Development of countermeasures for medical problems encountered in space flight
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Adv. Space Res. Vol. 12, No. 1, pp. (1)329-(1)337, 1992 0273-1177/92 $15.00 Printed in Great Britain. 1991 COSPAR DEVELOPMENT OF COUNTERMEASURES FOR MEDICAL PROBLEMS ENCOUNTERED IN SPACE FLIGHT Arnauld E. Nicogossian,* John D. Rummel,* Lauren Leveton** and Ron Teeter** * Life Sciences Division, NASA Headquarters, Washington, DC 20546, U.S.A. ** Life Sciences Program Office, Lockheed Corporation, Washington, DC 20024, U.S.A. ABSTRACT By the turn of this century, long-duration space missions, either in low Earth orbit or for got early planetary missions, will become commonplace. From the physiological standpoint, exposure to the weightless environment results in changes in body function, some of which are adaptive in nature and some of which can be life threatening. Important issues such as environmental health, radiation protection, physical deconditioning, and bone and muscle loss are of concern to life scientists and mission designers. Physical conditioning techniques such as exercise are not sufficient to protect future space travellers. A review of past experience with piloted missions has shown that gradual breakdown in bone and muscle tissue, together with fluid losses, despite a vigorous exercise regimen can ultimately lead to increased evidence of renal stones, musculoskeletal injuries, and bone fractures. Biological effects of radiation can, over long periods of time increase the risk of cancer development. Today, a vigorous program of study on the means to provide a complex exercise regimen to the antigravity muscles and skeleton is under study. Additional evaluation of artificial gravity as a mechanism to counteract bone and muscle deconditioning and cardiovascular asthenia is under study. New radiation methods are being developed. This paper will deal with the results of these studies. INTRODUCTION The continued definition, development, validation and fine-tuning of countermeasures to the medical problems associated with long-duration space flight is critical as NASA advanced planners, engineers, scientists, and physicians begin planning the human exploration missions. Sometime near the turn of the century crews will live and work on Space Station Freedom for 6 months to a year, and possible longer. Early in the next century the first crews will establish a permanent Lunar Outpost, and some time beyond that we will begin to undertake a 2- to 3-year voyage to Mars. Taken, together, these exploration missions present a set of challenges to maintain the health and fitness of the crews in several reduced gravity environments. In addition, crews must be able to return to normal life on Earth with a minimum rehabilitation period after prolonged exposure to space. The United States has had extremely limited experience with long duration missions. Crews have flown for up to 3 months on Skylab, although the sample is small (N=3) and no females have been flown for a comparable duration. Even in short-duration U.S. missions there have been several serious incidents of medical and life-support problems, including toxic contamination, bends, cardiac arrhythmias, and environmental control problems. Space Station Freedom will involve crews that are larger (i.e., initially crews of four followed by crews of eight) and more heterogeneous (i.e. males and famclao Go wall ~o rflffarant notnnolil4ao\ In oAeUthmn i-ha inn.smantollt, inrraadna ,1i.rotinno few rra~n,
Transcript of Development of countermeasures for medical problems encountered in space flight
Adv.SpaceRes. Vol. 12, No. 1,pp. (1)329-(1)337,1992 0273-1177/92$15.00Printedin GreatBritain. 1991 COSPAR
DEVELOPMENTOFCOUNTERMEASURESFORMEDICAL PROBLEMSENCOUNTEREDIN SPACEFLIGHT
ArnauldE. Nicogossian,*JohnD. Rummel,*LaurenLeveton** andRonTeeter**
* Life SciencesDivision, NASAHeadquarters, Washington,DC 20546, U.S.A.
** Life SciencesProgram Office,LockheedCorporation, Washington,DC 20024,U.S.A.
ABSTRACT
By theturn of this century,long-durationspacemissions,either in low Earth orbit or for got earlyplanetarymissions,will becomecommonplace.From the physiological standpoint,exposureto theweightlessenvironmentresultsin changesin body function, someof which are adaptivein natureandsomeof which canbelife threatening.Importantissuessuchasenvironmentalhealth,radiationprotection,physicaldeconditioning,andboneandmusclelossare of concernto life scientistsand missiondesigners. Physical conditioning techniquessuch as exercise arenotsufficient to protectfuture spacetravellers. A reviewof pastexperiencewith piloted missionshasshownthat gradualbreakdownin bone andmuscletissue, togetherwith fluid losses,despitea vigorousexerciseregimencan ultimatelyleadto increasedevidenceof renalstones,musculoskeletalinjuries, andbonefractures. Biologicaleffectsof radiationcan, over long periodsof time increasetherisk of cancerdevelopment.Today, a vigorous programof study on themeans to provide a complex exerciseregimen to theantigravity musclesand skeletonis under study. Additionalevaluation of artificial gravity as a mechanismto counteractboneand muscledeconditioning and cardiovascularastheniais under study. New radiationmethodsare being developed.This paperwill dealwith the resultsof thesestudies.
