Rib cage shape and motion in microgravity MARC ESTENNE, MASSIMO GORINI, ALAIN VAN MUYLEM, VINCENT NINANE, AND MANUEL PAIVA Respiratory Research Unit, Chest Service, and Institute of Interdisciplinary Research, Erasme University Hospital, Brussels School of Medicine, 1070 Brussels, Belgium

ESTENNE, MARC, MASSIMO GORINI, ALAIN VAN MUYLEM, VINCENT NINANE, AND MANUEL PAIVA. Rib cageshape and motion in microgravity. J. Appl. Physiol. 73(3): 946-954, 1992.We studied the effect of microgravity (0 G,) on the anteroposterior diameters of the upper (URC-AP) and lower (LRC-AP) rib cage, the transverse diameter of the lower rib cage (LRC-TR), and the xiphipubic distance and on the electromyographic (EMG) activity of the scalene and parasternal intercostal muscles in five normal subjects breathing quietly in the seated posture. Gastric pressure was also recorded in four subjects. At 0 G,, end-expiratory LRC-AP and xiphipubic distance increased but LRC-TR invariably decreased, as did end-expiratory gastric pressure. No consistent effect was observed on tidal LRCTR and xiphipubic displacements, but tidal changes in URCAP and LRC-AP were reduced. Although scalene and parasternal phasic inspiratory EMG activity tended to decrease at 0 G,, both muscle groups demonstrated an increase in tonic activity. We conclude that during brief periods of weightlessness 1) the rib cage at end expiration is displaced in the cranial direction and adopts a more circular shape, 2) the tidal expansion of the ventral rib cage is reduced, particularly in its upper portion, and 3) the scalenes and parasternal intercostals generally show a decrease in phasic inspiratory EMG activity and an increase in tonic activity. weightlessness; pressure

space

medicine;

respiratory

muscles;

gastric

is known to exert a major influence on the respiratory system (1,ll). Previous studies during immersion (19) and during changes from the upright (1 G,) to the supine (1 G,) posture have demonstrated substantial alterations in rib cage configuration and in the pattern of rib cage motion during quiet breathing. However, no information is available on the way that rib cage mechanics are influenced by gravitational unloading. In two previous studies (10, 15) we have described the effects of weightlessness on lung and chest wall mechanics. In this paper we extend this work to include measurements of rib cage shape, rib cage motion, and pattern of scalene and parasternal intercostal muscle activation in microgravity. GRAVITY

METHODS

The effect of microgravity on rib cage shape and motion was studied in five normal male subjects seated in a Caravelle 6 R aircraft. The trajectory flown by the aircraft and the pattern of G, changes are illustrated in Figs. 1 and 2. The parabola commenced with a period of in946

0161-7567/92

$2.00 Copyright

0

creased gravity lasting -20 s and reaching - 1.8 G, (this period will be referred to as 2 G,). The onset of the subsequent period of microgravity (0 G,) was abrupt, reaching 0.1 G, within 3 s. The duration of-microgravity was --20 s. There were three flights on consecutive days. Each flight lasted -2.5 h and consisted of 30 parabolas. The five subjects studied were physiologists trained in respiratory maneuvers and two of them (ME and MP) had previous experience in parabolic flights. Two subjects were studied on each of the first two flights, one subject changing places with the other during the 6-min interval at 1 G, after the 15th parabola. The last subject was studied on the third flight. Great care was taken to stabilize body position and minimize changes in spinal attitude with changes in gravity. Subjects were seated in an armchair with a firm back and headrest. They were secured by a lap belt and by strapping of the thighs, legs, and arms to the frame of the chair. In addition, the upper part of the dorsal spine was supported by a metal frame moulded to the natural shape of the trunk at 1 G,, and cranial motion of the trunk was prevented by metal rods positioned above the shoulders. The back in the lumbar region was also supported by a rounded metal frame. Flow was measured at the mouth with a no. 3 Fleish pneumotachograph and a Validyne pressure transducer and was integrated to give tidal volume (VT).The transducer was oriented vertically with the membrane along the longitudinal axis of the aircraft. The flow signal was frequently zeroed electrically during brief breath-hold periods. The dead space of the mouthpiece and flowmeter was increased to 200 ml to enhance VT. Three pairs of linearized magnetometers were attached slightly to the right of the midline to measure the anteroposterior diameter of the upper rib cage (URCAP) at the level of the manubrium sterni, the anteroposterior diameter of the lower rib cage (LRC-AP) at the level of the fifth intercostal space, and the distance between the xiphoid process and the pubis. The axes of these coils were oriented horizonta JlY A fourth pair of magn etometers was used t ‘0 measure the transverse dia meter of the lower rib cage (LRC-TR); it was attached 1 cm anterior to the midaxil lary lin .e(anterior to the latissimus dorsi) and 3-5 cm caudal to the coils used to measure the LRC-AP diameter. The axes of the coils measuring the transverse di ameter were oriented vertically. All coils were attached to the body surface using twosided adhesive disks. Before the flight, plots of voltage

1992 the American

Physiological

Society

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 10, 2019.

