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COUNTERMEASURES TO LONG-DURATION SPACEFLIGHT
Tamarack R. Czarnik, MD
Resident, Aerospace Medicine Wright State University
Over the course of 38 years of progressively longer spaceflights, numerous adjustments to the operations of the human body have been documented. While most (if not all) of these seem to be functional adaptations to the new environment, many of these operational acclimatizations can and do cause problems, both in-flight and on return to Earth. The most important (by prevalence and impact) of these are Space Motion Sickness, Sleep Disturbance, Fluid Redistribution, Loss of Bone Mineral Density, Muscle Atrophy, Cardiac Deconditioning, Vestibular Disorientation and Psychological Asthenia.
This paper reviews, in chronological fashion, the countermeasures attempted to date for each of these adaptations, examines the success of each, and briefly explores future measures being tested.
Before Man first ventured into space, dire predictions were made concerning the inability of the human body to withstand the stresses of spaceflight. The 1958 National Research Council Committee on Bioastronautics made ominous and contradictory predictions: increased or decreased sleepiness, diuresis or urinary retention, dangerously high or low blood pressure, euphoria or depression, inability to swallow, cardiac arrhythmias, motion sickness and muscle atrophy, loss of bowel function, and so on (1). Some of these predictions were later borne out; many were not. But the space race between the US and the Soviet Union left little time for careful research, and problems were instead addressed a priori, as they developed.
The Soviet Vostok program first uncovered Space Motion Sickness (SMS), and the subsequent Voskhod program (including the first EVA) launched a number of biosatellites (the Kosmos series), studying animals responses to spaceflight. Project Mercury flights revealed some orthostatic intolerance, cardiovascular changes and hemoconcentration, while the Gemini program data (while focussing on Mercurys cardiac findings) showed fluid and bone mineral loss, post-flight exercise intolerance and loss of red blood cells, as well as testing the first tether-generated artificial gravity. The Soyuz flights included tests of blood pressure, EKG and vision, and collected blood and urine samples for post-flight analysis, as well as the first exercise program to attempt to counteract muscle weakness and orthostatic intolerance. The Apollo program, while emphasizing prevention of back-contamination (bringing extraterrestrial organisms to Earth) and in-flight illness, added vestibular disturbance, dehydration, and cardiac arrhythmias to the list.
The Space Station era, begun in 1967 as Almaz and launched in 1971 as Salyut, tested numerous countermeasures still in use today, such as resistance-type suits, scheduled resistive and aerobic exercise (introducing treadmills and cycle ergometers), lower-body negative-pressure (LBNP), saline rehydration, thigh-occlusive cuffs and a compressive anti-g suit for reentry. Americas Skylab Program, housing astronauts in relatively huge quarters (built from the third stage of an unused Saturn V booster rocket) for 28, 59 and 84 days, allowed detailed studies of physiologic adaptations. Countermeasures to SMS, muscle atrophy and bone loss, exercise intolerance and cardiovascular deconditioning were tested, with varied success. The Apollo-Soyuz project, slated to study numerous adaptations, lost much of its data when American crewmembers were exposed to toxic nitrogen tetroxide, causing chemical pneumonitis and requiring intensive therapy and hospitalization. For the last 18 years the Space Shuttle program has systematically conducted extensive human and animal testing of all areas of physiologic adaptation and evaluated numerous pharmacologic and activity-based countermeasures.
Since 19 Feb 1986, the Mir space station has been almost continuously manned, collecting extensive data on human adaptation to long-duration spaceflight, despite the collision with the Progress resupply ship and subsequent depressurization and operational loss of the American science module Spektr in 1997. An on-board EKG and automated capillary blood analyzer (the Reflotron) have allowed in-flight analysis of changes (earlier confined to pre- and post-flight analysis). On-board study of these and other data (including blood, urine and fecal samples sent back to Earth for examination) have provided invaluable information on human response to the space environment, including trigeminy (a potentially lethal heart irregularity) on EVA requiring antiarrhythmic medication, near-toxic levels of carbon dioxide, and psychological asthenia (withdrawal and lethargy).
