INTRODUCTION

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IN NORMAL GRAVITY, LUNG DIFFUSING capacity (DL_CO), pulmonary capillary blood volume, lung tissue volume (LTV), and central blood volume change in concert during, for example, transitions between supine and upright postures and between rest and exercise (4, 10, 11, 21). This is so, because with increased central blood volume and cardiac output in supine compared with upright or in exercise compared with rest, there is a more homogenous distribution of pulmonary capillary blood volume in relation to the alveolar volume in the human lung (5). By analogy, therefore, the peripheral blood pooling and the decreased cardiac output that occur when sitting humans are exposed to hypergravity might be expected to be accompanied by similar decreases of pulmonary capillary blood volume, DL_CO, and LTV. Indeed, arterial desaturation is found in sitting humans in hypergravity (1, 9), pointing to an impairment of the lung as a gas exchanger. However, we reasoned that a simple relationship between DL_CO and LTV may not be found if some of the loss of effective circulating blood volume is due to pooling in the dependent parts of the lung circulation and not only in their systemic counterparts. We hypothesized that LTV then would increase despite a falling DL_CO, the latter caused by uneven distribution of pulmonary capillary blood. To test this hypothesis, we determined DL_CO and LTV during short-lasting hypergravity and we found differential changes of DL_CO and LTV in support of the notion of significant sequestration of blood in the pulmonary circulation in hypergravity.

	MATERIALS AND METHODS

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Subjects. Seven men and two women were studied. Their ages, heights, and body masses ranged from 22 to 32 yr, 169 to 193 cm, and 64 to 90 kg, respectively. They had no history of cardiopulmonary disease, and they were not taking any medication at the time. They were also instructed not to drink coffee or use nicotine-containing products on the day of the experiment. The experimental protocol used in the present study had been approved by the Ethics Committee of Karolinska Institutet.

Equipment and measurements. The experiments were conducted in the human centrifuge at Karolinska Institutet, Stockholm, Sweden. The rotational radius to the center of the centrifuge gondola was 7.2 m, and the roll angle of the gondola was automatically adjusted so that the gondola floor was perpendicular to the resultant of the normal G vector and the centrifugal G vector. Because of the 28° tilt of the backrest, the magnitude of the G vector in the head-to-feet direction (G_z+) was in reality 1 (0.88 g), 2 (1.77 g), and 3 g · cos 28° (2.65 g), respectively. The small errors introduced by rounding off to the nearest integer for G have been neglected throughout the text. Multiple slip rings at the center of rotation allowed for audiovisual monitoring, power supply, and transmission of physiological signals between the gondola and a control room. The instrumentation for respiratory measurements included a quadrupole mass spectrometer (AMIS 2000, Innovision, Odense, Denmark) and a manually operated rotary valve assembly with a 4-liter rebreathing bag. The subjects breathed through a mouthpiece and wore a nose clip. Between the mouthpiece and the rotary valve there was a unidirectional impeller flowmeter (KL Engineering, Northridge, CA) for measurements of expired flow and an inlet for gas sampling through a 10-m-long capillary tube to the mass spectrometer located at the center of the centrifuge. The volume of the instrumental dead space was 150 ml. Electrocardiogram was monitored from chest electrodes with a clinical monitoring system (type AS2, Datex, Helsinki, Finland). An accelerometer was positioned at heart level behind the backrest of the seat. Continuous signals were recorded at 20 Hz per channel in a digital data-acquisition system. Before and after each experiment, the mass spectrometer was calibrated against gases of known composition. The flowmeter was calibrated with a 3-liter syringe within the experimental flow range. The response latency of the mass spectrometer was determined from a sudden, simultaneous change of gas composition and flow direction at the inlet of the sampling capillary. The latency between the flowmeter and the mass spectrometer signals was ~3 s. The 95% rise time for the response to a step change in gas concentration was 150 ms. Light diodes were located at 60° lateral in the visual field of the subject. Subjects were instructed to report if their peripheral vision was decreased as an early sign of cerebral hypoperfusion.

Experimental procedures. The subjects came to the laboratory on two occasions, a first session for familiarization and a few days later for the experiment. The experiments were performed at 1, 2, and 3 G_z+, four times at each level and with a randomized order between G levels.

