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Human health during space travel: An overview
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.259123
Keywords: Health in outer space, space medicine, space travel
To a country where just 4 years ago, there was not a single neurologist or neurosurgeon living in areas where the majority of Indians lived,[1] a review of human health in outer space may seem incongruous. Physicians however should not live in silos confined to their narrow sub-specialties. Rudyard Kipling once remarked, “What do they know of England who only England know”. Awareness of healthcare issues in space broadens one's outlook and draws attention to taking a fresh look at health problems on earth. In 31 months from now.[2] India will hopefully become the 4th nation in the world to send humans into space. It is not impossible that during the lifetime of the younger readers of this journal, space travel from India will be a reality. A future-looking emerging economy like India should no longer consider space medicine as being esoteric. Understanding health issues in space will eventually result in better healthcare on earth, as proven by multiple prior NASA technologies.[3] Space travellers need liveable environments and protection from the exacting conditions of microgravity and radiation in space. The degree of risk to health and the mitigation needed will depend upon the distance and duration of travel into space.[4] Human health and welfare depend on knowledge gained from appropriate ground and space-based research. In the past 50 years, much has been learnt about human physiology and medical care in Low Earth Orbit (LEO) and on short lunar missions. During the US Apollo space missions, immediate hazards on relatively short voyages to the Moon - microgravity and radiation [5] – were understood 'well enough' (though not necessarily fully mitigated) to enable successful journeys. This was achieved using state-of-the-art technologies of the day focusing on spacecraft design, life-support systems, control systems and communication devices. Protective measures included utilizing appropriate shielding while crossing through Earth's radiation belts. Basic understanding of human physiologic responses to and countermeasures against the intense and varying levels of forces, such as during high acceleration on ascent, microgravity in space and partial gravity (1/6 G) on the Moon, the fiery deceleration on descent through Earth's atmosphere, and finally readjustment back to Earth's gravity, helped to exquisitely manage major risks during lunar missions. There is no evidence that the environmental hazards encountered caused the subsequent death of any astronaut. Much was already known or learnt about the risks of space travel by early aerospace pioneers. Decompression, which has been understood since the early days, can sometimes still be fatal, as occurred with the death of 3 cosmonauts on USSR's Soyuz 11, in 1971, during re-entry to Earth. A partial decompression also occurred in April 2017 on the return of Soyuz MS-02. Thus, decompression remains a concern not only during launch and landing, but also during extra-vehicular activities (EVA) in spite of smart engineering. However, the risks associated with today's ambitions for prolonged and distant space travel are far more complex. The interested reader is referred to a recent NASA self-audit on preparedness, which categorized environment-based hazards (particularly radiation and micro-gravity) that pose risks to human health and performance, specific to location in space and distance from Earth.[6] Einstein's relativity theory predicts that astronauts, travelling at a very high speed in space, would age less chronologically than their counterparts on Earth. Scott Kelly, who travelled for a year on the International Space Station (ISS) at about 17,000 mph, aged about 8.6 milliseconds (just less than a hundredth of a second) less than his Earth-bound brother, Mark, and the rest of us.[7] Medically speaking, this amount of temporal slowing of aging is clearly of no advantage to an astronaut's well-being. Importantly, during prolonged exposure to the environment of space, the 'net' chronic effect on human physiology from all identified health hazards, including radiation and microgravity, is pro-inflammatory and pro-senescence, in short – accelerated biological deterioration and ageing relative to those bound to Earth![8] Upon his recent return to Earth, Scott Kelly remarked, “Fruits and vegetables seem to rot much faster here [in space] than on Earth. I am not sure why, and seeing the process makes me worry that the same thing is happening to my own cells”.[8]
Nearly everything understood about life today is in the context of its continual development and existence at Earth's gravity (1G). Earth's gravity field decreases inversely with the square of the distance from it, so its exertion is only about 10% less in the Low Earth Orbit (LEO), where the ISS orbits.[9] However, since the spacecraft and the astronauts within, are in freefall towards Earth, they experience a sensation of weightlessness—a state termed 'microgravity'—in which everything is falling towards Earth together. This freefall microgravity changes several factors that affect human physiology. In microgravity, phenomena such as buoyancy, sedimentation and convection are absent. Conversely, properties of liquids like cohesion, adhesion and surface tension play a more important role in microgravity. Since gravity-dependent fluid properties and phenomena are severely altered or otherwise absent, flow and distribution of complex physiological fluids such as blood and other biological fluids - such as interstitial, lymphatic, and cerebrospinal - are also influenced.[10] By interfering with body functions, both mechanical and ultimately metabolic that depend on Earth's gravity, microgravity poses a health hazard.
