PHYSIOLOGY OF ADULT HOMO SAPIENS - FLYING AND IMMERSION PHYSIOLOGY

aviation medicine : that branch of medicine which has to do with the physiological, medical, psychological, and epidemiological problems involved in aviation

• commercial air flight : the provision of medical assistance to passengers during flights aboard commercial aircraft is a matter of concern to most physicians^{ref1, ref2, ref3, ref4}. Although close to 2 billion people travel on commercial airlines each year, there has been little study of medical issues related to air travel^ref. As the number of air travelers increases and as the population ages, the number of medical events aboard commercial aircraft will increase. In this article, we review the environmental, clinical, and legal aspects of in-flight medical care aboard commercial aircraft.
• epidemiologic features
• incidence and types of events : determining the incidence of in-flight medical events aboard commercial aircraft is difficult, because there are currently no regulatory reporting requirements. As a consequence, the available data are fragmentary. In the 1980s, 2 studies examined in-flight medical events among all the passengers arriving at 2 major U.S. airports. One of these studies reported an incidence of in-flight medical events of 1 per 33,600 passengers, and the other reported an incidence of 1 per 39,600 passengers — rates that correspond to about 33 and 30 in-flight medical events per day, respectively^{ref1, ref2}. In 2000, a study by the Federal Aviation Administration (FAA) of 1132 in-flight medical events on domestic flights during 1996 and 1997 found a rate of 13 events per day (DeJohn CA, Veronneau SJ, Wolbrink AM, et al. The evaluation of in-flight medical care aboard selected U.S. air carriers: 1996 to 1997. Washington, D.C.: Federal Aviation Administration, Office of Aviation Medicine, 2000. (Technical report no. DOT/FAA/AM-0013)). This rate may be lower than those reported previously because only events serious enough to have involved ground-based medical support were included. Together, these results suggest that fewer than half the medical events that occur during flight are serious enough to require ground-based medical assistance. The frequency of in-flight medical events aboard international flights is increasing^{ref1, ref2, ref3} (DeJohn CA, Veronneau SJ, Wolbrink AM, et al. The evaluation of in-flight medical care aboard selected U.S. air carriers: 1996 to 1997. Washington, D.C.: Federal Aviation Administration, Office of Aviation Medicine, 2000. (Technical report no. DOT/FAA/AM-0013.)) and was recently estimated to be one in-flight medical event per 14,000 passengers worldwide^ref. However, this estimate is probably high, since it would correspond to a rate of more than 350 events a day worldwide. Most in-flight medical events are not serious^{ref1, ref2, ref3, ref4, ref5, ref6} (Hordinsky JR, George MH. Utilization of emergency kits by air carriers. Washington, D.C.: Federal Aviation Administration, Office of Aviation Medicine, 1991. (Technical report no. DOT/FAA/AM-91/2.)). Vasovagal episodes (fainting, near-fainting, dizziness, and hyperventilation) are the most common events. Cardiac, neurologic, and respiratory problems make up the most serious events and account for the majority of instances in which an aircraft must be diverted and make an unscheduled landing
• physicians' participation : According to physicians' accounts^{ref1, ref2, ref3} and the results of a 1998 questionnaire^ref, it appears that physicians are comfortable providing voluntary assistance during in-flight medical events. Fear of liability is cited as a major reason for physicians' reluctance to offer assistance^ref. In the 2000 study by the FAA, 69% of all in-flight medical events aboard U.S.-registered aircraft were attended by a health care professional: physicians volunteered in approximately 40% of the instances, nurses in 25%, and paramedics in 4%. Moreover, there was 79% agreement between the in-flight medical diagnosis and the subsequent hospital diagnosis, and the passenger's condition improved in 60% of the cases, findings that suggest that the in-flight treatment provided was appropriate (DeJohn CA, Veronneau SJ, Wolbrink AM, et al. The evaluation of in-flight medical care aboard selected U.S. air carriers: 1996 to 1997. Washington, D.C.: Federal Aviation Administration, Office of Aviation Medicine, 2000. (Technical report no. DOT/FAA/AM-0013.))
