Chest volume and shape and intrapleural pressure in microgravity

The following is the abstract of the article discussed in the subsequent letter:

	ABSTRACT

White, Ronald J., and C. Gunnar Blomqvist. Central venous pressure and cardiac function during spaceflight. J. Appl. Physiol. 85(2): 738-746, 1998.Early in spaceflight, an apparently paradoxical condition occurs in which, despite an externally visible headward fluid shift, measured central venous pressure is lower but stroke volume and cardiac output are higher, and heart rate is unchanged from reference measurements made before flight. This paper presents a set of studies in which a simple three-compartment, steady-state model of cardiovascular function is used, providing insight into the contributions made by the major mechanisms that could be responsible for these events. On the basis of these studies, we conclude that, during weightless spaceflight, the chest relaxes with a concomitant shape change that increases the volume of the closed chest cavity. This leads to a decrease in intrapleural pressure, ultimately causing a shift of blood into the vessels of the chest, increasing the transmural filling pressure of the heart, and decreasing the central venous pressure. The increase in the transmural filling pressure of the heart is responsible, through a Starling-type mechanism, for the observed increases in heart size, left ventricular end-diastolic volume, stroke volume, and cardiac output.

	LETTER

Chest volume and shape and intrapleural pressure in microgravity

To the Editor: In their theoretical modeling study, White and Blomqvist (5) concluded that "during weightless spaceflight, the chest relaxes with a concomitant shape change that increases the volume of the closed chest cavity. This leads to a decrease in intrapleural pressure ... decreasing the central venous pressure." However, the authors do not refer to experimental data that have been reported on these topics, and these data do not appear to support their conclusions.

Lung volume measurements have not been made in the transition from 1 G to microgravity (µG) in subjects in the supine position with legs raised (i.e., the posture during launch). However, Elliott et al. (2) showed that, in the usual supine position, µG causes an increase in lung volume of only ~15% or 500 ml compared with 1 G. In the standing posture, µG causes a reduction in lung volume of about the same amount compared with 1 G (2), and essentially the same decrease was reported in seated subjects by Paiva et al. (4).

Shape changes of the chest in µG compared with 1 G in seated subjects were reported by Michels et al. (3), who found a modest headward movement of the diaphragm and widening of the rib cage.

Intrapleural pressure has not been measured directly, but a change in lung volume of ~500 ml would cause the pressure to change by <2 mmHg if lung compliance is normal. Consistent with this, Edyvean et al. (1) measured esophageal pressure in the subjects in the seated posture and showed that the difference between 1 G and µG was <1 mmHg.

Thus the conclusion from the experimental data is that the change in intrapleural pressure appears to be far too small to explain the observed reduction in central venous pressure of ~9 mmHg.

	REFERENCES

1.   Edyvean, J., M. Estenne, M. Paiva, and L. A. Engel. Lung and chest wall mechanics in microgravity. J. Appl. Physiol. 71: 1956-1966, 1991[Abstract/Free Full Text].

2.   Elliott, A. R., G. K. Prisk, H. J. Guy, and J. B. West. Lung volumes during sustained microgravity on Spacelab SLS-1. J. Appl. Physiol. 77: 2005-2014, 1994[Abstract/Free Full Text].

3.   Michels, D. B., P. J. Friedman, and J. B. West. Radiographic comparison of human lung shape during normal gravity and weightlessness. J. Appl. Physiol. 47: 851-857, 1979[Abstract/Free Full Text].

4.   Paiva, M., M. Estenne, and L. A. Engel. Lung volumes, chest wall configuration, and pattern of breathing in microgravity. J. Appl. Physiol. 67: 1542-1550, 1989[Abstract/Free Full Text].

5.   White, R. J., and C. G. Blomqvist. Central venous pressure and cardiac function during spaceflight. J. Appl. Physiol. 85: 738-746, 1998.

John B. West, G. Kim Prisk, Department of Medicine University of California, San Diego La Jolla, California 92093-0623