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Microgravity
Ultrasound in Med. & Biol., Vol. 26, Supplement 1, pp. S144 –S146, 2000 Copyright © 2000 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/00/$–see front matter
PII S0301-5629(00)00190-3
● Part II: Clinical Applications MICROGRAVITY L. POURCELOT, F. PATAT, M. DEFONTAINE, J. M. GREGOIRE and M. BERSON LUSSI, INSERM U316, GIP Ultrasons, University of Tours, Tours, France
thorax. A progressive oedema appears at the level of the face and trunk of the astronauts, accompanied by an increase in blood volume in the cardiac cavities, brain and lungs (Kirsch et al. 1984). This shift will lead to thirst reduction, increased urine excretion and, consequently, a reduction in the blood mass of the astronaut. The expected haemodynamic consequences of this fluid shift are a transient increase in cardiac volumes and cardiac output, followed, after a few h, by a decrease in these parameters below basal reference values (Bungo et al. 1985). The antigravity muscles do only relatively light work during microgravity activity (the body has no weight), leading to muscular atrophy (chicken legs) and bone demineralisation. Posture changes no longer stimulate the vasomotor reflexes needed to control the distribution of blood volume in the body (especially to the lower limbs) as under normal gravity conditions. In the absence of countermeasures, these phenomena lead to cardiac and vascular deconditioning, reduction of exercise capabilities and bone loss. In practice, orthostatic intolerance and maladaptation to exercise are frequently observed in astronauts during their recovery phase after short-term space flights. During long-term flights, several countermeasures are used by the astronauts to limit cardiovascular deconditioning and bone loss: isotonic and isometric exercise, lower body negative pressure and fluid loading.
INTRODUCTION Since the first manned space flight in 1961, approximately 250 astronauts (or cosmonauts) have been launched into space. Space flights have lengthened from the 1 h 48 min of Gagarin’s initial flight, to the world record stay of Valeri Poliakov: 438 consecutive days onboard the Mir orbital station. Sergueı¨ Avdeı¨ev has accumulated 748 d in microgravity during three space flights and Sergueı¨ Krikalev has spent 36 h outside the Mir station during seven extravehicular activity sessions. Space agencies are discussing a manned mission to Mars that could be launched in approximately 2015 and last for 2 y. In weightlessness, or microgravity, bodies are freefloating. In fact, microgravity is similar to free fall, and cannot be simulated correctly on Earth, except for very short periods of 20 –30 s during parabolic aircraft flights. During space flights, several physiological functions of the astronauts are impaired or modified (e.g., neurosensorial system; otholiths are no longer functional, for instance; cardiovascular deconditioning, bone demineralisation, muscular atrophy, and increased irradiation). The time-course of each of these changes is different, some changes appearing immediately at the beginning of the flight (motion sickness due to neurovestibular disturbances, fluid shift) and others being only detectable during long-term flights (calcium loss). MICROGRAVITY EFFECTS ON CARDIOVASCULAR SYSTEM, MUSCLES AND BONES
STUDY OF THE CARDIOVASCULAR SYSTEM IN SPACE To study in detail the modifications of the cardiovascular system in weightlessness, we specially designed two successive Doppler echocardiographs for space conditions. The first machine was used in space 3 times: on the Salyut 7 space station in 1982 (7-day Soviet-French mission) and 1984 (237-day flight by three Soviet astronauts), and in 1985 on the American space shuttle during a 7-day international mission.
During the early phase of microgravity exposure, an important volume of liquid (mostly blood) stowed in the inferior part of the body moves toward the head and the Address correspondence to: Professor L. Pourcelot, Service de Me´decine Nucleaire in vivo et Ultrasons, Centre Hospitalier Universitaire, 2 Bd Tonnelle´, 37044 Tours Ce´dex 1, France. S144
Microgravity ● L. POURCELOT et al.