• diversions : unscheduled landings for medical purposes are a serious problem for commercial air carriers. Delays in reaching the final destination, inconvenience to passengers, added costs, and increased risks to safety make the decisions about diverting an aircraft complex and difficult. The cost of a medical diversion typically ranges from $3,000 to $100,000, depending on whether fuel needs to be dumped before landing and whether overnight accommodations for passengers are arranged^{ref1, ref2}. According to the 2000 report by the FAA, 13% of all in-flight medical incidents aboard domestic aircraft resulted in an emergency diversion. Cardiac incidents accounted for the greatest percentage of the diversions (46%), followed in frequency by neurologic incidents (18%) and respiratory incidents (6%) (DeJohn CA, Veronneau SJ, Wolbrink AM, et al. The evaluation of in-flight medical care aboard selected U.S. air carriers: 1996 to 1997. Washington, D.C.: Federal Aviation Administration, Office of Aviation Medicine, 2000. (Technical report no. DOT/FAA/AM-0013.))
• environmental and physiological factors :
• cabin pressure : a frequently overlooked fact is that during flight, the cabin pressure on commercial aircraft is usually adjusted to be equivalent to the barometric pressure found at an altitude of 1500 to 2500 m (5000 to 8000 ft) above sea level. Variations in cabin pressure within this range depend on the type of aircraft, weather conditions, and the need for passenger comfort in turbulent conditions (Cottrell JJ. Altitude exposures during aircraft flight: flying higher. Chest 1988;92:81-84; Cummins RO. High-altitude flights and risk of cardiac stress. JAMA 1988;260:3668-3669). These barometric pressures result in a decrease in the partial pressure of arterial oxygen from about 95 mm Hg to about 56 mm Hg in healthy passengers. This represents only a 4% reduction in the oxygen carried by the blood, since a partial pressure of oxygen of 56 mm Hg lies on the flat part of the oxyhemoglobin dissociation curve. However, in many passengers with cardiopulmonary disease, the partial pressure of arterial oxygen at sea level is < 95 mm Hg and is on the steep portion of the oxyhemoglobin dissociation curve, and at ordinary cabin pressures the oxygen saturation may fall dramatically^{ref1, ref2} (Medical guidelines for airline travel. Alexandria, Va.: Aerospace Medical Association, 1997). For them, routine cabin pressures increase the risk of hypobaric hypoxia^{ref1, ref2, ref3, ref4}. Air and gas in body cavities expand in direct proportion to decreases in pressure, as described by Boyle's law. A cabin pressure equivalent to the pressure at an altitude of 1500 m results in expansion of air or gas volume by up to 30% (Cottrell JJ. Altitude exposures during aircraft flight: flying higher. Chest 1988;92:81-84; Fitness to travel by air. In: Harding RM, Mills FJ. Aviation medicine. 3rd ed. London: BMJ Publishing, 1993:30-42)^{ref1, ref2, ref3}. In healthy passengers, gas expansion causes only minor abdominal cramping or aural symptoms. However, passengers who have recently undergone surgical procedures are at increased risk for wound dehiscence in conditions that cause gas expansion^ref.25 Medical devices such as pneumatic splints, feeding tubes, urinary catheters, and cuffed endotracheal or tracheostomy tubes may be affected by the expansion of air or gas. Instillation of water rather than air can avert these problems^{ref1, ref2} (Medical guidelines for airline travel. Alexandria, Va.: Aerospace Medical Association, 1997). Because of the potential adverse consequences of gas expansion, many commercial airlines do not allow the use of pneumatic splints aboard their aircraft. Plaster casts applied within 48 hours before flight should be bivalved to reduce the chance of circulatory problems^ref
• cabin air quality : many scientific studies show that the air in commercial aircraft cabins is safe and poses no risk to passengers^{ref1, ref2, ref3, ref4}. However, the humidity in cabins is low, typically 10 to 20%^{ref1, ref2} (Medical guidelines for airline travel. Alexandria, Va.: Aerospace Medical Association, 1997). This low humidity has the propensity to exacerbate reactive airway disease and trigger other minor problems, such as dryness of the eyes. Little information regarding infectious diseases and their potential for person-to-person transmission in the cabin-air environment is available. The Centers for Disease Control and Prevention and the World Health Organization have established guidelines on when and how to notify passengers and flight crew of the need for antimicrobial chemoprophylaxis after certain types of exposure^{ref1, ref2}.