INTRODUCTION
The continueddefinition, development,validation andfine-tuningof countermeasuresto the medicalproblemsassociatedwith long-durationspaceflight is critical asNASA advancedplanners,engineers,scientists,andphysiciansbeginplanningthehumanexplorationmissions.Sometimeneartheturn ofthecenturycrewswill live andwork on SpaceStationFreedomfor 6 monthsto a year,andpossiblelonger.Early in the next centurythe first crews will establisha permanentLunar Outpost, andsome timebeyond that we will begin to undertakea 2- to 3-year voyageto Mars. Taken, together,theseexplorationmissionspresenta setof challengesto maintainthehealthandfitnessof thecrewsin severalreducedgravity environments.In addition,crews mustbe ableto returnto normal life on Earthwith aminimumrehabilitationperiodafterprolongedexposureto space.
The United Stateshashad extremelylimited experiencewith long durationmissions. Crewshaveflown for up to 3 monthson Skylab, although the sampleis small (N=3) andno femaleshavebeenflown for a comparableduration. Even in short-durationU.S. missionstherehavebeenseveralseriousincidents of medical and life-support problems, including toxic contamination, bends, cardiacarrhythmias,andenvironmentalcontrol problems. SpaceStationFreedomwill involve crewsthat arelarger (i.e., initially crews of four followedby crewsof eight) andmoreheterogeneous(i.e. malesandfamclao Go wall ~o rflffarant notnnolil4ao\ In oAeUthmn i-ha inn.smantollt, inrraadna ,1i.rotinno few rra~n,
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HISTORICALPERSPECTIVE
During the early 1960’s, inpreparationfor the first humanmissionsin theMercury andGemini series,NASA life sciencesactivities included developingcriteria for astronautselection and biomedicalresearchassessingcrew performanceduring weightlessness,and defining human life supportrequirements.All six oftheMercuryastronautsreturnedtoEarthin satisfactoryconditionaftertheshortdurationmissionsof severalhoursto threedays,thusminimizing manyoftheoriginal medicalconcerns.Still of primaryconcern,however,weretheproblemsof weightloss dueto dehydrationandimpairmentof cardiovascularfunction. Postflight, orthostatic intoleranceand neurovestibularproblemswereobserveduponstandingIll.
Investigationsconductedonthe Geminiaddressedthelimits of astronautendurancefor longerflights.Studieswere undertakenon Gemini to further assessthe cardiovascularchangesobservedduring theMercurymissions. Thesechangeswereviewedas adaptiveresponsesto conditionsof weightlessness;the key questioninvolvedthe degreeto whichthe observedcardiovasculardeconditioningwas aself-limiting response.The “doubling philosophy”regardingmedicalcertificationfor crewstaytimeswasreinforcedin the Gemini Programfor applicationin the forthcomingApollo missions. New evidencewasprovidedfor severalbiomedicalproblems,including loss of bone mineral, redblood cells (5-20percent),postflightorthostaticintolerance,loss of exercisecapacity,andbonedensitychanges.
The broad biomedical effort supporting the Apollo missions in the late 1960’s documentedcardiovasculardeconditioning,aswell as vestibularproblemsassociatedwith “spacemotionsickness.”Thebiomedicalprogramalsoprovidedtheopportunityto developnewtechniquesfor assessingsystemicchangesover the 14-dayApollo missions. Studiesyielded information on the cardiovascularsystemresponses,metabolic functioning, microbial loads, and radiobiology effects. The most significantbiomedicalproblemsevidencedin theApollo Programincludedvestibulardisturbances,lossof appetite,postflight dehydrationand weightloss, postifight orthostaticintolerance,reducedpostflight exercisetolerance,cardiacarrhythmias,anddecreasedredbloodcell mass(2-10percent)andplasmavolume (4-9 percent).