In microgravity, the heart and blood vessels are relieved of hydrostatic forces exerted on them by blood. The heart becomes rounded, potentially changing its pumping action. Blood is no longer pulled towards the feet. Blood pumped by the heart “floats” forward and remains in the head and upper torso. This causes a redistribution of blood and interstitial fluid towards the upper torso altering the physical shape of the astronaut's body by causing a puffy face, engorged neck veins, an enlarged thorax and reduced leg volume ('puffy face bird leg' syndrome). Physiologic sensors in the upper torso interpret this increase as “too much blood volume” in the body. The body's response is to correct this erroneous perception by decreasing the volume of blood pumped (and work done) by the heart, increasing fluid loss through the kidney and slowing the rate of red blood cell production.[11] In the absence of gravitational forces, heart muscles and blood vessels will atrophy relatively quickly. As less work is required for the heart in microgravity, cardiac myocyte atrophy could be substantial. In space, the compensatory drop in red blood cell production may not cause anemia. However, when a person returns to planetary gravity, the fluid volume within the blood vessels increases due to rehydration and a relative anemia (a drop in red bloods cell per unit volume) would occur until the marrow begins to produce enough red blood cells to maintain a normal oxygen blood carrying capacity. Another potential concern on returning to gravity, as blood pools back into the legs, is orthostatic intolerance, manifested as light-headedness caused by lowering of blood supply to the brain. Mitigations include pre- and post-landing re-hydration with normal saline, assuming a recumbent posture during re-entry and landing, and wearing a 'G-suit' to prevent blood pooling in the legs, enhancing blood return to the heart. The cardiovascular fluid volume recovers within 24 hours.[11],[12]
The intense gravitational forces exerted on muscles and bones on Earth do not exist in microgravity. This causes significant muscle and bone atrophy.[13] In microgravity, without adequate exercise, bones atrophy at 1-1.5% per month, 10 times faster than on Earth. Ensuring supplementary diet and exercise in space is difficult. Muscle atrophy occurs at 1% weekly, 50 times faster than after age 55 on Earth. Musculoskeletal atrophy continues until a new steady state work level occurs. Bone atrophy during space travel could lead to increased kidney stone formation and bone fractures. Spaceflight hardens the intervertebral discs leading to disc disease and pain.[14]
Space motion sickness occurs in 75% of those exposed to microgravity, probably due to lack of stimulation of the ear's hair cells during linear or angular momentum changes.[15] In general, 'motion sickness syndrome' lasts up to 72 hours initially in space and for an hour or so on return to Earth. Fluid redistribution, causes extra-cranial soft tissue edema that dulls the senses of smell, taste and possibly hearing.[16] Proprioception, monosynaptic and other muscle reflexes, have also been shown to diminish. Loss of proprioception and vestibular system dysfunction requires conscious re-orientation to exert appropriate effort needed for movement. Positioning and orientation of one's arms or legs in space is only through vision.[16] Furthermore, the presence in microgravity induces brain changes [17],[18] that may be mitigated by supine exercises.[19] Fine touch using fingers (which have the highest density of nerve endings), even without visual cues, is not disturbed. There is a risk of urinary retention, as the usual neuromuscular signals for voluntary urination are suboptimal in microgravity due to absence of the hydrostatic pull of Earth's gravity on a full bladder.
During prolonged missions, biomarkers, such as cytokines and other pro-inflammatory molecules, and increased reactivation of latent viruses (e.g., chickenpox and herpes) have been documented.[11],[20] On short missions, symptomatic infections have not been a problem though rashes have occasionally been documented. Following early experience by NASA, a pre-flight astronaut quarantine program was put in place after it was realized that pre-flight upper respiratory infections (URI) were picked up by the astronauts from their own young children. Nevertheless, microgravity and space radiation can adversely affect the human immune system, its microbiome and even wound healing. Additionally, increased expression of carcinogenic genes, disturbance of host-microorganism interactions and alteration of pharmacokinetics may occur.
Following the initial adaptation, the body eventually achieves homeostasis, for the new space conditions. Re-adaptation, leading to full recovery, occurs on return to Earth. The 'new normal' may sometimes lead to pathological changes during prolonged missions, as in the case of SANS (spaceflight associated neuro-ocular syndrome) and VIIP (vision impairment and intracranial pressure).[21] Some structural changes such as eyeball buckling, bone loss and cardiac architecture may not be reversible.