The second machine was launched in 1988 on the Soviet orbital station Mir. It was used during three flights in 1988 (Soviet-French 24-day mission Aragatz), 1992 (Russian-French 14-day mission Antares) and 1993 (Russian-French 21-day mission Altair). The same device called “As de Cœur” was also flown on the American space shuttle during the Spacelab D2 mission of 10 days in 1993. For peripheral vascular circulation monitoring during space flights, a special self-powered and portable Doppler system was developed to study blood flow velocities in aorta and femoral, carotid and intracranial vessels. This device was small enough for being used either during ascent and descent or onboard space vehicles. It was first tested during the Spacelab D2 mission 1993 and then integrated into the scientific hardware of missions on board the Mir station (Russian-French flight Cossiope´e in 1996) and on board the space shuttle (STS 93 flight in 1999). The astronauts were trained to use the B-mode, M-mode and Doppler probes on themselves during the flight. Preflight measurements were made 1 month and a few days before the space flight. Postflight data were collected as soon as possible after landing and during the first days of the recovery phase; the last recording session taking place 20 –30 days postflight. The following cardiovascular data were studied: left heart dimensions and function, left ventricular diastolic and systolic volumes, stroke volume, ejection fraction, heart rate, blood flow velocity and/or volume (carotid, intracranial, aortic, femoral, jugular, subhepatic, renal),
S145
size and compliance of arterial and venous system, arterial blood pressure, and cardiovascular responses to the lower body negative pressure, test exercise, and squat-up test. Changes in cardiovascular parameters are relatively similar for short-term (7–10-day) and midterm (1 month) flights with: 1. an increase in heart rate; 2. a transient increase in left ventricular diastolic volume during the first h of space flight, followed by a decrease during the rest of the flight; 3. a decrease in stroke volume and ejection fraction; and 4. a decrease in renal and cerebral circulatory resistances. The recovery phase is relatively short and, for most parameters, is only a few days (Pottier et al. 1988). During long-term flights (several months), countermeasures are systematically used by astronauts to prevent critical changes in muscular mass, blood volume, and vascular deconditioning. Consequently, there is a much larger range of variation in individual parameters compared with short-term flights, depending on the duration and intensity of the countermeasures for each individual and also personal responses for a given level of countermeasures. Generally, the left ventricular diastolic volume remains lower than reference values in flight, and the recovery phase lasts more than 1 month for the majority of the cardiac parameters (Arbeille et al 1987). Different stress tests are used to investigate the cardiovascular deconditioning after the flight. They show a large percentage of orthostatic intolerance during the first days of recovery, which is well demonstrated by
Fig. 1. View of an astronaut performing echocardiography on board the American space shuttle (1985).
S146
Ultrasound in Medicine and Biology
either the lower body negative pressure test, or the squat-up test. Also, the physical condition of the astronauts is modified, corresponding to a significant reduction in adaptation to exercise even after short-term flights of a few days. BONE DENSITOMETRY IN ASTRONAUTS A side effect of long-term exposure to weightlessness is loss of mass of weight-bearing bones. The loss of mass is similar to osteoporosis and can be detected by ultrasound (US). Indeed, US equipment is well suited for use in space since it is compact, nonionising and has low power consumption. We developed, in cooperation with the European Space Agency, a special device that was first flown on EuroMir 95 155-day flight onboard Mir orbital station (1995–1996). The system was designed to measure speed of sound (SOS) and broadband US attenuation (BUA) in the heel bone, with a reproducibility of 0.3% and 2%, respectively. Two transducers mounted on a springloaded mechanism are pushed against either side of the heel. One acts as a transmitter, the other as a receiver. The transmission delay and the transducer spacing are measured and used to compute the SOS. The received signal is digitised and processed to determine the BUA (Schmidt-Harms et al. 1998). This first experiment in microgravity showed a decrease in SOS and BUA values. However, changes in foot morphology were also observed, which could have affected the precision of foot repositioning and, consequently, the accuracy of sequential measurements. To solve this particular problem, a new system is under development, based on the acquisition of two-dimensional (2-D) images of BUA and SOS, allowing a precise follow-up of these two parameters. An ultrasonic osteodensitometer based on matrical scanning (beam scanner) has been designed under contract of the European Space Agency, for calcis bone and wrist imaging. The ultrasonic matrices (Vermon SA) consisting of 576 elements are placed on each side of the explored bone.
