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Microgravity

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Abstract

Gravity profoundly affects the overall mechanics of the respiratory system. Functional residual capacity, when measured in sustained microgravity, is intermediate to that present in the standing and supine postures in 1G, consistent with early modeling studies. This change occurs almost exclusively through changes in the abdominal compliance and thus in the volume of the abdominal compartment, with the rib cage being relatively unaffected by gravity. Microgravity leaves vital capacity unaltered once the initial translocation of blood into the thorax is corrected by homeostatic mechanisms, but residual volume is reduced, likely through a more uniform distribution of alveolar size permitting deflation to a lower overall lung volume. Expiratory flows are unaffected by microgravity provided they are measured following normalization of the intrathoracic blood volume. During sleep in microgravity, there is an almost complete abolition of obstructive sleep apnea events. © 2011 American Physiological Society. Compr Physiol 1:485‐497, 2011.

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Figure 1. Figure 1.

A theoretical analysis (from 1964) of the pressures on the rig cage and abdomen in the upright (left) and supine (middle) postures and in weightlessness (right). The changes in profile from the upright condition are shown as the dashed lines in the supine and weightless conditions for the purposes of comparison. The lung volume is assumed to be at 36% of vital capacity, approximating functional residual capacity (FRC) in the upright posture. Pressure across the rib cage is the sum of those across the abdomen and the diaphragm (Prc = Pab + Pdi), where Pab is the pressure on the abdominal side of the diaphragmatic dome and is itself the sum of the gravitational effects of the abdominal contents (Pabc) and the tension in the abdominal wall (Padw), that is, Pab = Pabc + Pabw. Note that before any measurements had ever been made in weightlessness, the prediction was for a reduction in FRC compared to upright. From , with permission.

Figure 2. Figure 2.

Lung outline tracings from five subjects from chest radiographs taken in 1G (solid lines) and in microgravity (dotted lines) during parabolic flight. Columns from left to right are anterior‐posterior views at residual volume (RV), functional residual capacity (FRC), and total lung capacity (TLC), and a left lateral view at TLC. From , with permission.

Figure 3. Figure 3.

(A) Strip chart recorder tracing from a subject performing an inspiratory capacity maneuver after 10 to 15 s in weightlessness during parabolic flight. The cabin G level in the aircraft is shown at the top with a period of level flight, the initial pull up at ∼1.8Gz, the period of microgravity, and the subsequent pullout at ∼1.8Gz. Changes in the volume of the abdominal compartment (Vab) and rib cage compartment (Vrc) were measured by inductive plethysmography, and changes in thoracoabdominal volume (Vw) by the sum of the two. Gas volume was measured by integration of a flowmeter. Note that changes in end expiratory Vw are almost entirely the result of changes in Vabd. From , with permission. (B) Changes in thoracoabdominal volume (termed Vw) as measured by inductive plethysmography, and in functional residual capacity as measured by the integration of a respiratory flowmeter in five normal subjects during parabolic flight. Note that the reduction in lung volume between 1G and microgravity was much greater than the increase in volume between 1G and 2G. From , with permission.

Figure 4. Figure 4.

Functional residual capacity (FRC) measured in four subjects before and during a sustained microgravity exposure of 9 days. Preflight measurements were in both the standing and supine postures. FRC in microgravity was intermediate to that in standing and supine as previously predicted (see Fig. ). From , with permission. *P < 0.05 compared to standing.

Figure 5. Figure 5.

Change in end‐expiratory rib cage (Vrc) and abdominal (Vab) volumes as a function of G level in five normal subjects during parabolic flight. The changes in Vrc were not significantly different from zero. From , with permission.

Figure 6. Figure 6.

Changes in end‐expiratory abdominal volume (A) and end‐expiratory gastric pressure (B) as a function of G level in five normal subjects during parabolic flight. Note the essentially parallel behavior between the two measurements. From , with permission.

Figure 7. Figure 7.

