Differential changes of lung diffusing capacity and tissue volume in hypergravity

Pulmonary nitric oxide in astronauts before and during long-term spaceflight

Introduction: During exploratory space flights astronauts risk exposure to toxic planetary dust. Exhaled nitric oxide partial pressure (PENO) is a simple method to monitor lung health by detecting airway inflammation after dust inhalation. The turnover of NO in the lungs is dependent on several factors which will be altered during planetary exploration such as gravity (G) and gas density. To investigate the impacts of these factors on normal PENO, we took measurements before and during a stay at the International Space Station, at both normal and reduced atmospheric pressures. We expected stable PENO levels during the preflight and inflight periods, with lower values inflight. With reduced pressure we expected no net changes of PENO. Material and methods: Ten astronauts were studied during the pre-flight (1 G) and inflight (µG) periods at normal pressure [1.0 ata (atmospheres absolute)], with six of them also monitored at reduced (0.7 ata) pressure and gas density. The average observation period was from 191 days before launch until 105 days after launch. PENO was measured together with estimates of alveolar NO and the airway contribution to the exhaled NO flux. Results: The levels of PENO at 50 mL/s (PENO50) were not stable during the preflight and inflight periods respectively but decreased with time (p = 0.0284) at a rate of 0.55 (0.24) [mean (SD)] mPa per 180 days throughout the observation period, so that there was a significant difference (p < 0.01, N = 10) between gravity conditions. Thus, PENO50 averaged 2.28 (0.70) mPa at 1 G and 1.65 (0.51) mPa during µG (-27%). Reduced atmospheric pressure had no net impact on PENO50 but increased the airway contribution to exhaled NO. Discussion: The time courses of PENO50 suggest an initial airway inflammation, which gradually subsided. Our previous hypothesis of an increased uptake of NO to the blood by means of an expanded gas-blood interface in µG leading to decreased PENO50 is neither supported nor contradicted by the present findings. Baseline PENO50 values for lung health monitoring in astronauts should be obtained not only on ground but also during the relevant gravity conditions and before the possibility of inhaling toxic planetary dust.

Cardiovascular Regulation During Acute Gravitational Changes with Exhaling on Exertion

During gravitational changes or changes in the direction of action in relation to the body, fluid displacements can be observed. In special cases different breathing maneuvers (e. g., exhaling on exertion; Ex-Ex) are used to counteract acute fluid shifts. Both factors have a significant impact on cardiovascular regulation. Eight healthy male subjects were tested on a tilt seat, long arm human centrifuge, and parabolic flight. The work aims to investigate the effect of exhaling on exertion on the cardiovascular regulation during acute gravitational changes compared to normal breathing. Possible interactions and differences between conditions (Ex-Ex, normal breathing) for the parameters V’O₂ , V’_E , HR, and SV were analysed over a 40 s period by a three-way ANOVA. Significant (p≤0.05) effects for all main factors and interactions between condition and time as well as maneuver and time were found for all variables. The exhaling on exertion maneuver had a significant influence on the cardiovascular response during acute gravitational and positional changes. For example, the significant increase of V’O2 at the end of the exhalation on exertion maneuver indicates an increased lung circulation as a result of the maneuver.

Interactions of the human cardiopulmonary, hormonal and body fluid systems in parabolic flight

Purpose

Commercial parabolic flights accessible to customers with a wide range of health states will become more prevalent in the near future because of a growing private space flight sector. However, parabolic flights present the passengers’ cardiovascular system with a combination of stressors, including a moderately hypobaric hypoxic ambient environment (HH) and repeated gravity transitions (GT). Thus, the aim of this study was to identify unique and combined effects of HH and GT on the human cardiovascular, pulmonary and fluid regulation systems.

Methods

Cardiac index was determined by inert gas rebreathing (CI_rb), and continuous non-invasive finger blood pressure (FBP) was repeatedly measured in 18 healthy subjects in the standing position while they were in parabolic flight at 0 and 1.8 G_z. Plasma volume (PV) and fluid regulating blood hormones were determined five times over the flight day. Eleven out of the 18 subjects were subjected to an identical test protocol in a hypobaric chamber in ambient conditions comparable to parabolic flight.