Long-term physiologicaladaptationto spacewasinvestigatedduringthe Skylabprogramin theearly1970’s. The threeSkylabmissionslasting 28, 59 and84 days,presentedthefirst sustainedopportunityto study the time courseof physiologicaladaptationto weightlessnessof space. Biomedical studiesincluded spacemotion sickness,cardiovasculardeconditioning,bone mineral loss, and metabolicchanges. Basedon the subjective information from the three Skylab crews and the vestibularexperimentsconductedduring ifight it was concludedthatground-basedtesting for predictionof spacemotion sickness(SMS) is not reliable; however, prophylactic administration of medications foralleviationof SMS waspossible.In theareaof cardiovasculardeconditioning,Skylabalsoprovidedthefirstopportunityto examineorthostaticstressresponsesin flight.
Astronautswere tested(pre-, post- andinflight) with a lower body negative(LBNP) device. It wasfoundthatcardiovasculardeconditioningeffectsstabilizedaftera4- to 6- weekperiodin ifight. Otherbiomedicalfindings includedbonelossin thelower extremitiesandsomemetabolicchanges,includingchangesin urinarycalcium,nitrogenandcalcium. Muscletissueloss was alsorevealedby leg volumemeasurements,with returnto normal within 4-dayspostifightIll.
During the Apollo-Soyuz Test Project,studies showedmuscle dysfunction after 9 days into themission. Short-termmission duration, appearedto be relatedto an increasein muscle fatigue. Inaddition,datacollectedimmediatelypostflightrevealeddecreasesinmusclereflex responses.
The first SpaceShuttle flight waslaunchedon April 12, 1981,andlasted54 hoursand21 minutes.Physiologicalchangescontinueto be studiedon the Shuttle. Headwardfluid shift begins almostimmediatelyupononsetof weightlessness.Crewsexperienceshiftsinbody fluids duringreentry,along
Medical ProblemsEncountered in SpaceFlight (1)331
measurements.Most of thephysiological changespersistfrom afew hoursto severaldayspostflight,returningto preflightbaselineafter3 days. No residualphysiologicaldeconditioningeffectshavebeenassociatedwith theshort-durationShuttleflights.
EXTENDEDDURATION ORBITERMEDICAL PROGRAM
In 1989,the ExtendedDuration Orbiter Medical Program(EDOMP) was initiatedin responseto adirectivefrom theNASA Administratorto graduallyextendthedurationof SpaceShuttlemissions.Theprogram’scurrentgoalis to ensurecrewhealthandsafetyon SpaceShuttlemissionsof up to 16 days.Progresswill be incremental:recommendationsfor operationalcountermeasureswill be madefor 13-dayflights (the first suchmissionis scheduledfor March 1991),andoncethesearevalidatedin flight,biomedicalmonitoringand countermeasuretesting will be directedtowardenablingthe first 16-daymission,scheduledfor April 1994. Specific objectivesare to optimize crewperformanceduring entry,landing,androutine andemergencyegress;andto maintainthe environmentalquality of thespacecraft/2/.
The SpaceShuttle is uniquefrom the spacevehicles usedin previousprogramsin that it must bemanually piloted through the reentryandlanding phases. So far, most shuttlemissionshave had anominal durationof 7 days(STSflew for 10 days). A vigorousprogramis evolving to supportpilotperformanceduring reentryandlanding,andto defineproceduresfor on-orbit maintenanceof pilotingskills. Focusing on these critical mission phases,studies will identify decrementsand developcountermeasuresfor neuromusculardysfunctionandvestibulardisturbances.Assuring thatorthostaticintoleranceandpotential loss of muscle strengthwill not compromisecrewmembers’capability forunaidedegress- routineandcontingency- after longerShuttlemissionsis aprimary focus. Orthostaticintolerance,disequilibriumandperceptualdisturbances,andmuscleatrophyare areasof investigation.Sampling andanalysiswill beperformedin investigatehabitabilityconcerns,which includeair quality,hygiene,toxic contaminants,andmicrobialbuild-up/3/.