Vital capacity (VC) (both inspiratory, IVC, and expiratory, EVC) measured in four subjects over the course of 9 days of sustained microgravity. Controls in 1G were measured multiple times in the months preceding flight in both the upright and supine postures. VC was reduced on flight day 2 (after ∼24 h of exposure to microgravity) but subsequently recovered to standing control values. *P < 0.05 compared to 1G standing. From , with permission.

Figure 8. Figure 8.

Residual volume (RV) measured in four subjects before and during a sustained microgravity exposure of 9 days. Preflight measurements were in both the standing and supine postures. RV in microgravity was lower than that observed in standing and supine in 1G. *P < 0.05 compared to 1G standing. From , with permission.

Figure 9. Figure 9.

Theoretical model to explain the reduction in residual volume (RV) in microgravity seen in Figure . At RV, alveolar size increases from base to apex in 1G (B), but in microgravity is uniform (C). If area 2 in panel A is less than area 1, then the sum of alveolar volumes in microgravity will be less than that in 1G. From , with permission.

Figure 10. Figure 10.

Graphic summary of the distribution of lung volumes in the upright and supine postures in 1G and in microgravity. In summary, in microgravity, vital capacity is maintained but there are reductions in residual volume, expiratory reserve volume, and functional residual capacity. From , with permission.

Figure 11. Figure 11.

Comparison of the descending limbs of the maximum expiratory flow volume curves in a single subject in 1G and in microgravity during parabolic flights. Each curve is the ensemble‐averaged sum of multiple measurements in each condition, with the curves aligned at residual volume (RV), and each curve shows its associated standard error. To the right is a statistical comparison between the curves as a function of expired flow. Note that the curve from microgravity is more “scooped” than that measured in 1G (the volume at a given flow rate is higher in microgravity near RV but lower at higher lung volumes). Similar changes are seen in water immersion. From , with permission.

Figure 12. Figure 12.

Parameters from maximum expiratory flow volume (MEFV) curves measured in 1G in the upright and supine postures and over the course of 9 days of sustained microgravity. Forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), and their ratio follow the same pattern as vital capacity (Fig. ) with an early inflight reduction followed by recovery. Peak expiratory flow rate (PEFR) was reduced to a much greater extent, perhaps as a result of difficulties in performing the forced expirations in the novel environment (see text for details) but also subsequently recovered. *P < 0.05 compared to 1G standing. From , with permission.

Figure 13. Figure 13.

The abdominal contribution to tidal breathing at different accelerations in five normal subjects (one, subject L.E., was studied in two flights) during parabolic flight. The results are the averages of between 4 and 10 periods of weightlessness. Increased gravity (labeled as 2Gz but typically ∼1.8Gz) slightly decreased the abdominal contribution to tidal volume, but microgravity resulted in a substantial increase in the abdominal contribution in all subjects. From , with permission.

Figure 14. Figure 14.

Abdominal compliance (Cab) as a function of Gz (A) in five normal subjects (some studied in more than one flight) and the relationship between Cab and Gz (B). As Gz is decreased, Cab increases. From , with permission.

Figure 15. Figure 15.

The abdominal contribution to tidal breathing in sustained microgravity measured in five subjects during the night, using full polysomnography. Each subject was studied four times over the course of flight (twice in the first week, twice in the second week). The preflight data are the average of multiple nights. The awake data are shown with the associated standard error for each measurement shown by the shaded area. Awake data are from periods of quiet wakefulness during the lights out period. Light sleep refers to sleep stages 1 and 2, deep sleep to stages 3 and 4, and rapid eye movement (REM) to REM sleep. Note that there were substantial differences in the abdominal contribution to sleep between different sleep stages that persisted essentially unchanged from 1G to microgravity. Furthermore, the abdominal contribution to tidal volume was maximal early in flight and subsequently decreased. *0.05 < P < 0.1 compared to preflight and **P < 0.05 compared to preflight. From , with permission.

Figure 16. Figure 16.