• violence aboard aircraft : the incidence of disruptive behavior in an aircraft passenger, or "air rage," is on the rise and places flight crew and passengers at high risk for injury (U.S. House of Representatives, Committee on Transportation and Infrastructure. Problems of passengers interference with flight crews and a review of H.R. 3064, the Carry-On Baggage Reduction Act of 1997. Washington, D.C.: 105th U.S. Congress, June 11, 1998). The number of in-flight incidents involving disorderly physical actions tripled between 1994 and 1997 aboard American Airlines alone, and alcohol was implicated in 25% of all such incidents (U.S. House of Representatives, Committee on Transportation and Infrastructure. Problems of passengers interference with flight crews and a review of H.R. 3064, the Carry-On Baggage Reduction Act of 1997. Washington, D.C.: 105th U.S. Congress, June 11, 1998; Seeing red over air rage. Vol. 15. No. 2. Consumer Reports Travel Letter 1999:15. (Washington, D.C.: Consumers Union.)). British Airways recorded 266 incidents in 1997 involving smoking, drunkenness, or abusive behavior. In some instances, physicians have assisted by chemically sedating agitated or violent passengers^ref (Kurkjian S, Tench M. Airliner bomb threat averted. Boston Globe. December 23, 2001:A1). The recent increases in vigilance with respect to aircraft security have led to stricter penalties for violent or disorderly behavior aboard commercial aircraft^ref (U.S. House of Representatives, Committee on Transportation and Infrastructure. Problems of passengers interference with flight crews and a review of H.R. 3064, the Carry-On Baggage Reduction Act of 1997. Washington, D.C.: 105th U.S. Congress, June 11, 1998)
• medical fitness for air travel :
• general principles : many passengers are unaware of the health implications of air travel. Physicians are increasingly expected to decide who is and who is not fit for air travel and to advise the public on matters relating to safe air travel. The Air Carrier Access Act of 1986 prohibits airlines from discriminating against passengers with disabilities, but airlines still have the right to refuse passengers who are not medically fit to travel on commercial aircraft (Air Carrier Access Act of 1986, 14 C.F.R. Part 382 (nondiscrimination on the basis of handicap in air travel)). A patient with special needs, such as a need for supplemental oxygen, may require a medical certificate from a physician (preferably one with training in aviation medicine) stating that the passenger is medically fit for commercial air travel at a cabin pressure equivalent to that at an altitude of 2500 m (Medical guidelines for airline travel. Alexandria, Va.: Aerospace Medical Association, 1997)^{ref1, ref2, ref3}. FAA security directives established since the terrorist acts of September 11, 2001, allow syringes and needles (with cap guards) if the passenger has a documented medical need for such equipment. The passenger must also have in his or her possession the medication that necessitates the use of a syringe, and the medication must have a pharmacy label identifying it^ref. There are numerous guidelines on air travel for persons with acute or chronic illnesses^{ref1, ref2, ref3, ref4, ref5, ref6, ref7, ref8, ref9, ref10, ref11, ref12} (Medical guidelines for airline travel. Alexandria, Va.: Aerospace Medical Association, 1997; Fitness to travel by air. In: Harding RM, Mills FJ. Aviation medicine. 3rd ed. London: BMJ Publishing, 1993:30-42)18,19,20,21,22,23,24,25,26,27,48,49,50 In general, travel on a commercial aircraft is contraindicated if a medical condition is adversely affected by hypoxia or pressure changes. A simple and useful test to assess a person's fitness for air travel is to determine whether he or she can walk 50 m (150 ft) or climb one flight of stairs without severe dyspnea or angina (Medical guidelines for airline travel. Alexandria, Va.: Aerospace Medical Association, 1997; Fitness to travel by air. In: Harding RM, Mills FJ. Aviation medicine. 3rd ed. London: BMJ Publishing, 1993:30-42). Partial list of contraindications to commercial air travel.