The EDOMPis designedas an operationallyorientedprogramto systematicallyinvestigatespecifictargetareas. Predictivemodelswill bedeveloped,wherepractical,to guide risk analysisfor astronautperformanceon longer flights. In the initial stagesof the program, Discipline Working Groupscomposedof expertsfrom NASA anduniversitiesidentified critical areasof operationalconcernandformulatedquestionsto be addressedthroughEDO investigations.Thescientificdisciplinescorrespondto themajor bodysystems,all of whichundergosomedegreeof adaptivechangeduring spaceslight.The disciplines includecardiopulmonary,musculoskeletal,biomedicalphysiology, neuroscience,andregulatory physiology. Discipline Working Groups have also definedquestions in the area ofenvironmentalhealth/2/.
The implementationapproachto theprogrammaybe summarizedin aseriesof steps:developcriticalquestions,establishsuccesscriteria, conductinflight data acquisitionand analysis, validate criteria,validatecountermeasures,and,finally, defineoperationalprocedures.Flight investigationsareprecededby ground-basedstudiesutilizing non-astronautsubjects. Ground-basedinvestigations(includingprolongedbed rest, used to simulate microgravity) will addressexercisephysiology to identify thesignificant parametersassociatedwith predicting andtestingegresscapability. Bed restwill alsobeusedin investigationsin the areaof cardiovascular/fluidandelectrolytephysiology,in whichthemostefficientcombinationof fluid loadingandLBNP countermeasureswill beidentified /2/.
Integrationof EDO investigationswith existingmissionpayloadsis now underway. EDO-relatedSpacelabexperimentswill investigate,amongothers,hypothesesrelatedto cardiovascularadaptationtoreducedgravity, cardiovasculardeconditioning,musclefunction, andthe vestibularsystem. However,due to the dynamic and evolutionary nature of the EDOMP (results will drive subsequentinvestigations), the Detailed SupplementaryObjective (DSO) processwill constitute the prime~ c..4.~., +5~..+ a,
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after two to threeflights anddeletedif an operationallyrelevant trend is not identified. Similarly,countermeasuresthat do not showpromisewill be dropped. Currently manifestedEDO DSO’s willinvestigatesuch parametersas cardiac size, exercisecapacity,blood pressurevariability, orthostaticfunction, lower body negativepressure,renal stonerisk, manualcontrol, emergencytask analysis,metabolicresponses,andvisual-vestibularintegrationasafunctionof adaptationto weightlessness.It isanticipatedthat EDO missionswill placegreatoperationalemphasison exerciseto preventphysicaldeconditioning. Exercisedeviceswill includea treadmill and/ora multifunctional exercisedevice toaccommodateaerobicandresistiveexercise,rowing andrunning. In addition,a numberof prototypeflight hardwareitems will be developed,ground tested, then flown for use in investigationsinmuscle/exercisephysiology,cardiovascular/fluidandelectrolytes,neurophysiology,andenvironmentalhealth/4/.
SOVIETEXPERIENCE
TheSoviet experiencewith long-durationspaceflight is muchmoreextensivethanthatof the UnitedStates:crewmembershaveremainedin orbit for up to 1 year,although this was on an experimental,ratherthan operational,basis. On the Mir spacestation, a variety of countermeasuresaredirectedtoward preventingcompleteadaptationto microgravity. Unfortunately,it is difficult to assesstheirefficacy. The rawdataare scarce,with somepostflightdataunavailableto the U.S. In addition,somedataareconflicting, with confoundingvariablesandinadequatecontrols, andmeasurementprotocolsbetweenthe U.S. andU.S.S.R.havenot beenstandardized,althoughstepshave beentakento correctthissituation/5/.
While it is true thatcosmonautshavecompletedlengthy staysonorbit, thephysiologicalcostsarebyno means insignificant, and the countermeasuresemployedimpose a high operationaloverhead,particularly in terms of crew time and resources. The time penalty involved with the Sovietcountermeasureswould be unacceptablein view of the work schedulesenvisionedfor SpaceStationFreedom. Cosmonautsexercisefrom 2 to 4 hoursperday (two sessions),usinga bicycle ergometer,extensors,and a treadmill equippedwith devicesfor pulling loads parallel andperpendicularto thelongitudinalaxis of the body. The treadmill operatesin two modes— powered,for light running,andunpowered,for very strenuousexercise (10 to 15 minutes maximum) /6/. A resistiveexerciser(expander)is usedfor strengthtraining, especiallyin preparationfor EVA. The type andextentofinflight exerciseis individually tailored according to flight duration andpreflight baselinephysicalfitness. OnMir, the“Penguin” suit is wornduring wakinghoursto loadthemusculoskeletalsystemandamelioratemuscledeconditioning. As the cosmonautmoves,bandsof rubberwoven into the fabricproducetensiononvariousmusclesin thelegsandtorso.