Data analysis. After intermediate storage in the AMIS 2000 system, off-line data analysis was performed with an Acknowledge 3.2 Biopac digital data handling system (Biopac, Goleta, CA). Off-line computations included algorithms for total dry pressure correction (23) and computation of calibrated values for all dry gas fractional concentrations. Also, concentration readings were corrected for the response latency of the mass spectrometer system, and gas volumes and flows were converted to BTPS (body temperature, ambient pressure, saturated with water vapor) when appropriate. During the REB maneuver the DL_CO was calculated from the rate of uptake of C¹⁸O and the pulmonary capillary blood flow was calculated as proportional to the uptake of C₂/H₂ (7, 20-22) (Fig. 1). We considered the values of pulmonary capillary blood flow to be equivalent to cardiac output because pure intrapulmonary shunting is negligible in normal subjects also at 3 G_z+ (19). The intercept of the CO regression line with the initially inspired CO level was used to define the instant when the inhaled gas mixture reached the alveoli, i.e., the onset of alveolocapillary exchange of the inhaled foreign gases or time zero (21) (Fig. 1). LTV was estimated as described by Sackner et al. (21) from the intercept of the back-extrapolated C₂H₂ disappearance curve with the ordinate at time zero (Fig. 1). FRC was calculated from the dilution of the insoluble gas Ar in the lung-bag system volume, where all Ar readings were offset by the Ar concentration in atmospheric air. O₂ uptake was calculated from the linear slope of end-tidal O₂ values during rebreathing and the lung-bag system volume (2).

View larger version (17K): [in this window] [in a new window]   Fig. 1.   Time courses of the blood-soluble gases acetylene (C2H2) and carbon monoxide (C18O) during rebreathing in 1 subject at 3 times normal gravity. FA is the instantaneous soluble gas concentration and FAO is the corresponding concentration initially inspired from the rebreathing bag. All data are normalized for concomitant concentrations of an insoluble gas component to adjust for gas mixing and volume shrinkage due to exchange of metabolic gases.

Statistical techniques. ANOVA (STATISTICA 5.5, Statsoft, Tulsa, OK) with repeated measures design with one dependent factor (G_z+) was used to test for differences between changing G levels with respect to respiratory variables. Planned comparison was used as post hoc test. The two contrasts used were linear and quadratic. Results were considered statistically significant if P < 0.05, and all tests were two sided. Data are presented as mean and SE, if not otherwise stated. To obtain percent changes, data were normalized to the average 1-G_z+ value of each subject.

	RESULTS

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Eight subjects completed the experiments, and one subject performed only one-half of the maneuvers because of nausea. No subject reported decreased peripheral vision. Figures 2 and 3 are typical recordings at 3 G_z+, illustrating the fact that no end-expiratory concentration deviations (phase IV phenomena) could be identified in any of the analyzed gases during the slow exhalation to residual volume at the end of the rebreathing. Cardiopulmonary variables are shown in Table 1. Data on LTV and DL_CO in percentage of 1 G_z+ control are shown in Fig. 4. All variables changed significantly with increased G level, except for FRC, which remained within 3% of control in both hypergravity conditions. Cardiac output was decreased by 11% at 2 G_z+ and by 16% at 3 G_z+ compared with 1-G_z+ control. DL_CO was reduced by 20 and 33% and LTV was increased by 26 and 38% at 2 G_z+ and 3 G_z+, respectively. All subjects had increased LTV values and decreased DL_CO values at 3 G_z+ compared with control. O₂ uptake was increased by 5% at 2 G_z+ and 16% at 3 G_z+. The arteriovenous O₂ difference computed as O₂ uptake/cardiac output increased by 19 and 39%, respectively (P < 0.001). All variables, except for FRC, were demonstrated as a linear effect in the planned comparison.

View larger version (9K): [in this window] [in a new window]   Fig. 2.   Typical recording at 3 times normal gravity of the insoluble gas Ar during the rebreathing maneuver and the following slow expiration to residual volume at the end of the maneuver. Note that there is an artifact caused by presence of air in the rotary valve unit, indicating the transition from the end of the rebreathing maneuver to the start of the slow expiration into the cabin air. The lack of phase IV phenomena is indicating a homogenous distribution of Ar throughout the lung also during hypergravity.

View larger version (9K): [in this window] [in a new window]   Fig. 3.   Typical recording of the soluble gas acetylene (C2H2) during the same conditions as in Fig. 3.

View larger version (10K): [in this window] [in a new window]   Fig. 4.   Lung diffusing capacity (DLCO) and lung tissue volume (LTV) in percentage of control ± SE as a function of gravity in the head-to-feet direction (Gz+). Data from 2 and 3 Gz+ are from the present study. Data from microgravity (0 G) are from Ref. 24 and were obtained during parabolic flight. LTV data were derived from pulmonary capillary blood volume data (VC), assuming an ~4:1 relationship between LTV and VC at 1-Gz+ sitting rest (21). Because of this computation, no error bars for LTV at 0 G are included in the figure.

	DISCUSSION

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The major finding in the present study was the differential responses of lung diffusing capacity and lung tissue volume with increased G_z+ in resting humans. The findings of decreased cardiac output and maintained FRC with increased G_z+ are in agreement with previous studies by Rosenhamer (19) and Glaister (8), respectively.