• oxygen therapy : any passenger with a partial pressure of arterial oxygen of < 70 mm Hg at sea level at rest requires supplemental oxygen during air travel^{ref1, ref2} (Medical guidelines for airline travel. Alexandria, Va.: Aerospace Medical Association, 1997). Airlines have become familiar with transporting passengers with cardiopulmonary disease, and the use of in-flight therapeutic oxygen is increasing by 10 to 12% each year^ref. Supplemental oxygen for in-flight medical use can be arranged with commercial carriers but requires at least 48 hours' advance notice and a prescription for oxygen. Passengers cannot use their own equipment during flight, because oxygen is considered a hazardous material. Passengers are responsible for arranging for their own oxygen supply at their departure and arrival terminals.
• medical liability : in general, airlines are legally liable for gross neglect or willful misconduct during in-flight medical events, as stipulated by international treaties^{ref1, ref2}. The threat of lawsuits is one of the factors that has recently motivated airlines to improve their resources for handling in-flight medical events. To our knowledge, no litigation has been brought to date against a physician who has rendered assistance during an in-flight medical event. Does a physician who is a passenger have a duty to volunteer medical assistance? In the United States, Canada, and the United Kingdom, physicians do not have a legal duty to render assistance unless there is a preexisting physician–patient relationship. In contrast, many European countries and Australia do impose such a legal obligation^{ref1, ref2, ref3}. By international law, the country in which the aircraft is registered has legal jurisdiction^{ref1, ref2, ref3}.52,53,54 However, the country in which the incident occurs or the country of citizenship of the plaintiff or defendant can also have jurisdiction^{ref1, ref2, ref3, ref4}. An important step that reduced physicians' concern about liability was taken in 1998, when the Aviation Medical Assistance Act was signed into law^{ref1, ref2}. The act provides limited "good Samaritan" protection to any medically qualified passenger who provides medical assistance aboard an aircraft^ref. In addition to being medically qualified, the assisting passenger must be a volunteer, render care in good faith, and receive no monetary compensation. Gifts in the form of travel vouchers, wine, or seat upgrades are not considered compensation. The assisting passenger must also render medical care similar to the care that others with similar training would provide under such circumstances. Physicians should be aware of the provisions of the Aviation Medical Assistance Act and recognize its limitations.
• medical resources aboard commercial aircraft
• emergency medical kit : since 1986, the FAA has required all commercial aircraft with > 30 passenger seats to carry an emergency medical kit^{ref1, ref2}. Table 3 lists the contents of the emergency medical kit currently mandated by the FAA. In a recent reevaluation (DeJohn CA, Veronneau SJ, Wolbrink AM, et al. The evaluation of in-flight medical care aboard selected U.S. air carriers: 1996 to 1997. Washington, D.C.: Federal Aviation Administration, Office of Aviation Medicine, 2000. (Technical report no. DOT/FAA/AM-0013.)), it was discovered that bronchodilator inhalers, oral antihistamines, and nonnarcotic analgesics obtained from other passengers were used frequently enough to warrant the inclusion of these items in the on-board medical kits (DeJohn CA, Veronneau SJ, Wolbrink AM, et al. The evaluation of in-flight medical care aboard selected U.S. air carriers: 1996 to 1997. Washington, D.C.: Federal Aviation Administration, Office of Aviation Medicine, 2000. (Technical report no. DOT/FAA/AM-0013)), and a final rule issued by the FAA in April 2001 required the addition of these items to the kits by April 2004^ref. Several airlines have augmented their on-board medical kits to include cardiac resuscitative medications, sedatives, diuretics, and intubation equipment. A basic first-aid kit containing adhesive bandages, dressings, elastic bandages, and splints is also available on board commercial aircraft to handle minor injuries. Enhanced on-board emergency medical kit mandated by the Federal Aviation Administration :
