being a research subject,
• MRI contraindications:
• History or active claustrophobia,
• Osteosynthesis material, presence of metallic implants or any other contra-indication for MRI,
• Allergy to Gadolinium.
• Vulnerable persons according to law "Code de la Santé Publique" (L1121-5 to L1121-8) :
• Persons deprived of their liberty by an administrative or judicial decision,
• Persons under involuntary psychiatric care,
• Persons admitted in a health or social establishment for purposes other than research,
• Minors,
• Adults subject to legal protection (subject under guardianship or trusteeship) or unable to express their consent.