Although we are not aware of any previous determinations of DL_CO in hypergravity, we expected from previously demonstrated hypergravity-induced impairments of the upright human lung as a gas exchanger (1, 8, 9, 19) that DL_CO would be reduced in hypergravity. Accordingly, DL_CO was gradually reduced by up to 33% in the present 3-G_z+ experiments. The hypergravity-induced decrements in DL_CO (Fig. 1) can be compared with corresponding measurements in weightlessness. Verbanck et al. (25) determined DL_CO in four astronauts using an identical rebreathing technique; DL_CO was increased by 12% in sustained weightlessness compared with upright ground control, and because FRC was reduced, the concomitant increase of DL_CO per unit alveolar volume was 32%. Prisk et al. (18) obtained similar data in sustained weightlessness but with a slightly different method; DL_CO was determined during breath holding at total lung capacity (TLC). Vaïda et al. (24), also determining DL_CO after breath holding at TLC, found a much larger increase of DL_CO per unit alveolar volume in transient weightlessness during parabolic flight, 66% compared with 1 G_z+. Because the DL_CO measurements performed in sustained microgravity (18, 25) were obtained 24 h or more into the microgravity period they are not necessarily comparable with those obtained during more transient G_z+ changes, such as in parabolic flight (24) and the present study. Thus it cannot be excluded that the initial DL_CO response in microgravity is a very large increase followed by a decrease to a lower level as plasma volume is reduced (14) as a consequence of adaptation to microgravity. Therefore, in Fig. 4 we used the data of Vaïda et al. (24) for comparison with the present data. Taken together, these studies of DL_CO vs. G_z+ indicate that in the G_z+ range 0-3 g, alveolocapillary diffusion of CO is a relatively simple function of gravity, although not linear. Just as the improvement of DL_CO in weightlessness has been ascribed to a more homogeneous distribution of alveolar gas volume in relation to pulmonary capillary blood volume (18, 25), the present marked decrements of DL_CO at 2 and 3 G_z+ can be ascribed to a less homogeneous distribution of alveolar volume with respect to pulmonary capillary blood volume. The physiological consequence of the reduced diffusing capacity was demonstrated by Rosenhamer (19), who showed that the alveolar-to-arterial PO₂ difference in resting, sitting men was almost doubled from 14 Torr at 1 G_z+ to 24 Torr at 3 G_z+. Corresponding data from weightlessness have so far not been obtained.

From the above-demonstrated inverse relationship between G_z+ and DL_CO it might be expected that, at the same time, the filling of the lung capillaries would be reduced. This is so, because at normal gravity, DL_CO, pulmonary capillary blood volume, central blood volume, and cardiac output change in concert, for example, during transitions between upright and supine and between rest and exercise (4, 10, 11, 21). Rosenhamer (19) studied sitting humans in 1-3 G_z+ and used central venous injection of indocyanine green and continuous radial arterial sampling and spectrometric analysis of blood to determine cardiac output, and he found a shortened transfer time for passage of dye through the interposed vascular segment, which included part of the axillary vein, the vena cava, the heart, the lung circulation, a section of the aorta, and the brachial and radial arteries. Rosenhamer cautioned that data could be biased by changes in transfer time in the arterial section. Nevertheless the data could be compatible with a reduced central blood volume at 3 G_z+ in sitting resting men, which seemed reasonable considering concomitant signs of peripheral venous pooling, such as a 50% reduction in stroke volume. Also, in another study from the same laboratory, Linnarsson and Rosenhamer (15) showed that at 3 G_z+, sitting resting subjects gradually developed arterial hypotension, which was instantaneously normalized when the subjects started to perform light, dynamic leg exercise, which markedly improved venous return by the action of the muscle pump (6, 13).

The above findings, all in favor of peripheral blood pooling in dependent parts of the body in resting humans at increased G_z+, appear to be at variance with the present finding of an increased LTV. The most rapidly changing component of LTV is the pulmonary capillary blood volume, and, therefore, the present finding of increased LTV after only minutes of exposure to elevated G_z+ can only have been caused by an equally increased pulmonary capillary blood volume. This is assuming that there is no rapidly developing lung tissue edema during the short-lasting high-G periods. The absence of time-dependent trends in LTV at normal G between the high-G runs speaks strongly against development of lung tissue edema in the present experiments.

Under normal 1-G_z+ conditions, an increased LTV could be expected to be associated with an overall increase of central blood volume. In the present high-G experiments, however, we propose that there is sequestration of blood in the dependent parts of the lung circulation, just as there is in the dependent part of the systemic circulation. The present data and those of Rosenhamer (19) taken together suggest that during hypergravity there were marked reductions of the blood volume in other segments of the central blood volume than in the pulmonary capillaries outweighing the ~200-ml increase of pulmonary capillary blood volume.

Effects of inhomogeneity. It should be considered whether the present findings of differential responses of DL_CO and LTV during hypergravity in sitting men are merely artifacts resulting from increased inhomogeneities of pulmonary ventilation and perfusion. We have, however, several reasons to believe that this is not the case.