To reduceorthostaticintoleranceonreturnto Earth,lower body negativepressureisregularlyappliedfor a2- to 4-weekperiod prior to reentry,andextendedduring the final 2 daysof flight. The “Chibis”suit applies negativepressure(-30 mmHg) to the lower half of the body, resulting in a downwardredistributionof bodyfluids. Elasticstockingsarealsowornprior toreentryandseveraldayspostflight.During the final 2 daysof flight cosmonautsdrink asalt andwatersolutionprior to donningthe anti-gsuit. Worn during reentryandpostflight, the adjustableanti-g suit is equippedwith inflatable bladdersthatplacepressureon legsandabdomento preventbloodpoolingandmaintaincirculationto thebrain.
Soviet experiencewith pharmaceuticalcountermeasuresis limited, but diphosphonateshaveofferedsome protectionagainstbonedemineralization.Centralanticholinergicagentsanddrugsthataugmentcentral sympatheticactivity are amongthe approachesbeing consideredfor relief of spacemotionsickness. Nutritional supplementsinclude multivitamins and calcium supplements(still in theexperimentalstage).
The lon~-clurationflights in the Sovietoro~ramnaturallyreauirea greaterinvolvementwith human
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extensiveprogramof social andpsychologicalsupportincludesregularcommunicationwith families,celebrities,andthepress/5,6/.
Theremainderof thispaperattemptsto summarizethecurrentknowledgeregardingmedicalconcernsandcountermeasuresfor long-durationspaceifight. Specific emphasisis on physiological systems(cardiovascular,musculoskeletal,immunological, hematological,and neurosensory),radiobiologicalcountermeasures,andenvironmentalmonitoringsystems. Someof theseareashavebeenidentified aspotential “showstoppers”for long-durationmissions. This paperalso attemptsto describethe majorstrategiesfor developingcountermeasuresin theseareas.
PHYSIOLOGICALCOUNTERMEASURES
During spaceflight, the major effectsof weightlessnesson the cardiovascularsysteminvolve fluidshiftsandloss,alteredcardiovasculardynamics,changesin pulmonaryfunctionandelectricalactivity,inflight loss of exercisecapacity,andpostflight orthostaticintoleranceandmild dehydration. Theseobservationshavebeendocumentedby echocardiographic,blood pressure,fluid balanceanddopplermeasurements/7/. TheSovietprogramhasfocusedon cardiovascularproblems,anddatareveala100-percentincidenceof orthostatichypotensionpostflight. The utilization of extensivecountermeasureshave helpedcosmonautsmaintaininflight exercisecapacity. Both the Soviet andthe U.S. programs(Skylab,Apollo, andShuttle)havedocumentedincidentsof cardiacarrhythmias;onecosmonautwasreturnedto Earthafter175daysof a324-daymissionbecauseof acardiacarrhythmia.
Theunknownswhichhaveto be addressedin the future includetheneuroendocrinefunction, theroleof electrolytesin controlling cardiacelectrical active, blood flow patternsandvascularpermeability.Thus far, the countermeasuresempirically and successfullyusedhave includedfluid andelectrolytesubstitutes,LBNP devices, concentricand eccentricexercise protocols, antigravity suits, and, onoccasion, medications. Future countermeasuresmay include electrical stimulation, autogenicbiofeedback, and the addition of glucose and other electrolytesto the normal saline to promoteabsorptionandretentionof fluids. Thesecountermeasureswill requirecareful studyto determinethemostefficaciousschedulesandmeansfor their administration,as well asprovisionsfor utilizing severalcountermeasuresin uniquecombinationsthatcanbetailoredto theindividual (e.g., LBNP andresistiveexercise).
