Astronauts complain about fluid shifts from their lower extremities to their head caused by weightlessness during their flight into space. For a study of this phenomenon, RWTH Aachen University and Charité University Berlin participated in a joint project on two parabolic flight campaigns of the German Aerospace Centre (DLR) in September 2005 and June 2006. During these campaigns, the characteristics of the rapid fluid shifts during hyper- and micro gravity were measured by a combination of PPG and PPGI optoelectronic sensor concepts.

Blanik, N.

Blazek, C. R.

Herzog, M.

Hülsbusch, M.

English

N. Blanik

M. Hülsbusch

M. Herzog

C. R. Blazek

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Acta Polytechnica

Assessment of Human Hemodynamics under Hyper- and Microgravity: Results of two Aachen University Parabolic Flight Experiments

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

1 IntroductionDuring manned flights into space fluid shifts from thelower extremities to the heads of the astronauts have been de-tected. This phenomenon is termed “puffy face syndrome” or“bird legs”, because of the optical impression of the subject(bloated face and thin legs). These effects are caused bythe unfamiliar influence of microgravity on the human body.The fluid shifts into the head result in motion sickness,concentration problems, vision degradation and other po-tentially dangerous symptoms during space flights. Undernormal conditions (on earth), gravitation arranges a fluid bal-ance in the human body. Thereby most of the blood volume isallocated to the veins of the lower extremities. Figure 1 showsthe distribution of the blood volume inside the human bodywhile standing on earth. Flying into space and escapingfrom the gravitational force disturbs this balance. Especiallyinteresting in this kind of fluid shifts are the dynamic charac-teristics of the blood flow immediately after the momentwhen the gravity changes. So far these phenomena are onlyknown qualitatively.To research these dynamic behaviors, the subjects wereexposed to a rapid change of gravity (between zero and1.8 times the normal gravitational force on earth, i.e. 0 g and1.8 g, respectively) on parabolic flights. During this time,sensors measured the relative change of the blood volume inthe upper skin layers of the legs and forehead.2 Parabolic flightStudies on gravitational effects requiring lower than nor-mal gravitational force on earth are impossible under usuallaboratory conditions. There are only a few ways to eliminatethe disturbing influence of gravity on earth: free fall towers,ballistic rockets, parabolic flights or just leaving the earth.Parabolic flights can be taken on board the Airbus A300ZERO-G owned by the European Space Agency (ESA). Eachyear the Germany Aerospace Centre (DLR) organizes para-bolic flight campaigns especially for research projects.©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 29Acta Polytechnica Vol. 47  No. 4–5/2007Assessment of Human Hemodynamicsunder Hyper- and Microgravity: Resultsof two Aachen University ParabolicFlight ExperimentsN. Blanik, M. Hülsbusch, M. Herzog, C. R. BlazekAstronauts complain about fluid shifts from their lower extremities to their head caused by weightlessness during their flight into space. For astudy of this phenomenon, RWTH Aachen University and Charité University Berlin participated in a joint project on two parabolic flightcampaigns of the German Aerospace Centre (DLR) in September 2005 and June 2006. During these campaigns, the characteristics of therapid fluid shifts during hyper- and micro gravity were measured by a combination of PPG and PPGI optoelectronic sensor concepts.Keywords: rapid fluid shifts, photoplethysmography, photoplethysmography imaging, parabolic flights, microgravity.Fig. 1: Distribution of blood volume for the different parts of thevascular system Fig. 2: Airbus A300 Zero-G directly before injecting the 0 g-phaseIn September 2005 and June 2006, our joint research pro-gram had the opportunity to fly our experiments on the 7thand 8th DLR parabolic flight campaigns in Bordeaux andCologne, respectively. For this purpose the specially preparedAirbus A300 is equipped with up to twelve separate ex-periments conducted by various participants. The campaigncomprises 4 to 5 flight days. During one flight the aeroplaneaccomplishes 31 parabola maneuvers. These flight maneu-vers (Fig. 4) follow a special trajectory which results in micro-gravity up to 22 seconds (Fig. 5) inside the aircraft. Onemaneuver can be separated into 3 phases. In the first one, theplane is pulled up from the horizontal to a climbing flight upto an angle of 47°. During this time (about 20 seconds) theoccupants of the plane are exposed to 1.8 g. In the next phasethe state of microgravity is achieved. Therefore the plane issteered in a ballistic trajectory, a parabola. Inside the plane areduced gravity of about 0.02 g can be achieved. This lastsanother 20 seconds. After that in the third phase the plane ispulled out to its normal flight level. In doing this 1.8 g isachieved for another 20 seconds. After a break of a fewminutes the maneuver is repeated.3 Optoelectronic sensorsOptoelectronic sensor concepts provide a good opportu-nity to assess the blood volume in the skin. They are basicallynon-invasive and thus allow measurements also under spaceconditions and provide an unproblematic way to obtain datafrom the subjects or even to exchange subjects, during flightin the time between two parabolas. A greater number of sub-jects can therefore be measured.3.1 Photoplethysmography sensorThe first sensor type used here is the photopletysmo-graphy (PPG) sensor. This sensor is stuck to the surface of theskin. It contains a monochromatic light source and a photodetector. It uses the fact that blood absorb light at a higherrate than the surrounding tissue. The backscattered or re-flected light intensity in well perfused tissues is lower than