Results

CI_rb in 0 G_z decreased significantly during flight (early, 5.139 ± 1.326 L/min; late, 4.150 ± 1.082 L/min) because of a significant decrease in heart rate (HR) (early, 92 ± 15 min⁻¹; late, 78 ± 12 min⁻¹), even though the stroke volume (SV) remained the same. HH produced a small decrease in the PV, both in the hypobaric chamber and in parabolic flight, indicating a dominating HH effect without a significant effect of GT on PV (−52 ± 34 and −115 ± 32 ml, respectively). Pulmonary tissue volume decreased in the HH conditions because of hypoxic pulmonary vasoconstriction (0.694 ± 0.185 and 0.560 ± 0.207 ml) but increased at 0 and 1.8 G_z in parabolic flight (0.593 ± 0.181 and 0.885 ± 0.458 ml, respectively), indicating that cardiac output and arterial blood pressure rather than HH are the main factors affecting pulmonary vascular regulation in parabolic flight.

Conclusion

HH and GT each lead to specific responses of the cardiovascular system in parabolic flight. Whereas HH seems to be mainly responsible for the PV decrease in flight, GT overrides the hypoxic pulmonary vasoconstriction induced by HH. This finding indicates the need for careful and individual medical examination and, if necessary, health status improvement for each individual considering a parabolic flight, given the effects of the combination of HH and GT in flight.

Electronic supplementary material

The online version of this article (doi:10.1007/s00421-014-2856-3) contains supplementary material, which is available to authorized users.

Regional lung ventilation in humans during hypergravity studied with quantitative SPECT

Recently we challenged the view that arterial desaturation during hypergravity is caused by redistribution of blood flow to dependent lung regions by demonstrating a paradoxical redistribution of blood flow towards non-dependent regions. We have now quantified regional ventilation in 10 healthy supine volunteers at normal and three times normal gravity (1G and 3G). Regional ventilation was measured with Technegas ((99m)Tc) and quantitative single photon emission computed tomography (SPECT). Hypergravity caused arterial desaturation, mean decrease 8%, p<0.05 vs. 1G. The ratio for mean ventilation per voxel for non-dependent and dependent lung regions was 0.81±0.12 during 1G and 1.63±0.35 during 3G (mean±SD), p<0.0001. Thus, regional ventilation was shifted from dependent to non-dependent regions. We suggest that arterial desaturation during hypergravity is caused by quantitatively different redistributions of blood flow and ventilation. To our knowledge, this is the first study presenting high-resolution measurements of regional ventilation in humans breathing normally during hypergravity.

Cerebral circulation during mild +Gz hypergravity by short-arm human centrifuge

We examined changes in cerebral circulation in 15 healthy men during exposure to mild +Gz hypergravity (1.5 Gz, head-to-foot) using a short-arm centrifuge. Continuous arterial pressure waveform (tonometry), cerebral blood flow (CBF) velocity in the middle cerebral artery (transcranial Doppler ultrasonography), and partial pressure of end-tidal carbon dioxide (ETco(2)) were measured in the sitting position (1 Gz) and during 21 min of exposure to mild hypergravity (1.5 Gz). Dynamic cerebral autoregulation was assessed by spectral and transfer function analysis between beat-to-beat mean arterial pressure (MAP) and mean CBF velocity (MCBFV). Steady-state MAP did not change, but MCBFV was significantly reduced with 1.5 Gz (-7%). ETco(2) was also reduced (-12%). Variability of MAP increased significantly with 1.5 Gz in low (53%)- and high-frequency ranges (88%), but variability of MCBFV did not change in these frequency ranges, resulting in significant decreases in transfer function gain between MAP and MCBFV (gain in low-frequency range, -17%; gain in high-frequency range, -13%). In contrast, all of these indexes in the very low-frequency range were unchanged. Transfer from arterial pressure oscillations to CBF fluctuations was thus suppressed in low- and high-frequency ranges. These results suggest that steady-state global CBF was reduced, but dynamic cerebral autoregulation in low- and high-frequency ranges was improved with stabilization of CBF fluctuations despite increases in arterial pressure oscillations during mild +Gz hypergravity. We speculate that this improvement in dynamic cerebral autoregulation within these frequency ranges may have been due to compensatory effects against the reduction in steady-state global CBF.