The rate of electroencephalogram‐based arousals from sleep in five subjects before, during, and after spaceflight of ∼2 weeks duration. The rate of arousal is partitioned into those arousals preceded by a respiratory event (an apnea or hypopnea), as shown by the shaded area, and arousals bearing no such correlation. Note that in microgravity, respiratory arousals, which were primarily obstructive in nature, were almost absent. From , with permission.



Figure 1.

A theoretical analysis (from 1964) of the pressures on the rig cage and abdomen in the upright (left) and supine (middle) postures and in weightlessness (right). The changes in profile from the upright condition are shown as the dashed lines in the supine and weightless conditions for the purposes of comparison. The lung volume is assumed to be at 36% of vital capacity, approximating functional residual capacity (FRC) in the upright posture. Pressure across the rib cage is the sum of those across the abdomen and the diaphragm (Prc = Pab + Pdi), where Pab is the pressure on the abdominal side of the diaphragmatic dome and is itself the sum of the gravitational effects of the abdominal contents (Pabc) and the tension in the abdominal wall (Padw), that is, Pab = Pabc + Pabw. Note that before any measurements had ever been made in weightlessness, the prediction was for a reduction in FRC compared to upright. From , with permission.



Figure 2.

Lung outline tracings from five subjects from chest radiographs taken in 1G (solid lines) and in microgravity (dotted lines) during parabolic flight. Columns from left to right are anterior‐posterior views at residual volume (RV), functional residual capacity (FRC), and total lung capacity (TLC), and a left lateral view at TLC. From , with permission.



Figure 3.

(A) Strip chart recorder tracing from a subject performing an inspiratory capacity maneuver after 10 to 15 s in weightlessness during parabolic flight. The cabin G level in the aircraft is shown at the top with a period of level flight, the initial pull up at ∼1.8Gz, the period of microgravity, and the subsequent pullout at ∼1.8Gz. Changes in the volume of the abdominal compartment (Vab) and rib cage compartment (Vrc) were measured by inductive plethysmography, and changes in thoracoabdominal volume (Vw) by the sum of the two. Gas volume was measured by integration of a flowmeter. Note that changes in end expiratory Vw are almost entirely the result of changes in Vabd. From , with permission. (B) Changes in thoracoabdominal volume (termed Vw) as measured by inductive plethysmography, and in functional residual capacity as measured by the integration of a respiratory flowmeter in five normal subjects during parabolic flight. Note that the reduction in lung volume between 1G and microgravity was much greater than the increase in volume between 1G and 2G. From , with permission.



Figure 4.

Functional residual capacity (FRC) measured in four subjects before and during a sustained microgravity exposure of 9 days. Preflight measurements were in both the standing and supine postures. FRC in microgravity was intermediate to that in standing and supine as previously predicted (see Fig. ). From , with permission. *P < 0.05 compared to standing.



Figure 5.

Change in end‐expiratory rib cage (Vrc) and abdominal (Vab) volumes as a function of G level in five normal subjects during parabolic flight. The changes in Vrc were not significantly different from zero. From , with permission.



Figure 6.

Changes in end‐expiratory abdominal volume (A) and end‐expiratory gastric pressure (B) as a function of G level in five normal subjects during parabolic flight. Note the essentially parallel behavior between the two measurements. From , with permission.



Figure 7.

Vital capacity (VC) (both inspiratory, IVC, and expiratory, EVC) measured in four subjects over the course of 9 days of sustained microgravity. Controls in 1G were measured multiple times in the months preceding flight in both the upright and supine postures. VC was reduced on flight day 2 (after ∼24 h of exposure to microgravity) but subsequently recovered to standing control values. *P < 0.05 compared to 1G standing. From , with permission.



Figure 8.

Residual volume (RV) measured in four subjects before and during a sustained microgravity exposure of 9 days. Preflight measurements were in both the standing and supine postures. RV in microgravity was lower than that observed in standing and supine in 1G. *P < 0.05 compared to 1G standing. From , with permission.



