Pulmonary diffusing capacity, capillary blood volume, and cardiac output during sustained microgravity

Abstract

We measured pulmonary diffusing capacity (DL), diffusing capacity per unit lung volume, pulmonary capillary blood volume (Vc), membrane diffusing capacity (Dm), pulmonary capillary blood flow or cardiac output (Qc), and cardiac stroke volume (SV) in four subjects exposed to 9 days of microgravity (weightlessness, 0 G). The same subjects were studied standing and supine numerous times preflight and in the week immediately after return from space. DL in microgravity was elevated (28%) compared with preflight standing values and was higher than preflight supine because of the elevation of both Vc (28%) and Dm (27%). The elevation in Vc was comparable to that measured supine in 1 G, but the increase in Dm was in sharp contrast to the supine value (which was unchanged). We postulate that, in 0 G, pulmonary capillary blood is evenly distributed throughout the lung, providing for uniform capillary filling, leading to an increase in the surface area available for diffusion. By contrast, in the supine 1-G state, the capillaries are less evenly filled, and although a similar increase in blood volume is observed, the corresponding increase in surface area does not occur. DL and its subdivisions showed no adaptive changes from the first measurement 24 h after the start of 0 G to 8 days later. Similarly, there were no trends in the postflight data, suggesting that the principal mechanism of these changes was gravitational. The increase in Dm suggests that subclinical pulmonary edema did not result from exposure to 0 G. Qc was modestly increased (18%) inflight and decreased (9%) post-flight compared with preflight standing. Compared with preflight standing, SV was increased 46% inflight and decreased 14% in the 1st wk postflight. There were temporal changes in Qc and SV during 0 G, with the highest values recorded at the first measurement, 24 h into the flight. The lowest values of Qc and SV occurred on the day of return.

Effect of 60° head-down tilt on peripheral gas mixing in the human lung

“…However, the longest period of time for which G DL CO data are available is 10 days (50); whether an elevated DL CO will persist after several weeks or months in G is not presently known. It does not seem unreasonable to speculate that "subclinical" fluid accumulation in the pulmonary interstitial matrix may be reflected in the changes in SBW and MBW observed during steep HDT, and perhaps those observed during G. However, this is not to say that either HDT or spaceflight results in alveolar fluid accumulation (e.g., pulmonary edema), for indeed the evidence to date does not indicate that pulmonary edema occurs in humans during either HDT or spaceflight (42).…”

“…However this may not provide a straightforward explanation, because acute changes in body position, e.g., upright to supine, are more generally found to increase DL CO (42,45). However, the acute increases in DL CO appear primarily due to an increase in pulmonary capillary blood volume and to a lesser extent to changes at the membrane level (42,45). Thus it is possible, despite evidence of acute increases in DL CO , that an increase in lung water content, i.e., fluid retention in the pulmonary interstitium, could occur without overt clinical symptoms over the 60 min of steep HDT.…”

“…The observation that significant reductions in the diffusing capacity for carbon monoxide (DL CO ) occur during long-term 6°head-down bed rest (23,30,44) is compatible with the notion of greater lung water during HDT. However this may not provide a straightforward explanation, because acute changes in body position, e.g., upright to supine, are more generally found to increase DL CO (42,45). However, the acute increases in DL CO appear primarily due to an increase in pulmonary capillary blood volume and to a lesser extent to changes at the membrane level (42,45).…”

Microgravity and the respiratory system

“…As the lung receives virtually the entire cardiac output, it provides a useful window into cardiac function, something that has been exploited extensively [43][44][45]. In summary, cardiac output is elevated (compared with standing) by ,35% after 1 day in microgravity due to a large (60-70%) increase in stroke volume and a concomitant bradycardia.…”

Vasorelaxation in Space

“…2,3 In these studies, blood pressure was not measured. In another study, Fritsch-Yelle et al 4 observed in 12 astronauts over several flights that the mean 24-hour diastolic arterial pressure, but not systolic pressure, was significantly decreased in space by some 5 mm Hg.…”