The musculoskeletalsystemundergoessignificantchangesas a resultof exposureto weightlessness.Thesechangesmay affect the performanceof strenuousextravehicularor intravehicularactivity (EVAor IVA). Inflight findings includeanthropometricandposturalchanges,while postifight findings revealweightloss,reductionin exercisecapacity,decreasesin musclestrengthandmotorcoordination. Datafrom Skylabshowed20-25percentleg strengthlossinspiteof countermeasures,anddatafrom the 1987Mir flights of 160-, 175-, and326-daymissionsdemonstratedmusclelossin therangeof 25-40percentpostflight. Issuesto be addressedin thefutureincludechangesin musclecytology, rateof recoveryofboneloss,if any, neuroendocrinefunction andmodulationof therecoveryprocessin boneandmuscle/7/. In addition,it will beimportantto determinethecontributionof leg musclelossto the orthostasisproblemandtaskperformanceduring flight andpostflight. Osteoporosis-relatedeffectshaveincludedtotal bodycalciumloss of .3-.4percentpermonthandcalcaneousboneloss at therateof approximately5 percentpermonth/8/. Anothersignificantunknownisthe extentto which bonelossmayor may notplateauduring spaceflight: therecouldbe asteadybonelossthat increasesproportionallyto the lengthof the mission. It is alsouncertainwhethera thresholdexists for bonedecalcificationbeyondwhichthereisno recovery. Bonequality afterprolongedexposureto spacemustbeaddressedaswell.
Medical countermeasuresfor musculoskeletaldeconditioning in long-durationspaceflight mustincludetechniquesfor treatmentof potentialbonefracturesand/orinjury, repairor muscleinjury, andabsorptionof substancesfrom the gastrointestinaltract (suchas vitamins, amino acids, electrolytes).fltharcnneidarotinne Gm i-ha man,~lotinn nf tl~aknAt,’o tnatnhnlli,n, nnrtin.ilcrl~, fnr ,lacinnina tharonaiitir
(1)334 A. E. Nicogossianeta!.
musculoskeletaldeconditioningincludeexercise,compressionalforces,electricalstimulation,nutritionalregimens,medications(i.e.,calciumsubstitute),crewscreening,andintegratedexerciseregimes.
Theweightlessenvironmentof spaceresultsin functionaldisturbancesin theneurosensorysystemthatcausevariousside effects,themost significant of which is spacemotion sickness(SMS). Additionalneurosensorysystem adjustmentshave been implicated in the appearanceof reported illusions,disturbancesin sleeppatternsand postflight changesin the reflex mechanism,task performance,movementcoordination,and postural stability. The acute neurovestibularadaptivephenomenaareevidentduring thefirst few daysof spaceflight, unfortunatelycoinciding with an operationallycriticalperiod /9/. (The activity of donning an EVA suit could also potentially induce SMS.) The majorsymptomsof SMS includenausea,vomiting(projectile),lethargy,malaiseandanorexia.Thechangesinthe neurosensorysystemhave thus far appearedto be transientin nature, with rapid adaptationtoweightlessnessandreadaptationto Earth’s gravity /7/. During spaceflight, thesesymptomsusuallydisappearwithin 2 to 4 days,although‘Sovietdataindicatethatneurovestibularsymptomscanoccuratlatter phasesof extendedmissions. One of the major issuesis the relationshipamongneurosensorychanges,shiftsin cardiovascularfluids,andlossofmusclestrength. Thisrelationshipis importantin thereadaptationprocessto a onegravity environment,whereproblemsof movementcoordinationcouldimpairability foremergencyegress. Seriousquestionsalso remainconcerningthelong-termeffectsofthesechangesat the level of the neurotransmittersandreceptors,with clinical implicationsfor drugeffectsin themicrogravityenvironment/7/.
The development,in one gravity, of predictive indicatorsof predispositionto SMS has not beeneffective/9/. Therefore,greateremphasishasbeenfocusedon thedevelopmentof countermeasuresthatcan preventthe onsetor reducethe severityof the symptomsduring spaceflight. Countermeasuresinclude preflight habituation (visual) to acclimatize crews to conflicting neurovestibularcues,medications,biofeedback,exposureto compressionalforces,andwaterimmersiontraining/10/. In viewof the limited capability to simulate the microgravity environmentwith a high degreeof fidelity,vestibularresearchon SpaceStationFreedomwill play animportantrole in fosteringanunderstandingof thebasicmechanisms.
