FLIGHT PHYSIOLOGY

Flight Physiology

Flight Physiology

Introduction

Acute Variations In Gravity [head-to-foot acceleration (Gz)] induce dramatic fluid shifts in pilots and astronauts as a consequence of changes in hydrostatic pressure gradients. The resulting hemodynamic alterations are responsible for many of the consequences associated with acute gravity changes (hyper- or microgravity) (5, 30). For example, combat-aircraft pilots experience reduction in their cerebral blood flow resulting from acute acceleration and have to use anti-G devices or maneuvers to prevent loss of consciousness (22). During microgravity, venous blood shifts from the legs toward the central compartment (14, 27), resulting in a variety of hemodynamic changes, including increased pulmonary capillary blood volume (23, 29) and stroke volume (10, 15, 19). The role of the venous system as a blood reservoir is crucial for these rapid cardiovascular adaptations (4, 7, 8, 20). However, the exact contribution of the leg venous compartment to volume redistribution has never been quantified directly.

To evaluate venous volume shifts associated with acute changes in gravity, we measured leg volume changes occurring during the succession of micro- and hypergravity phases generated by parabolic flight profiles (29). The leg volume measurements were performed by using strain-gauge plethysmography in healthy, standing volunteers.

Materials And Methods

Five healthy volunteers, free of venous pathology, were enrolled in this study. Individual anthropometric data are presented in Table 1. All subjects underwent special flight physical examination and gave written, informed consent to participate in this study. Subjects were not taking any medication before and during flights. This study was approved from an ethical point of view by the French Space Agency [Centre National d'Etudes Spatiales (CNES)].

Equipment and protocol

A total of six parabolic flight sessions were organized by CNES, the National Aeronautics and Space Administration (NASA), and NOVESPACE (trade society of CNES, France) at the Société Girondine d'Equipements, de Réparation, et de Maintenance Aéronautique center in Bordeaux, France, during two separate campaigns, in November 1995 and November 1996. A NASA-Johnson Space Center KC-135A aircraft was used to perform the parabolic flight profiles. Flights were managed on three consecutive days. Each flight session lasted 2.5-3 h and incorporated 30 parabolas.

Instantaneous gravity was measured continuously by using the aircraft's accelerometer. Gravity variations during a parabolic flight profile include four consecutive phases (Fig. 1): normogravity (1 Gz) before the parabola began; mild hypergravity (1.6-1.7 Gz) during the ascending phase of the parabola (20-25 s); microgravity (0 Gz), lasting for 20 s and corresponding to the top of the parabola; and a second period of mild hypergravity (1.6-1.8 Gz) during the descending phase of the parabola (20-25 s), followed by a new steady state at 1 Gz.

Plethysmography measurements were performed with two Perivein apparatuses (Jansen Scientific Instruments, Liège, Belgium) by using gauges made of highly elastic tubing filled with mercury. The changes in resistance of the gauges are proportional to changes in length when variations in length are <10% of the unstretched gauge (see Plethysmography calibration).

Subjects were studied in the upright position. To keep them in a near-standing position during microgravity, they were fastened to a saddle with a seat ...