• automated external defibrillators : evaluations of the use of automated external defibrillators on board U.S. commercial aircraft show survival rates ranging from 27% to 40%^{ref1, ref2, ref3}. In general, automated external defibrillators analyze cardiac rhythm and automatically deliver a defibrillatory shock if ventricular fibrillation or rapid ventricular tachycardia is detected^ref. Some on-board automated external defibrillators have an electrocardiographic display that shows the cardiac rhythm. One study found on-board automated external defibrillators useful as cardiac monitors in that they allow better decision making in cases in which a passenger has chest pain, palpitations, dyspnea, or lightheadedness^ref. According to the recent FAA ruling, all commercial aircraft traveling with at least one flight attendant must carry an automated external defibrillator by April 2004^ref. Many airlines have already voluntarily placed these devices on their aircraft and train flight attendants in their use. Several airlines allow only trained flight attendants — rather than volunteering physicians, who may be unfamiliar with the equipment — to attach and operate on-board automated external defibrillators^ref. Advanced telemetry systems that transmit video images, vital signs, 12-lead electrocardiograms, and oxygen-saturation data to ground-based physicians have been tested^ref. However, only one airline (Virgin Atlantic) has installed such systems on board its aircraft.
• ground-based medical assistance : many airlines no longer rely on the chance that a physician will be on board their aircraft if a medical event occurs. Flight crews on most major airlines now have direct links to some form of ground-based medical assistance. Several companies provide 24-hour, ground-to-air medical consultation and are staffed by physicians who are board certified in emergency medicine and have additional training in aviation medicine. When contacted, the ground-based physician advises the flight crew, and any medically qualified passenger who volunteers to assist, on treatment. Although the ultimate decision to divert or not to divert the aircraft rests with the captain, the ground-based physician can provide medical advice on this point as well as information about diversion locations. At least one major airline maintains its own ground-based medical staff. One possible advantage of the availability of ground-based medical assistance is that it may reduce unnecessary aircraft diversions. Such diversions are inconvenient and costly and often are not as safe as a routine landing at the planned destination. A 1997 study by the Air Transport Association found that ground-based medical assistance resulted in a 70% decrease in medical diversions (Air Transport Association medical kit survey. Washington, D.C.: Air Transport Association, 1997)
General approach to in-flight medical events : there are no federal regulations or guidelines on the management of in-flight medical events, and each airline has its own policy. The flight crew is responsible for responding to a passenger who becomes acutely ill. The role of any passenger who is a health care professional and volunteers is to assist the flight crew with the medical event, not to take control^ref. Physicians should feel comfortable assisting during an in-flight medical event; fear of liability should not prompt reluctance to offer assistance. Medicolegal Recommendations for Physicians Volunteering Assistance during an In-Flight Medical Event. The goal of in-flight medical assistance is to stabilize the condition of the ill passenger until the aircraft has landed. The health care professional who volunteers to assist has several options for basic management: he or she may provide oxygen; use medications and supplies in the emergency medical kit; ask the flight crew to lower the altitude of the aircraft to increase cabin pressure; consult with and obtain advice from ground-based medical support personnel; and suggest diversion of the aircraft^{ref1, ref2}. Unresponsiveness in a patient warrants application of the automated external defibrillator (if available on board), use of oxygen, establishment of intravenous access, and administration of 50% dextrose. Angina should be treated with aspirin and nitrates. A bronchodilator should be considered for shortness of breath in a passenger with asthma or chronic obstructive pulmonary disease. Vasovagal syncope can usually be managed simply by raising the legs and applying cold compresses to the forehead. Acute allergic reactions can be managed with diphenhydramine and (in severe cases) subcutaneous epinephrine. Increasing the cabin pressure, if possible, may alleviate altitude-related chest pain, shortness of breath, and abdominal pain by resolving relative hypoxia and decreasing the expansion of gas. An acutely agitated, psychotic, or violent passenger may be sedated with a benzodiazepine if it is available in the on-board medical kit. A recommendation to divert the aircraft should be considered if a passenger has chest pain, shortness of breath, or severe abdominal pain that does not improve with use of the recommended initial interventions. A recommendation to divert should also be considered if a passenger is persistently unresponsive or has cardiac arrest, an acute coronary syndrome, severe dyspnea, stroke, refractory seizure, or severe agitation.