Radiation exposure is a major hazard by itself, but it may also compound the ill effects of microgravity. The actual radiation dose depends on its source, type, intensity and duration. Ionizing radiation carries enough energy to knock out electrons from atoms or molecules in its path.[4],[22] Both ionizing and non-ionizing (e.g., ultraviolet rays from sunlight) radiation are well-known hazards to living organisms. Ionizing radiation from space (primarily high-energy solar particles) is trapped within Earth's radiation belts (van Allen belts), and by the fragile envelope of Earth's atmosphere. However, outside the Earth's protection, radiation consists of solar energetic particles (SEP; arising from the Sun), and galactic cosmic radiation (GCR; from deep space). The SEP radiation is made up of energetic protons, electrons, X-rays, gamma rays, alpha particles (including far ultraviolet light), and other decaying particles.[4] GCR is composed of high-energy protons and heavy atomic nuclei. The high-energy, high-charge particle radiation in space can cause damage to both replicating and non-replicating cells (e.g., nerve cells which generally duplicate less frequently). Ionizing radiation can cause damage to both the cellular structure and to its deoxyribonucleic acid (DNA).[22] The health effects of acute radiation exposure are reasonably well understood. Chronic radiation exposure may cause cataracts, cancer, cardiovascular problems such as atherosclerosis, and neuro-degenerative disorders such as dementia.[23] In the US, the average annual human radiation exposure from all radiation sources (solar, cosmic ray and medical radiation) is about 7 milli-Sieverts (mSv). The astronauts on the ISS, orbiting at 200 to 275 miles from Earth—well below the Van Allen Belts—receive about 80 mSv over a 6-month period.[24] Astronauts cannot avoid cumulative radiation exposure during their spaceflight. 15 US astronauts, including 2 of 24 Apollo astronauts and 3 of the first 7 astronauts, have succumbed to cancer. However, it is impossible to prove that spaceflight radiation was the causative factor.
As extra-terrestrial space is a hostile environment, the spacecraft and equipment must protect the astronauts within, during extra-vehicular activities (EVA), and from common and uncommon trauma. The cloistered internal space within the vehicle is a potentially hostile environment, both physically and psychologically.[25] The International Space Station, supporting 6 astronauts, has a habitable volume of 15,000 cubic feet (two Boeing 747 airliners), far more voluminous than early spacecrafts or those being currently contemplated for deep space exploration. Long duration missions result in prolonged isolation from family, friends, and daily activities of living on Earth. This could lead to anxiety and depression, alter interpersonal dynamics and cause behavioral changes, leading to inadequate team performance and mission failure.[26],[27] Isolation is a factor that may alter the immune system, decreasing longevity and increasing the possibility of transmission of infections among the closely clustered crew. Spacecraft must be engineered to compensate for and optimize both human-human and human-system interaction.[28] During long duration missions, a mission-threatening risk from unknown infections is possible. Prions are the normally present cellular proteins abundant in the brain. They undergo an abnormal folding, for unknown reasons, and become proteinacious infectious particles that replicate and are transmissible. Prions are not life forms but brain proteins that are normally present but go haywire and cause fatal infectious encephalopathies. If similar new unanticipated biological risks are encountered in space, an environment that is known to suppress immune responses, and increase the virulence of certain bacteria (e.g., salmonella, pseudomonas aeruginosa) and fungi (candida albicans) and reactivate dormant viruses, is created. Thus, astronauts will face potentially fatal risks endangering the mission. The human microbiome consists of billions of various, rapidly multiplying, microorganisms that live symbiotically on several of the body's barrier sites. They have a profound beneficial influence on our health. The microbiota is vulnerable to radiation and microgravity in space. A disturbance in this normal symbiosis could turn these protectors themselves against the body, increasing susceptibility to infections, metabolic disorders, and cancer.[29],[30] An astronaut's immune system suffers additional deterioration due to work overload, and in the low-earth orbit, from circadian de-synchronization and sleep loss.[31],[32] A spacecraft orbiting Earth, experiences sunrises and sunset roughly every 90 minutes. This frequent variation in zeitgeibers—cues from the external world, such as light or temperature, that synchronize our internal clocks—has the potential to disturb sleep patterns, duration, and quality. NASA has identified several hazards that could affect human health in space due to one's distance from Earth. Distance adds substantially to any risk, but especially during emergencies. Astronauts are selected for their optimal health and stamina. Nevertheless, acute illnesses requiring intense medical care may still occur. Such emergencies might include minor and major accidents, burns, injuries, trauma, heart attacks, strokes, emboli and infections. Acute emergencies requiring immediate care include obstructing kidney stones, cholecystitis and appendicitis. Some may be treated by medication alone, but others may need surgical interventions. Thus, preventive measures, including diligent apriori selection and pre-flight