Volume 26, Supplement 1, 2000
In the meantime, the first ultrasonic osteodensitometer was used during a long duration bed-rest study performed in Moscow in 1997. A total of 10 subjects were involved in this experiment: 4 subjects (group A) for a 2-month confinement in bed, and 6 subjects (group B) for a 4-month confinement. BUA and SOS parameters measurements were made every 2 weeks. This study showed clearly a stability in SOS values, and a decrease in BUA values of 4% and 6% in group A and group B, respectively. CONCLUSIONS In conclusion, US techniques have been accepted in the early 1980s as the best method for studying the cardiovascular response of astronauts during space flights and the subsequent recovery phase. Such techniques are now very helpful in the development of countermeasures for manned very long-term space flights and for the monitoring in microgravity of parameters such as bone calcium content, orthostatic tolerance, and response of the cardiovascular and muscular systems to exercise. The ultrasonic techniques also give fundamental data for researchers interested in modeling the regulation of physiological parameters in weightlessness, a particular situation that cannot be simulated correctly on earth. REFERENCES Arbeille Ph, Pottier JM, Patat F, et al. Cardiovascular adaptation to zero g during a long-term flight (237 days) on board the Salyut VII soviet space station. ESA SP-27 1987;143–146. Bungo MW, Charles JB, Johnson PC. Cardiovascular deconditioning during space flight and the use of saline as a countermeasure to orthostatic intolerance. Aviat Space Environ Med 1985;56:985– 990. Kirsch KA, Roecker L, Gauer OH, et al. Venous pressure in man during weightlessness. Science 1984;225:218. Pottier JM, Arbeille Ph, Patat F, et al. Comparative study of the cardiovascular adaptation to zero g during 7 days space flights. Physiologist 1988;31(1):4 –15. Schmidt-Harms C, Boutry J, Dubois I, et al. Using ultrasound for bone densitometry in space: the BDM instrument. Microgravity News 1998;8:2–5.
PII S0301-5629(00)00190-3
● Part II: Clinical Applications MICROGRAVITY L. POURCELOT, F. PATAT, M. DEFONTAINE, J. M. GREGOIRE and M. BERSON LUSSI, INSERM U316, GIP Ultrasons, University of Tours, Tours, France
thorax. A progressive oedema appears at the level of the face and trunk of the astronauts, accompanied by an increase in blood volume in the cardiac cavities, brain and lungs (Kirsch et al. 1984). This shift will lead to thirst reduction, increased urine excretion and, consequently, a reduction in the blood mass of the astronaut. The expected haemodynamic consequences of this fluid shift are a transient increase in cardiac volumes and cardiac output, followed, after a few h, by a decrease in these parameters below basal reference values (Bungo et al. 1985). The antigravity muscles do only relatively light work during microgravity activity (the body has no weight), leading to muscular atrophy (chicken legs) and bone demineralisation. Posture changes no longer stimulate the vasomotor reflexes needed to control the distribution of blood volume in the body (especially to the lower limbs) as under normal gravity conditions. In the absence of countermeasures, these phenomena lead to cardiac and vascular deconditioning, reduction of exercise capabilities and bone loss. In practice, orthostatic intolerance and maladaptation to exercise are frequently observed in astronauts during their recovery phase after short-term space flights. During long-term flights, several countermeasures are used by the astronauts to limit cardiovascular deconditioning and bone loss: isotonic and isometric exercise, lower body negative pressure and fluid loading.