• space medicine : that branch of aviation medicine concerned solely with conditions to be encountered by man in space.
• microgravity : the minute amount of gravitational force existing in outer space; it results in a weightless condition and enhances the likelihood of certain diseases, e.g. osteoporosis. Spinal injury patients provide a much better model than those with osteoporosis for the weightless conditions in space because they lose bone at a similar rate to astronauts (lose about 2% every month, even with exercise, vs. 2-3% every decade in age-related osteoporosis). In a study, 8 patients who did not take zoledronate lost 16-18% of their femur bone mass over a year, while 7 patients using the drug lost only 6%.

Returning astronauts have experienced altered immune function and increased vulnerability to infection during spaceflights dating back to Apollo and Skylab. Lack of immune response in microgravity occurs at the cellular level. Differential gene expression was analyzed to find gravity-dependent genes and pathways. Inhibited induction of 91 genes was found in the simulated freefall environment of the random positioning machine. Altered induction of 10 genes regulated by key signaling pathways was verified using real-time RT-PCR. Impaired induction of early genes regulated primarily by transcription factors NF-kB, CREB, ELK, AP-1, and STAT after crosslinking the T-cell receptor contributes to T-cell dysfunction in altered gravity environments. PKA and PKC are key early regulators in T-cell activation. Since the majority of the genes were regulated by NF-kB, CREB, and AP-1, we studied the pathways that regulated these transcription factors. The PKA pathway was down-regulated in vg. In contrast, PI3-K, PKC, and its upstream regulator pLAT were not significantly down-regulated by vectorless gravity. Since NF-kB, AP-1, and CREB are all regulated by PKA and are transcription factors predicted by microarray analysis to be involved in the altered gene expression in vectorless gravity, the data suggest that PKA is a key player in the loss of T-cell activation in altered gravity^ref.
Unloading of weight bearing bones as induced by microgravity or immobilization has significant impacts on the calcium and bone metabolism and is the most likely cause for space osteoporosis. During a 4.5 to 6 month stay in space most of the astronauts develop a reduction in bone mineral density in spine, femoral neck, trochanter, and pelvis of 1%-1.6% measured by Dual Energy X-ray Absorption (DEXA). Dependent on the mission length and the individual turnover rates of the astronauts it can even reach individual losses of up to 14% in the femoral neck. Osteoporosis itself is defined as the deterioration of bone tissue leading to enhanced bone fragility and to a consequent increase in fracture risk. Thinking of long-term missions to Mars or interplanetary missions for years, space osteoporosis is one of the major concerns for manned spaceflight. However, decrease in bone density can be initiated differently. It either can be caused by increases in bone formation and bone resorption resulting in a net bone loss, as obtained in fast looser postmenopausal osteoporosis. On the other hand decrease in bone formation and increase in bone resorption also leads to bone losses as obtained in slow looser postmenopausal osteoporosis or in anorexia nervosa patients. Biomarkers of bone turnover measured during several missions indicated that the pattern of space osteoporosis is very similar to the pattern of Anorexia Nervosa patients or slow looser postmenopausal osteoporosis. However, beside unloading, other risk factors for space osteoporosis exist such as stress, nutrition, fluid shifts, dehydration and bone perfusion. Especially nutritional factors may contribute considerably to the development of osteoporosis. From earthbound studies it is known that calcium supplementation in women and men can prevent bone loss of 1% bone per year. Based on these results the calcium intake was studied during several European missions and performed an experiment during the German MIR 97 mission where researchers investigated the effects of high calcium intake (>1000 mg/d) and vitamin D supplementation (650 IU/d) on the calcium and bone metabolism during 21 days in microgravity. In the MIR 97 mission high calcium intake and vitamin D supplementation led to high ionized calcium levels and a marked decrease in calcitriol levels together with decreased bone formation and increased bone resorption markers. Tghe conclusion from the MIR 97 mission is that an adequate calcium intake and vitamin D supplementation during space missions is mandatory but, in contrast to terrestrial conditions, does not efficiently counteract the development of space osteoporosis^ref.