Pulmonary circulation in extreme environments

The pulmonary circulation is subject to direct challenge from both altered pressure and altered gravity. To efficiently exchange gas, the pulmonary capillaries must be extremely thin-walled and directly exposed to the alveolar space. Thus, alterations in ambient pressure are directly transmitted to the capillaries with the potential to alter pulmonary blood flow. To produce ventilation, the mammalian lung must expand and contract, and so it is a highly compliant structure. Thus, because the capillaries are contained in the alveolar walls, alterations in the apparent gravitational force deform the lung and directly affect pulmonary blood flow both through lung deformation and through changes in the hydrostatic pressure distribution in the lung. High gravitational forces are encountered in the aviation environment, while gravity is absent in spaceflight. Diving subjects the lung to large increases in ambient pressure, while large reductions in pressure occur, often associated with alterations in oxygen level and airway pressure, in aviation. This article reviews the effects of alterations in both gravity and ambient pressure on the pulmonary circulation.

Microgravity decreases and hypergravity increases exhaled nitric oxide

Inhalation of toxic dust during planetary space missions may cause airway inflammation, which can be monitored with exhaled nitric oxide (NO). Gravity will differ from earth, and we hypothesized that gravity changes would influence exhaled NO by altering lung diffusing capacity and alveolar uptake of NO. Five subjects were studied during microgravity aboard the International Space Station, and 10 subjects were studied during hypergravity in a human centrifuge. Exhaled NO concentrations were measured during flows of 50 (all gravity conditions), 100, 200, and 500 ml/s (hypergravity). During microgravity, exhaled NO fell from a ground control value of 12.3 ± 4.7 parts/billion (mean ± SD) to 6.6 ± 4.4 parts/billion ( P = 0.016). In the centrifuge experiments and at the same flow, exhaled NO values were 16.0 ± 4.3, 19.5 ± 5.1, and 18.6 ± 4.7 parts/billion at one, two, and three times normal gravity, where exhaled NO in hypergravity was significantly elevated compared with normal gravity ( P ≤ 0.011 for all flows). Estimated alveolar NO was 2.3 ± 1.1 parts/billion in normal gravity and increased significantly to 3.9 ± 1.4 and 3.8 ± 0.8 parts/billion at two and three times normal gravity ( P < 0.002). The findings of decreased exhaled NO in microgravity and increased exhaled and estimated alveolar NO values in hypergravity suggest that gravity-induced changes in alveolar-to-lung capillary gas transfer modify exhaled NO.

Paradoxical redistribution of pulmonary blood flow in prone and supine humans exposed to hypergravity

We hypothesized that exposure to hypergravity in the supine and prone postures causes a redistribution of pulmonary blood flow to dependent lung regions. Four normal subjects were exposed to hypergravity by use of a human centrifuge. Regional lung perfusion was estimated by single-photon-emission computed tomography (SPECT) after administration of (99m)Tc-labeled albumin macroaggregates during normal and three times normal gravity conditions in the supine and prone postures. All images were obtained during normal gravity. Exposure to hypergravity caused a redistribution of blood flow from dependent to nondependent lung regions in all subjects in both postures. We speculate that this unexpected and paradoxical redistribution is a consequence of airway closure in dependent lung regions causing alveolar hypoxia and hypoxic vasoconstriction. Alternatively, increased vascular resistance in dependent lung regions is caused by distortion of lung parenchyma. The redistribution of blood flow is likely to attenuate rather than contribute to the arterial desaturation caused by hypergravity.

Pattern of diffusion disturbance related to clinical diagnosis: The K(CO) has no diagnostic value next to the DL(CO)

AIM OF THE STUDY

The diffusion capacity of the lung for carbon monoxide (DL(CO)) is an important tool in the diagnosis and follow-up of patients with pulmonary diseases. In case of a decreased DL(CO) the K(CO), defined as DL(CO)/V(A) (V(A) is alveolar volume), can differentiate between normal alveolocapillary membrane (normal K(CO)) and abnormal alveolocapillary membrane (low K(CO)). The latter category consists of decreased surface of the membrane, increased thickness or decreased perfusion of ventilated alveoli. The V(A)/TLC (TLC is total lung capacity determined by whole body plethysmography) can partially differentiate between these categories. The aim of this study was to investigate the diagnostic value of the specific diffusion disturbances, which can be constructed by combining the DL(CO), K(CO) and V(A)/TLC.