RIB CAGE MECHANICS Altitude (meters)

8000

I I 0

I

I I

I I

I 1.6 G, during the first hypergravity period and < 0.05 G, during the microgravity period; 3) a minimum of four breaths at each gravity level; and 4) quiet tidal breathing throughout the parabola. These criteria permitted the selection of 5-11 parabolas in each subject. The measurements for hypergravity were selected during the period of increased acceleration before weightlessness. The changes in rib cage dimensions and gastric pressure at end expiration were computed from the 0- and 2-G, periods that showed at least three to four breaths with stable end-expiratory levels. Tidal changes in rib cage dimensions were normalized by VT. The height of the integrated EMG activity was measured in arbitrary units above electrical zero to give tonic activity, which was taken as the plateau of EMG activity during expiration. Phasic inspiratory activity was calculated as the amplitude of the deflection occurring simultaneously with inspiration. This deflection was also measured in arbitrary units and was normalized by VT. The effect of G, was tested using a repeated-measures analysis of variance. When the F ratio was statistically significant, a modified t test was used to assess the difference between 1 vs. 2 G, and 1 vs. 0 G,. The level of statistical significance was taken as P < 0.05. Unless otherwise specified, data are reported as means t SE. 2

I

Gz

l-

UK-AP

1 cm

LRC-AP

lcm[rn

LRC-Tr

lun[C

XIPHI-PIBIC 1 cm 1

4 20 set

FIG. 2. Changes in acceleration (G,), lung volume (VL), and rib cage dimensions during quiet tidal breathing in a representative parabola. URC-AP, upper rib cage anteroposterior diameter; LRC-AP, lower rib cage anteroposterior diameter; LRC-TR, lower rib cage transverse diameter. See text for discussion.

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 10, 2019.

948 TABLE

RIB CAGE MECHANICS

IN WEIGHTLESSNESS

1. Effect of gravity on rib cage dimensions at end expiration Subjects

Dimension,

cm

URC-AP

G*

VN

ME

AM

MG

MP

0

18.1

1

17.8 17.5"

16.6” 17.8 18.2” 24.6' 24.0 24.1' 3o.ot‘ 30.1 30.5* 31.6" 29.9 29.4*

NA NA NA

17.7* 17.8 17.8 22.8" 22.3 22.3 30.7" 31.7 32.3" 30.6" 29.6 29.1”

16.2” 15.9 15.7’ 22.0' 21.7 21.8 32.1" 32.2 32.6* 29.2* 28.5 28.5

2

LRC-AP LRC-TR

0

21.9”

1

2

21.5 21.4

0

31.3”

1

31.8 31.9t

2

Xiphipubic

0

26.8" 26.4 26.4

1

2

23.0 23.0 23.4* 28.2" 28.4

28.9* 29.5” 28.6 28.3"

Means

17.2t0.5 17.3t0.5 17.3k0.6 22.9t0.5 22.5k0.5 22.6k0.5 30.5t0.7 30.8t0.7 31.220.7 29.5kO.G 28.6t0.6 28.320.5

URC-AP, upper rib cage anteroposterior diameter; LRC-AP, lower rib cage anteroposterior diameter; LRC-TR, diameter; NA, not available. P values refer to the whole group of subjects. * P < 0.01; t P < 0.05 vs. 1 G,. RESULTS

End-expiratory rib cage dimensions sure. The effect of G, on end-expiratory

and gastric pres-

rib cage dimensions is illustrated for one representative subject in Fig. 2. Individual data for all subjects are given in Table 1 and average changes are displayed in Fig. 3. For technical reasons, data for the URC-AP diameter were not available in one subject (AM). In the others, this diameter often failed to show stable end-expiratory levels during periods of stable micro- and hypergravity; as a result, values presented in Table 1 were computed from a limited number of parabolas (range 2-8). Although generally consistent within subjects, changes in end-expiratory URC-AP diameter were very variable between subjects and statistically nonsignificant for the group. We attribute this variability to involuntary changes in the position of the subject’s head and neck with changes in acceleration. The LRC-AP diameter at end expiration increased at 0 G, in four subjects and did not change in one. For the five subjects, the increase averaged 4 t 1 mm (P < 0.025). In contrast, no consistent change was observed at 2 G,. Un-

P Value

co.025

co.05