Figure 9.

Theoretical model to explain the reduction in residual volume (RV) in microgravity seen in Figure . At RV, alveolar size increases from base to apex in 1G (B), but in microgravity is uniform (C). If area 2 in panel A is less than area 1, then the sum of alveolar volumes in microgravity will be less than that in 1G. From , with permission.



Figure 10.

Graphic summary of the distribution of lung volumes in the upright and supine postures in 1G and in microgravity. In summary, in microgravity, vital capacity is maintained but there are reductions in residual volume, expiratory reserve volume, and functional residual capacity. From , with permission.



Figure 11.

Comparison of the descending limbs of the maximum expiratory flow volume curves in a single subject in 1G and in microgravity during parabolic flights. Each curve is the ensemble‐averaged sum of multiple measurements in each condition, with the curves aligned at residual volume (RV), and each curve shows its associated standard error. To the right is a statistical comparison between the curves as a function of expired flow. Note that the curve from microgravity is more “scooped” than that measured in 1G (the volume at a given flow rate is higher in microgravity near RV but lower at higher lung volumes). Similar changes are seen in water immersion. From , with permission.



Figure 12.

Parameters from maximum expiratory flow volume (MEFV) curves measured in 1G in the upright and supine postures and over the course of 9 days of sustained microgravity. Forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), and their ratio follow the same pattern as vital capacity (Fig. ) with an early inflight reduction followed by recovery. Peak expiratory flow rate (PEFR) was reduced to a much greater extent, perhaps as a result of difficulties in performing the forced expirations in the novel environment (see text for details) but also subsequently recovered. *P < 0.05 compared to 1G standing. From , with permission.



Figure 13.

The abdominal contribution to tidal breathing at different accelerations in five normal subjects (one, subject L.E., was studied in two flights) during parabolic flight. The results are the averages of between 4 and 10 periods of weightlessness. Increased gravity (labeled as 2Gz but typically ∼1.8Gz) slightly decreased the abdominal contribution to tidal volume, but microgravity resulted in a substantial increase in the abdominal contribution in all subjects. From , with permission.



Figure 14.

Abdominal compliance (Cab) as a function of Gz (A) in five normal subjects (some studied in more than one flight) and the relationship between Cab and Gz (B). As Gz is decreased, Cab increases. From , with permission.



Figure 15.

The abdominal contribution to tidal breathing in sustained microgravity measured in five subjects during the night, using full polysomnography. Each subject was studied four times over the course of flight (twice in the first week, twice in the second week). The preflight data are the average of multiple nights. The awake data are shown with the associated standard error for each measurement shown by the shaded area. Awake data are from periods of quiet wakefulness during the lights out period. Light sleep refers to sleep stages 1 and 2, deep sleep to stages 3 and 4, and rapid eye movement (REM) to REM sleep. Note that there were substantial differences in the abdominal contribution to sleep between different sleep stages that persisted essentially unchanged from 1G to microgravity. Furthermore, the abdominal contribution to tidal volume was maximal early in flight and subsequently decreased. *0.05 < P < 0.1 compared to preflight and **P < 0.05 compared to preflight. From , with permission.



Figure 16.

The rate of electroencephalogram‐based arousals from sleep in five subjects before, during, and after spaceflight of ∼2 weeks duration. The rate of arousal is partitioned into those arousals preceded by a respiratory event (an apnea or hypopnea), as shown by the shaded area, and arousals bearing no such correlation. Note that in microgravity, respiratory arousals, which were primarily obstructive in nature, were almost absent. From , with permission.

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Further Reading
 1. Agostoni E, Mead J. Statics of the respiratory system. In: Fenn WO, and Rahn H, editors. Handbook of Physiology, Section 3: Respiration. Washington, DC: American Physiological Society, 1964, Vol 1, Chapt 13, p. 387–428.

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How to Cite

G. Kim Prisk. Microgravity. Compr Physiol 2011, 1: 485-497. doi: 10.1002/cphy.c100014