METHODS

In 460 patients the diagnosis made by clinicians were fitted into five diagnostic categories: asthma, chronic obstructive pulmonary disease (COPD), treatment effects of haematologic malignancies, heart failure and diffuse parenchymal lung diseases (DPLD). These categories were linked to the pattern of diffusion disturbance.

RESULTS

Almost all patients with asthma have a normal DL(CO), most patients in the other groups do not have the expected pattern of diffusion disturbance, especially in the group with DPLD a bad match is observed.

CONCLUSION

In this study the pattern of diffusion disturbance is of limited use in establishing a diagnosis. The use of the K(CO) next to the DL(CO) has no additional diagnostic value. Regional ventilation-perfusion inequality probably forms an important underlying mechanism of decreased DL(CO).

Effects of gravity and blood volume shifts on cardiogenic oscillations in respired gas

During the cardiac cycle, cardiogenic oscillations of expired gas (x) concentrations (COS([x])) are generated. At the same time, there are heart-synchronous cardiogenic oscillations of airway flow (COS(flow)), where inflow occurs during systole. We hypothesized that both phenomena, although primarily generated by the heartbeat, would react differently to the cephalad blood shift caused by inflation of an anti-gravity (anti-G) suit and to changes in gravity. Twelve seated subjects performed a rebreathing-breath-holding-expiration maneuver with a gas mixture containing O2 and He at normal (1 G) and moderately increased gravity (2 G); an anti-G suit was inflated to 85 mmHg in each condition. When the anti-G suit was inflated, COS(flow) amplitude increased (P = 0.0028) at 1 G to 186% of the control value without inflation (1-G control) and at 2 G to 203% of the control value without inflation (2-G control). In contrast, the amplitude of COS of the concentration of the blood-soluble gas O2 (COS([O2/He])), an index of the differences in pulmonary perfusion between lung units, declined to 75% of the 1-G control value and to 74% of the 2-G control value (P = 0.0030). There were no significant changes in COS(flow) or COS([O2/He]) amplitudes with gravity. We conclude that the heart-synchronous mechanical agitation of the lungs, as expressed by COS(flow), is highly dependent on peripheral-to-central blood shifts. In contrast, COS([blood-soluble gas]) appears relatively independent of this mechanical agitation and seems to be determined mainly by differences in intrapulmonary perfusion.

Cardiovascular effects of anti-G suit inflation at 1 and 2 G

We sought to determine to which pressure a full-coverage anti-G suit needs to be inflated in order to obtain the same stroke volume during a brief exposure to twice the normal gravity (2 G) as that at 1 G without anti-G suit inflation. Nine sitting subjects were studied at normal (1 G) and during 20 s of exposure to 2 G. They wore anti-G suits, which were inflated at both G-levels to the following target pressures: 0, 70, 140 and 210 mmHg. Stroke volume was computed from cardiac output, which was measured by rebreathing. Heart rate and mean arterial pressure at heart level were recorded. Inflation to 70 mmHg compensated for the decrease in stroke volume and cardiac output caused by hypergravity. Mean arterial pressure at heart level was comparable at 1 G and at 2 G and increased gradually and similarly with inflation (P<0.001) at both gravity levels. Thus, anti-G suits act by increasing both preload and afterload but the two effects counteract each other in terms of cardiac output, so that cardiac output at 2 G is maintained at its 1 G level. This effect is reached already at 70 mmHg of inflation. Greater inflation pressure further increases mean arterial pressure at heart level and compensates for the increased difference in hydrostatic pressure between heart and head in moderate hypergravity.