This paper, then, will separate problems related to physiologic adaptation to spaceflight by symptoms, then examine historically the attempts to correct the problems caused, and finally explore the treatment options being tested in the near future. I will not attempt here to document the many in-flight accidents, emergencies and progressing medical conditions which have been dealt with in space; a planned subsequent paper, Spaceflight Emergencies, will cover these. Rather, our focus here is limited to the bodys attempts to appropriately adapt to the space environment, how these adaptations cause problems, and what efforts have been made to prevent these adaptations. Indeed, from this perspective the wisdom of overriding the bodys physiologic adaptations might well be questioned, and this is the central point of the papers final section, Rehabilitation in Space.
Space Motion Sickness
In 1961, Gherman Titov in Vostok-2 became the first spacefarer to report space motion sickness (SMS). No symptoms of SMS were reported during the US Mercury or Gemini programs, possibly due to the much smaller capsule size; early astronauts could manage little more than small hand and head movements. This lends credence to the Sensory-Conflict theory, which states that nausea is caused by mismatch between data from the vestibular organs (which record acceleration and tilt) and the visual system (which records body orientation with respect to the visual scene); basically, if your eyes tell you youre tilting and your vestibular apparatus says youre straight and level, you get sick. But no one truly knows what causes SMS; another prominent theory, the Otolith Tilt-Translation Reinterpretation (OTTR) Hypothesis, appears to have been disproven by Neurolab (data not yet published).
Roughly 2/3rds to 3/4ths of Shuttle astronauts suffer from SMS to varying degrees in the first 2 days of spaceflight; symptoms of dizziness, sweating, nausea and vomiting resolve spontaneously, usually within 48 hours of entering space, but can be so debilitating that no EVAs or delicate maneuvers are planned during this period. Occurrence in space is currently impossible to predict from the ground: neither the early Coriolis Sickness Susceptibility Index, ocular torsion test nor provocation tests (as with maneuvers on high-performance fighter aircraft) have successfully predicted who will get sick and who will not.
Early Soviet attempts to adapt astronauts to visuo-vestibular mismatch (called the Coriolis effect) involved exposing subjects to hours of nausea-provoking rotation in a rotating chair (the Barany chair); although largely unsuccessful, these attempts have persisted to present-day Mir cosmonauts. The DOME (Device for Orientation and Motion Environments) trainer in the US uses virtual reality and a rotating platform to negate gravitys contribution to spatial orientation, thus training sensory-motor responses appropriate to microgravity. Biofeedback training has been attempted by the US, and selection screening by Russia, neither with great success.
Various devices have been used to try to prevent SMS. Load suits (the Russian Penguin suits), using adjustable elastic bands to produce tension over the chest, back abdomen, side and leg seams, were worn by the crews of Soyuz-13 and 14, but were unsuccessful in stopping fluid shift and SMS. Sea-bands (utilizing acupressure points on the wrists to alleviate nausea) have been unsuccessful. The Neck Pneumatic Shock Absorber, using rubber cords to load the cervical vertebrae and neck muscles (thus restraining excessive head movements which provoke SMS), was used in many Soviet flights with some success, but limited head movements too greatly. Weak electrical currents, whether through the frontal lobe of the brain (electroanalgesia) or along standard acupuncture pathways (electroacupuncture) have been tested by Russian researchers.
Pharmacologic treatments of SMS have focussed on anticholinergics (e.g. scopolamine), antihistamines (e.g. meclizine, promethazine), sympathomimetics (e.g. amphetamine), sympatholytics (e.g. chlorpromazine) and drug combinations (e.g. scopolamine and amphetamine). Transdermal scopolamine patches were used to combat SMS in early flights, but was found to delay adaptation to microgravity: once the astronaut stopped taking it, he/she would once again get nauseous. Scope-dex (scopolamine and amphetamine) has been used, with some success. The Shuttle Orbiter Medical kit carries Phenergan (in oral, rectal and intramuscular injectable forms), Scolopamine and Dexedrine for SMS; a 1995 list of medications flown on the Mir space station does not include drugs specifically for SMS, but includes methyl valerate for sympathicotonic disturbances.
A breakthrough of sorts was reached in 1990, when physician astronaut Bagian treated SMS with a 50-mg injection of promethazine (Phenergan), the first intramuscular injection in space. IM promethazine has been found to successfully treat most SMS; it does not delay adaptation to microgravity, and there is some evidence that side effects are lower in-flight than in ground-based studies.