Spaceflight effectson the immunological/hematological/endocrinesystemhaveseriousimplicationsfor long-durationmissions. Significantdecreasesin red blood cell mass havebeenreportedin theGemini,Apollo, SkylabandSoyuzprograms/ill.Thereis apartial recoveryof redcell massafter30 to60 days,evenwith continuedexposureto microgravity. Lossof electrolytescontinuesthroughoutflight,alongwith morphologicalandwhite bloodcell changes. Neuroendocrinechangesin spaceflight arecomplex and not well understood. These changes may be related to cardiac arrhythmias orgastrointestinaldysfunction. Body weight reductionin flight is the resultof loss of fluid, leanbodymass,andfat stores. Thechangesin hormonesmaycontributeto bodymassloss/11/. Spaceffight mayalso affect blood clotting capabilities /8/. Countermeasuresare primarily focused on nutritionalsupplements,preventivehealth stabilization, and pharmacologicalmeans (i.e., inoculation of theintestinal system,prophylactically, after the crew returns). The major clinical implications for long-durationspaceflight include the effects of injury anddisease/10/. The Soviets have reportedanincreasedincidenceof illnessfollowing crewexchangeon MIr /5/.
PSYCHOLOGICALCOUNTERMEASURES
Long-durationmissionswill involve small groupsof humansliving andworkingin isolated,confinedand dangerousenvironments. The successof theseextendedmissionswill dependon establishingcountermeasuresthat enhancegroup cohesionand productivity. The Sovietshavereportedcrewinterpersonal conflicts, and both the U.S. and the Soviets have reported symptoms that affectproductivityandhealth(e.g.,sleepdisorders,morale,stressandfatigue)/13/.
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leadershipwill be important. Time managementandthe sequencingandarrangementof interactionsand activities will be fundamentalto crew compatibility and motivation. Workload assessment,effective human-machineinterface,andtraining will be important for maintainingcrew productivitythroughouttheextendedmissions. Crewsupportsystems,suchasprovisionofprivacy, communicationswith family andfriends, individualchoiceof recreationalactivitiesandfood selection,will productivityandpsychologicalhealth/12/.
ENVIRONMENTAL HEALTH COUNTERMEASURES
Environmental health countermeasuresand environmental monitoring systems with extendedreliability mustbe developedto accommodatethehealthandsafetyof the crew overlongperiodsin aclosed environment. The major purpose of the environmentalmonitoring systemis to preventcontaminationevents,but whensucheventsoccur,thesesystemsmustdetectthem (within asufficienttime period)andwarncrewmembers.Toxicologyconcernsmaybeassociatedwithspacecraftmaterials,water treatmentchemicals,materialsprocessing,and biological andexperimentalactivities. Thesefactorsincreasetheprobabilitiesthatcontaminantswill be releasedinto the closedenvironment/14/. Itwill alsobeimportantto determinetheimpactof continualenvironmentalmonitoringandmaintenanceof systemsoncrewwork schedulesandproductivity for long-durationmissions.
SpaceStationFreedomwill require routine monitoringof air andwaterquality as well as microbialbuildup, vibroacoustics,fire detectionandsuppressionandenvironmentaldebris. Real-timemonitoringsystems,particularly sensors,must detectand characterizecontaminationlevels from a toxic spill;potential hazardsassociatedwith synergisticeffects of cleaning materialsand other substancesmayincurmedicalproblems.It is importantto characterizetherangeofpossiblecontingenciesandestablishthe capability to manage a contaminationevent. It will also be necessaryto understandhowenvironmentalquality requirementschangefor different missions(e.g.,Lunar Outpost,MarsMission)/15/.
Spacecrafthabitability on long-term missionsinvolves the designof environmentsto support andenhancecrew productivity, health, safety, and comfort. Early researchin spaceflight habitabilityfocusedon suchfactorsas temperatureandhumidity, sensorydeprivation,and variable acceleration/16/. Currentefforts focuson technologyandhumanfactorsconcerns/17/. Habitability factorsincludehabitable volume, temperature and humidity, lighting, vibroacoustics, personal hygiene, wastemanagement,privacy, environmentalaesthetics,food systems,andrecreationalactivities. Inadequatefood and water systems, inappropriatenoise, vibration and humidity levels, privacy constraints,recreationalactivities or schedulesthat are incompatible with individual preferences,as well as anaestheticallymonotonousenvironment,canhaveprofoundinfluencesonproductivityandhealth/15/.