Astronauts may soon have another weapon in the fight against the muscle-wasting effects of living in space. And it's a surprisingly low-tech one: a cycle-powered centrifuge that creates its own 'gravity'. The contraption, called the Space Cycle, spins to create a force that mimics the pull of gravity. The device consists of a central spindle with a pair of attached harnesses, one of which has pedals that drive the machine's rotation. As the cyclist pedals furiously, the centrifuge's spin throws the cages out and produces a force on the 2 occupants. The person sitting across from the cyclist can then perform exercises such as squats while 'weighted down' by the force of the rotation. The invention could be a simple solution to the problem of maintaining an astronaut's muscle bulk during long periods in space, such as stints on the International Space Station or a mission to Mars. Astronauts currently undergo a rigorous fitness regime when in space, but it is still difficult to mimic the effects of gravity, and long-term space passengers face losing up to 25% of their body muscle. Astronauts risk losing muscle mass and function because their muscles are not bearing enough weight. It is important to find ways to increase load-bearing activity so astronauts can maintain strength. The researchers have also been using the device to investigate the effects of different gravitational forces on muscle development. Participants undertake exercise at various levels of centrifugal force, after which their rates of muscle growth are evaluated. The study is part of a wider programme on space health run by the US National Space Biomedical Research Institute, in Houston, Texas. The European Space Agency's life-science unit, based in Noordwijk, the Netherlands, is developing a similar centrifuge to test in 2006. More ground testing is needed to prove that centrifugation really does help bones and muscle, although its benefits for the circulation system are more clear. Other teams are looking into devices that lower the pressure around an astronaut's lower extremities, helping to suck fluid into the lower limbs and giving the heart much needed exercise in pumping blood around the body. Both kinds of device could potentially benefit the body even if the astronaut doesn't exercise inside. What's important is to trigger the body's systems with short pulses of 'hypergravity'. But for astronauts who are feeling energetic, the Space Cycle can be fitted with a range of exercise gizmos, including a treadmill or even another cycle, so that two spacefarers can pedal their way to fitness together. Actually installing such devices on the International Space Station will require much further study and consideration. The presence of a spinning wheel inside a spacecraft can establish a small torque, which could potentially shunt it off course or put strain on the joints holding it together. Transporting and installing the Space Cycle is also such a complicated job that it may only be considered for future craft^ref.
Web resources : ESA's Bone Loss study
• deconditioning : a change in cardiovascular function after prolonged periods of weightlessness, probably related to a shift of a quantity of blood from the lower limbs to the thorax, resulting in reflex diuresis and a reduction of blood volume.
• hibernation would help astronauts to cope with the psychological demands of decades-long return journeys to destinations such as Saturn. And because less space and food would be needed on such missions, the spacecraft would be lighter and easier to launch. An injection of D-Ala,D-Leu-enkephalin (DADLE), a substance with opium-like properties, is known to trigger hibernation in ground squirrels during the summer season, when the animals would normally be awake. It also seems to send cultures of human cells to sleep: the cells divide more slowly and their gene activity drops when the molecule is applied. One downside of hibernation is that it leads to loss of muscle strength, a problem that also afflicts patients confined to bed after an operation : such bedridden patients retain more strength if they receive dobutamine, a drug used to boost the strength of heart muscles^ref, so a similar treatment might work during hibernation. The Madagascan fat-tailed dwarf lemur (Cheirogaleus medius) was revealed in 2004 as the first primate known to hibernate^ref.
• See also astronomy and astrophysics.

astrobiology :

Web resources : The Astrobiology Web

• National Space Biomedical Research Institute (NSBRI)
• Space Medicine
• Space Physiology
• Musculoskeletal Physiology
• Gravitational Biology
• Advanced Concepts Team at ESA

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