Thus, despite an inability to say what causes it or to predict who will get it, an effective treatment for SMS appears to have been found.
The next most common reason for medications taken in-flight is sleep disturbance. Desynchronosis (disruption of the bodys normal circadian rhythms) in space missions has been associated with disrupted sleep or work schedules: completing an orbit every 90 minutes cycles astronauts light-dark phases much faster than on Earth. Artificial lighting in the flight deck is low, doing little to improve the situation.
During early missions (through Apollo 9), members sleep schedules were staggered (so at least one member would always be awake), resulting in shifting of 6 to 10 hours from Earth-based sleep periods. Soviet scientists took note of this problem first, synchronizing crew schedules with ground control, keying the sleep-wake-work cycle to normal Moscow time during the Salyut program. Nowadays, Shuttle crews start shifting their sleep cycles several days before the launch, using bright lights and scheduled meal and exercise periods.
But sleep disturbance is still a common complaint in-flight. Whether or not the space environment intrinsically degrades sleep architecture was studied on Skylab, using EEGs, EOGs (electro-oculograms, recording eye movements) and motion detectors; they found no adverse effects on sleep in-flight, but sleeping medications were still often needed. More recently, this question was studied on Neurolab, but this time adding light levels to the study. While once again no degradation in sleep architecture was noted, it was found that lights were turned on exactly at the scheduled morning time, but stayed on 30 to 45 minutes past scheduled lights out (as astronauts strove to complete their assigned tasks). When astronauts were covered in monitors, probes, and wires, they slept longer and better than normal; when sleep was made a mission objective, sleep improved. Thus, space does not itself degrade sleep; rather, the intense work schedule on orbit forces sleep disturbances. Melatonin was not found to improve sleep when given at scheduled bedtime, but might still provide a necessary cue when shifting sleep cycles. Today, the Shuttle Orbiter Medical kit carries Dalmane and Restoril as sleep aids.
As discussed in Aerospace Medicine 101: Adaptations to Spaceflight, microgravity negates the bodys hydrostatic column: gravity no longer pools fluids in the lower extremities, and both intracellular and extracellular fluids rapidly redistribute evenly, shifting to the upper body and head (similar to hanging upside-down on a gym bar). About 1 liter of fluid is lost from each leg; this results in what astronauts call bird legs. Excess fluid in the head results in puffy faces, nasal congestion and may be involved in blunting crewmembers taste sensitivity (crew typically complain of dulling of taste, and hot spices are sent with every mission). More seriously, this excess fluid is rapidly removed by the kidneys, resulting in a 3% loss of total body water. This rapid diuresis causes a dehydration, appropriate to a microgravity environment but dangerously low at 1.0 g; on return to Earth, this dehydration contributes to SMS, orthostatic intolerance and dizziness (which could be lethal in an emergency), and the concomitant loss of potassium can contribute to cardiac arrhythmias.
Cuffs on the arms and/or thighs, tight enough to occlude venous return and thus preserve peripheral fluid, have been tried by Russian and American programs, with some success. Various Lower-Body Negative-Pressure (LBNP) devices (which use a vacuum around the lower body to keep fluids in the legs) have also shown some success, but are bulky and only temporary measures.
Both the American and Russian space programs currently use a fluid-loading regimen prior to re-entry: crewmembers drink 32 ounces of water or juice and 8 salt tablets (for fluid retention) about 1 hour before re-entry, producing 1 liter of isotonic saline in the stomach. This has the effect of lowering heart rate, decreasing cardiovascular deconditioning and alleviating much of the presyncope (faint feeling) returning astronauts typically have. But so much salt is hard on the stomach, and some astronauts vomit the solution back up. Research is being considered on a glycerin-based rehydration fluid.
This concludes part 1 of "Countermeasures to Long-Duration Spaceflight". Part 2 will cover countermeasures to Loss of Bone Mineral Density, Muscle Atrophy, Cardiac Deconditioning, Vestibular Disorientation and Psychological Asthenia, and discuss the appropriateness of countermeasures versus a Rehabilitation paradigm.
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