intissues with lower perfusion. The scattered light is received bythe detector, and conclusions are drawn from the perfusion ofthe skin in the area below the sensor. The achievable penetra-tion depth lies between one and threemillimeters, dependingon the wavelength of the light source that is used.3.2 Photoplethysmography imagingPhotopletysmography imaging (PPGI) is an advancementof classical PPG. It uses the same underlying circumstances.In contrast to the PPG-sensor, an external light source anda high sensitive camera are used. Like the PPG-sensor, thiscamera is also able to detect in its view small changes oflight intensity caused by the shifting of blood in the skin.Using a camera as detector, the analysis of the blood volumeshift is not limited to a single spot but measures a larger partof the skin surface. An additional benefit is the contactlessmeasurement principle.4 ResultsAfter the flights, the data from the different subjects wereanalysed and it was first ascertained that fluid shifts occurredto all subjects. This proves that even the short time span of30 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 47  No. 4–5/2007Fig. 3: Experimental configuration on the 8th DLR parabolicflight campaignFig. 4: Flight phases of parabolic flightFig. 5: Typical development of g-forces during a parabolamaneuver (1: g-forces vertically, 2: g-forces in flightdirection)Fig. 6: PPG sensor (left) and PPGI camera with surrounding lightsources (right)about 20 seconds of a single parabola suffices for clear detec-tion. Furthermore, a clear shift from the lower body parts tothe head could be verified, as had been predicted.Fig. 7 shows three typical PPG measurements. They con-tain the characteristics of one subject each. These recordingswere taken with sensors working with green and infrared lightsources. They are placed on the subjects’ foreheads. Thedetected light intensity is displayed in curve 1 (green light)and curve 2 (infrared light). The lower black curve shows thedevelopment of the g-forces during the parabola. The smalland fast oscillations seen in the curves are caused by the sub-jects’ heartbeats (clearly visible in the second blue curve) andbreathing. However, the main focus of this research programwas on the characteristics of the low-frequency components.It is clear to see that immediately after the injection of zerogravity the detected backscattered light intensity declinesrapidly. This corresponds to an increasing volume of bloodin the skin under the sensor.A comparison of the data for the different subjects showsthat all of them had a very quick reaction to the altered gra-vitational force. The time span between the change of gravityand the minimum in the detected light intensity (or themaximum if measured at the legs) was very short (only afew seconds). However, this also differs among the subjects.These individual characteristics are detectable with all typesof sensors used here. A statistical analysis of the data is shownin Fig. 8. It shows the relative shift of the light intensitydetected by the sensor during the 0g-phase in contrast to thestart of the parabola maneuver. The differences in the rangeof different light colors are caused by the individual penetra-tion depth of the light wave length that is used.An additional observation of the time that the blood takesto flow out of the vessels in the leg (during a change of gravityfrom 1.8 g to 0 g) with the time it takes to flow back (duringchange from 0 g to 1.8 g) shows that the former time span isnoticeably shorter than the latter one. An explanation for thisis that under microgravity conditions the resistance for theblood flow out of the leg vessels towards the heart (and head)is very little. In addition this flow is assisted by the venousvalves. Healthy venous valves prevent a direct flow backwardsthrough the veins. The flow towards the legs is thus limited tothe heart pumping volume and the arterial blood flow.©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 31Acta Polytechnica Vol. 47  No. 4–5/2007Fig. 7: Different kinetics of blood volume shifts of three sub-jects (measuring points on the forehead using a green (1)and an infrared light source (2), normalized at start ofmeasurement)Fig. 8: Statistical analysis of skin blood volume changes measuredwith PPG sensors on subjects’ foreheadsAcknowledgmentsThe experiment was conducted in cooperation withthe Centre for Space Medicine, Department of Physiology,Faculty of Medicine of the Charité University, Berlin(Prof. Gunga, principal investigator) and the Institute of HighFrequency Technology, RWTH Aachen (Prof. Blazek).References[1] Blazek, V.: Optoelektronische Systemkonzepte fürnichtinvasive Kreislaufdiagnostik,, Optoelek Magazin,1991, p. 212–219.[2] Space Travel and the Effects of Weightlessness on the HumanBody, Canadian Space Agency. http://www.space.gc.ca/asc/pdf/educator-microgravity_science_stu.pdf[3] Golenhofen, K.: Physiologie. Urban und SchwarzenbergVerlag, München 1997.[4] Blazek, V., Schultz-Ehrenburg, U. (Eds.): QuantitativePhotoplethysmography – Basic Facts and Examination Testsfor Evaluating Peripheral Vascular Functions, VDI Verlag,Düsseldorf 1996.[5] Blazek, V.: Funktionelle Beinvenendiagnostik mit der quan-titativen, digitalen Photoplethysmographie, Viavital Verlag,Essen 2005, p. 49–58.Nikolai Blanike-mail: blanik@ihf.rwth-aachen.deMarkus Hülsbusche-mail: huelsbusch@ihf.rwth-aachen.deMarkus Herzoge-mail: herzog@ihf.rwth-aachen.deInstitute of High Frequency TechnologyRWTH Aachen UniversityGermanyClaudia R. Blazeke-mail: claudia.blazek@gmx.deDepartment of DermatologyMedical FacultyRWTH Aachen University, Germany32 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 47  No. 4–5/2007

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Assessment of Human Hemodynamics under Hyper- and Microgravity: Results of two Aachen University Parabolic Flight Experiments

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