RADIATION PROTECTIONCOUNTERMEASURES
Protectionfrom the hazardsdue to ionizing radiation in spaceis critical to the safetyof extendedhumanspaceifight. A broadspectrumof radiation,rangingfrom infraredto galacticcosmicradiation,exists in the physical environmentof space,external to the enclosuresin which humanswill live andwork (habitats,spacecraft,andspacesuits). The biological effectsof exposureto heavyion particles,includingpossiblecarcinogenesisandgeneticdamage,areunknown. Also, theinternalenvironmentoftheselife-supportingenclosurescontainradiationproductsproducedby theinteractionof spaceradiationwith spacecraftmaterials. Similarly, the radiation field to which crewmembers’internal tissuesandorgansareexposedcontainsadditionalproductsdue to the interactionsof radiationwith individual’sbody/18/.
The managementof risks associatedwith exposureto ionizing radiation is best accomplishedbyreiiieti~n nf dneao~~Thie can he achipvM thrnuoh iiidiciniie echpiliilino nf n mieei~11 £,r PVA (I e ti~
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otherbiomedicalstrategies(e.g., nutritionalapproaches).(Someradioprotectivedrugshavebeentestedin theSovietprogram,but serioussideeffectshavebeenobserved.)
ARTIFICIAL GRAVITY COUNTERMEASURES
As discussedpreviously,spaceflight resultsin significantphysiologicalchanges,suchas bonemineralloss, muscle atrophy,cardiovasculardeconditioning,neurovestibulardisturbances,andimmunologicalchanges. Exercise,nutrition, pharmacologicalandotherdevicesandtechniques(e.g., “penguin” suit,lower body negative pressure,training, electrostimulation,and anti-gravity suits) show promise incontrolling the adversephysiological effects of long duration exposure to microgravity. Threeadditionaloptionsexistif reduced-gravitycountermeasuresfor long durationmissionsproveineffective:providing artificial gravity within the spacecraft,harnessingnuclearpropulsion to shortenthe timeinvolved in a Mars mission, and accepting and abort consequenceswith the best availablecountermeasures/19/.
A reduced-gravitycountermeasuresprogramwill testandevaluateaseriesof strategieson the SpaceShuttleandExtendedDurationOrbiter during theearly 1990’s; this will befollowed by lengthiertestson SpaceStationFreedom. Sinceit is highly likely thatartificial gravity alonemay not be sufficient(even on Earth in a one-gravity environmentsedentaryindividuals decondition),artificial gravitycombinedwith exercisemay be themost effective solution. Optionsfor artificial gravity includenon-rotating designs involving intermittent centrifugation(i.e., short-radiuscentrifugeor a long-radiuscentrifuge)anda rotating spacecraftthatmay betethered,or arigid structure. Oneof the problemsofrotating environmentsis that they frequentlycauseundesirablesideeffects, including disorientation,nausea,fatigue, mood fluctuations,andsleepdisturbances. However, individuals appearto adapttotheseproblemsif therotationrateis lessthan6 revolutionsper minute/20/. Trainingproceduresmayhabituate crews to problems associatedwith transitioning between a rotating and non-rotatingenvironmentOthersideeffectsneedto bedetermined.
CONCLUSION
TheNASA Life SciencesDivision is pursuing medicalcountermeasuresto preventand/orcontrol theknown physiologicaleffectsof weightlessness,to betterunderstandthe unknowns,andto reducetheuncertaintiesassociatedwith potentially life-threateningproblemsassociatedwith radiation exposureandthe spacecraftenvironmental. The initial approachis to conductsystematictestingof potentialcountermeasuresin groundfacilities andon theSpaceShuttleEDOMPmissions. This experiencewillhelp refine countermeasuresfor Space Station freedom, wherea reduced-gravitycountermeasuresprogramwill furthertesttheir applicability to subsequentspacemissions. Decisionsregardingthelevelor typeof artificial gravity on aMars spacecraftwill be basedon dataobtainedform theSpaceStationFreedomandfurther definedon the LunarOutpostat a one-sixthgravity datapoint. In somecases,analog environments,such as the Antarctic and underseahabitats,will be used to refine and testcountermeasures,including new conceptsand individual protocols, and investigatehuman factorsquestions.
The selectedsystemof medicalcountermeasureswill needto addressphysiological,psychological,environmentalandradiationprotectionrequirementsfor all missions. While humanshaveproventobeextremelyresilient andadaptive,thechallengeof permanentlyoccupyingspacemay testthelimits ofourcapabilities.
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