Effects of hypergravity on the distributions of lung ventilation and perfusion in sitting humans assessed with a simple two-step maneuver

Increased gravity impairs pulmonary distributions of ventilation and perfusion. We sought to develop a method for rapid, simultaneous, and noninvasive assessments of ventilation and perfusion distributions during a short-duration hypergravity exposure. Nine sitting subjects were exposed to one, two, and three times normal gravity (1, 2, and 3 G) in the head-to-feet direction and performed a rebreathing and a single-breath washout maneuver with a gas mixture containing C2H2, O2, and Ar. Expirograms were analyzed for cardiogenic oscillations (COS) and for phase IV amplitude to analyze inhomogeneities in ventilation (Ar) and perfusion [CO2-to-Ar ratio (CO2/Ar)] distribution, respectively. COS were normalized for changes in stroke volume. COS for Ar increased from 1-G control to 128 ± 6% (mean ± SE) at 2 G ( P = 0.02 for 1 vs. 2 G) and 165 ± 13% at 3 G ( P = 0.002 for 2 vs. 3 G). Corresponding values for CO2/Ar were 135 ± 12% ( P = 0.04) and 146 ± 13%. Phase IV amplitude for Ar increased to 193 ± 39% ( P = 0.008) at 2 G and 229 ± 51% at 3 G compared with 1 G. Corresponding values for CO2/Ar were 188 ± 29% ( P = 0.02) and 219 ± 18%. We conclude that not only large-scale ventilation and perfusion inhomogeneities, as reflected by phase IV amplitude, but also smaller-scale inhomogeneities, as reflected by the ratio of COS to stroke volume, increase with hypergravity. Except for small-scale ventilation distribution, most of the impairments observed at 3 G had been attained at 2 G. For some of the parameters and gravity levels, previous comparable data support the present simplified method.

Physiological evaluation of a new quantitative SPECT method measuring regional ventilation and perfusion

We have developed a new quantitative single-photon-emission computed tomography (SPECT) method that uses (113m)In-labeled albumin macroaggregates and Technegas ((99m)Tc) to estimate the distributions of regional ventilation and perfusion for the whole lung. The multiple inert-gas elimination technique (MIGET) and whole lung respiratory gas exchange were used as physiological evaluations of the SPECT method. Regional ventilation and perfusion were estimated by SPECT in nine healthy volunteers during awake, spontaneous breathing. Radiotracers were administered with subjects sitting upright, and SPECT images were acquired with subjects supine. Whole lung gas exchange of MIGET gases and arterial Po(2) and Pco(2) gases was predicted from estimates of regional ventilation and perfusion. We found a good agreement between measured and SPECT-predicted exchange of MIGET and respiratory gases. Correlations (r(2)) between SPECT-predicted and measured inert-gas excretions and retentions were 0.99. The method offers a new tool for measuring regional ventilation and perfusion in humans.

Effects of gravity on lung diffusing capacity and cardiac output in prone and supine humans

Both in normal subjects exposed to hypergravity and in patients with acute respiratory distress syndrome, there are increased hydrostatic pressure gradients down the lung. Also, both conditions show an impaired arterial oxygenation, which is less severe in the prone than in the supine posture. The aim of this study was to use hypergravity to further investigate the mechanisms behind the differences in arterial oxygenation between the prone and the supine posture. Ten healthy subjects were studied in a human centrifuge while exposed to 1 and 5 times normal gravity (1 G, 5 G) in the anterioposterior (supine) and posterioanterior (prone) direction. They performed one rebreathing maneuver after approximately 5 min at each G level and posture. Lung diffusing capacity decreased in hypergravity compared with 1 G (ANOVA, P = 0.002); it decreased by 46% in the supine posture compared with 25% in the prone (P = 0.01 for supine vs. prone). At the same time, functional residual capacity decreased by 33 and 23%, respectively (P < 0.001 for supine vs. prone), and cardiac output by 40 and 31% (P = 0.007 for supine vs. prone), despite an increase in heart rate of 16 and 28% (P < 0.001 for supine vs. prone), respectively. The finding of a more impaired diffusing capacity in the supine posture compared with the prone at 5 G supports our previous observations of more severe arterial hypoxemia in the supine posture during hypergravity. A reduced pulmonary-capillary blood flow and a reduced estimated alveolar volume can explain most of the reduction in diffusing capacity when supine.