INTRODUCTION Since the first manned space flight in 1961, approximately 250 astronauts (or cosmonauts) have been launched into space. Space flights have lengthened from the 1 h 48 min of Gagarin’s initial flight, to the world record stay of Valeri Poliakov: 438 consecutive days onboard the Mir orbital station. Sergueı¨ Avdeı¨ev has accumulated 748 d in microgravity during three space flights and Sergueı¨ Krikalev has spent 36 h outside the Mir station during seven extravehicular activity sessions. Space agencies are discussing a manned mission to Mars that could be launched in approximately 2015 and last for 2 y. In weightlessness, or microgravity, bodies are freefloating. In fact, microgravity is similar to free fall, and cannot be simulated correctly on Earth, except for very short periods of 20 –30 s during parabolic aircraft flights. During space flights, several physiological functions of the astronauts are impaired or modified (e.g., neurosensorial system; otholiths are no longer functional, for instance; cardiovascular deconditioning, bone demineralisation, muscular atrophy, and increased irradiation). The time-course of each of these changes is different, some changes appearing immediately at the beginning of the flight (motion sickness due to neurovestibular disturbances, fluid shift) and others being only detectable during long-term flights (calcium loss). MICROGRAVITY EFFECTS ON CARDIOVASCULAR SYSTEM, MUSCLES AND BONES
STUDY OF THE CARDIOVASCULAR SYSTEM IN SPACE To study in detail the modifications of the cardiovascular system in weightlessness, we specially designed two successive Doppler echocardiographs for space conditions. The first machine was used in space 3 times: on the Salyut 7 space station in 1982 (7-day Soviet-French mission) and 1984 (237-day flight by three Soviet astronauts), and in 1985 on the American space shuttle during a 7-day international mission.
During the early phase of microgravity exposure, an important volume of liquid (mostly blood) stowed in the inferior part of the body moves toward the head and the Address correspondence to: Professor L. Pourcelot, Service de Me´decine Nucleaire in vivo et Ultrasons, Centre Hospitalier Universitaire, 2 Bd Tonnelle´, 37044 Tours Ce´dex 1, France. S144
Microgravity ● L. POURCELOT et al.
The second machine was launched in 1988 on the Soviet orbital station Mir. It was used during three flights in 1988 (Soviet-French 24-day mission Aragatz), 1992 (Russian-French 14-day mission Antares) and 1993 (Russian-French 21-day mission Altair). The same device called “As de Cœur” was also flown on the American space shuttle during the Spacelab D2 mission of 10 days in 1993. For peripheral vascular circulation monitoring during space flights, a special self-powered and portable Doppler system was developed to study blood flow velocities in aorta and femoral, carotid and intracranial vessels. This device was small enough for being used either during ascent and descent or onboard space vehicles. It was first tested during the Spacelab D2 mission 1993 and then integrated into the scientific hardware of missions on board the Mir station (Russian-French flight Cossiope´e in 1996) and on board the space shuttle (STS 93 flight in 1999). The astronauts were trained to use the B-mode, M-mode and Doppler probes on themselves during the flight. Preflight measurements were made 1 month and a few days before the space flight. Postflight data were collected as soon as possible after landing and during the first days of the recovery phase; the last recording session taking place 20 –30 days postflight. The following cardiovascular data were studied: left heart dimensions and function, left ventricular diastolic and systolic volumes, stroke volume, ejection fraction, heart rate, blood flow velocity and/or volume (carotid, intracranial, aortic, femoral, jugular, subhepatic, renal),
S145
size and compliance of arterial and venous system, arterial blood pressure, and cardiovascular responses to the lower body negative pressure, test exercise, and squat-up test. Changes in cardiovascular parameters are relatively similar for short-term (7–10-day) and midterm (1 month) flights with: 1. an increase in heart rate; 2. a transient increase in left ventricular diastolic volume during the first h of space flight, followed by a decrease during the rest of the flight; 3. a decrease in stroke volume and ejection fraction; and 4. a decrease in renal and cerebral circulatory resistances. The recovery phase is relatively short and, for most parameters, is only a few days (Pottier et al. 1988). During long-term flights (several months), countermeasures are systematically used by astronauts to prevent critical changes in muscular mass, blood volume, and vascular deconditioning. Consequently, there is a much larger range of variation in individual parameters compared with short-term flights, depending on the duration and intensity of the countermeasures for each individual and also personal responses for a given level of countermeasures. Generally, the left ventricular diastolic volume remains lower than reference values in flight, and the recovery phase lasts more than 1 month for the majority of the cardiac parameters (Arbeille et al 1987). Different stress tests are used to investigate the cardiovascular deconditioning after the flight. They show a large percentage of orthostatic intolerance during the first days of recovery, which is well demonstrated by
Fig. 1. View of an astronaut performing echocardiography on board the American space shuttle (1985).