preventive regimens for maintaining a healthy crew, are critical to reducing risk to the mission. During the mission, new healthcare information can be uploaded in real time onto onboard computers. Each traveler's medical record could be stored within a digital 'super-doc' capable of diagnosis and therapeutic triage. Unobtrusive onboard remote sensing devices that can continuously monitor vital signs, blood cell levels, blood biochemistry and behavioral changes could send alerts to rest of the crew, ground personnel, and the onboard 'super-doc', when deviations from baseline health are detected. Diagnostic information should ideally be followed by appropriate intervention, which could even be administration of available medications. However, onboard medications will undergo chemical deterioration and lose potency when exposed to prolonged space radiation. So, chemical synthesizers or even organisms, such as genetically modified bacteria or fungi, could be used to tailor make drugs on a small scale.[33] However, if unprotected, these organisms themselves will be susceptible to the ravages of microgravity and radiation. Remote physical examination, laboratory tests and medical imaging may be necessary. Modified POCD (Point of Care Diagnostics) and hand held ultrasound scanners could be useful for diagnosing calculi in the kidneys or gallbladder and even abscesses. Ultrasound could be diagnostic and therapeutic. For example, although symptomatic renal stones have not been reported during spaceflight, there have been 14 reported cases of renal stones in astronauts after their return to Earth.[34] If these should occur during a mission, oral and parenteral hydration and medication may be sufficient. However, should obstruction of urine outflow occur, high frequency ultrasound energy may be useful for dislodging or disrupting smaller stones. Ultrasound image-guided percutaneous catheter drainage using minimally invasive draining techniques, modified for microgravity, could eventually be employed.[35] At present, general anesthesia administration or performing conventional surgical procedures in microgravity is not possible. Providing intavenous (IV) sedation and analgesia will require modified airtight techniques and administration of degasified liquid medications. Administering basic life support (BLS) in microgravity is available. However, a robust ventilator—other than a resuscitative breathing bag—is not available. Experimental trauma support procedures (such as endotracheal intubation, percutaneous tracheostomy, chest tube insertion, gastric tube placement, urinary bladder, and vascular access) have only been artificially simulated to date. Minor surgical procedures, such as vessel and wound closures, have only been performed during research studies in space on small animals.[36] On Earth, telediagnostics [37] and telesurgery [38] is possible. Theoretically, such remote surgery is also possible, with an acceptable brief time lag, if the patient is in a low earth orbit. These technologies need to be further developed for use in microgravity. Prophylactic removal of gallbladders and appendices while the crew is still on earth has been suggested. For minor surgical interventions, e.g., incising abscesses, cross-training the crew for simple percutaneous procedures should be possible.[35] Modifications in imaging diagnostic criteria may be needed as even a simple pathognomonic earth-based imaging sign for an abscess – the air-fluid level (air rises to the top and fluid rests on the bottom of a cavity) depends on gravity and cannot be expected to occur in space. Lightweight medical robots could enhance medical skills of cross-trained astronauts. In the future, earth-bound computers, equipped with artificial Intelligence and deep learning capabilities, could possibly predict which individuals would be physiologically best suited for space travel.
All awe-inspiring successes in space have been through high-risk endeavors. Explorers on earth braved transoceanic journeys to unchartered destinations with far less information than is available today. Human history is full of risk taking and setbacks. The latter has not deterred humankind. The challenges that humans face today are global and will require international cooperation, especially intellectual and financial, for their resolution. The International Space Station (ISS) successfully exemplifies the benefits of trans-national collaboration. Future collaborations should include emerging space faring nations such as China and India. Ultimately, the gains to humanity from such collaborations will be far greater than just safe space travel. Betterment of human life on Earth, and enrichment of human development, will be a by-product. For example, NASA placed men on the moon and also designed the space shuttle. Decades later, the shuttle's large main-engine pump was downsized into a four-ounce cardiac assist device for patients in heart failure awaiting heart transplants. Cross-fertilization of space and Earth-based research has resulted in many benefits. In attempting to solve challenges to human survival and longevity in extra-terrestrial environments, seemingly unrelated “out of the box” scientific questions, which we might not otherwise have pursued, could advance knowledge, in human health and disease.[39] In space, experiments can be performed in conditions not available on Earth—microgravity, near-perfect vacuum, near-absolute zero temperature, low to non-existent atmospheric pressures, and varying types and intensities of radiation. The sky is no longer the limit! Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.
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