S146
Ultrasound in Medicine and Biology
either the lower body negative pressure test, or the squat-up test. Also, the physical condition of the astronauts is modified, corresponding to a significant reduction in adaptation to exercise even after short-term flights of a few days. BONE DENSITOMETRY IN ASTRONAUTS A side effect of long-term exposure to weightlessness is loss of mass of weight-bearing bones. The loss of mass is similar to osteoporosis and can be detected by ultrasound (US). Indeed, US equipment is well suited for use in space since it is compact, nonionising and has low power consumption. We developed, in cooperation with the European Space Agency, a special device that was first flown on EuroMir 95 155-day flight onboard Mir orbital station (1995–1996). The system was designed to measure speed of sound (SOS) and broadband US attenuation (BUA) in the heel bone, with a reproducibility of 0.3% and 2%, respectively. Two transducers mounted on a springloaded mechanism are pushed against either side of the heel. One acts as a transmitter, the other as a receiver. The transmission delay and the transducer spacing are measured and used to compute the SOS. The received signal is digitised and processed to determine the BUA (Schmidt-Harms et al. 1998). This first experiment in microgravity showed a decrease in SOS and BUA values. However, changes in foot morphology were also observed, which could have affected the precision of foot repositioning and, consequently, the accuracy of sequential measurements. To solve this particular problem, a new system is under development, based on the acquisition of two-dimensional (2-D) images of BUA and SOS, allowing a precise follow-up of these two parameters. An ultrasonic osteodensitometer based on matrical scanning (beam scanner) has been designed under contract of the European Space Agency, for calcis bone and wrist imaging. The ultrasonic matrices (Vermon SA) consisting of 576 elements are placed on each side of the explored bone.
Volume 26, Supplement 1, 2000
In the meantime, the first ultrasonic osteodensitometer was used during a long duration bed-rest study performed in Moscow in 1997. A total of 10 subjects were involved in this experiment: 4 subjects (group A) for a 2-month confinement in bed, and 6 subjects (group B) for a 4-month confinement. BUA and SOS parameters measurements were made every 2 weeks. This study showed clearly a stability in SOS values, and a decrease in BUA values of 4% and 6% in group A and group B, respectively. CONCLUSIONS In conclusion, US techniques have been accepted in the early 1980s as the best method for studying the cardiovascular response of astronauts during space flights and the subsequent recovery phase. Such techniques are now very helpful in the development of countermeasures for manned very long-term space flights and for the monitoring in microgravity of parameters such as bone calcium content, orthostatic tolerance, and response of the cardiovascular and muscular systems to exercise. The ultrasonic techniques also give fundamental data for researchers interested in modeling the regulation of physiological parameters in weightlessness, a particular situation that cannot be simulated correctly on earth. REFERENCES Arbeille Ph, Pottier JM, Patat F, et al. Cardiovascular adaptation to zero g during a long-term flight (237 days) on board the Salyut VII soviet space station. ESA SP-27 1987;143–146. Bungo MW, Charles JB, Johnson PC. Cardiovascular deconditioning during space flight and the use of saline as a countermeasure to orthostatic intolerance. Aviat Space Environ Med 1985;56:985– 990. Kirsch KA, Roecker L, Gauer OH, et al. Venous pressure in man during weightlessness. Science 1984;225:218. Pottier JM, Arbeille Ph, Patat F, et al. Comparative study of the cardiovascular adaptation to zero g during 7 days space flights. Physiologist 1988;31(1):4 –15. Schmidt-Harms C, Boutry J, Dubois I, et al. Using ultrasound for bone densitometry in space: the BDM instrument. Microgravity News 1998;8:2–5.