Interactions of the human cardiopulmonary, hormonal and body fluid systems in parabolic flight

Abstract

Purpose: Commercial parabolic flights accessible to customers with a wide range of health states will become more prevalent in the near future because of a growing private space flight sector. However, parabolic flights present the passengers' cardiovascular system with a combination of stressors, including a moderately hypobaric hypoxic ambient environment (HH) and repeated gravity transitions (GT). Thus, the aim of this study was to identify unique and combined effects of HH and GT on the human cardiovascular, pulmonary and fluid regulation systems.

Methods: Cardiac index was determined by inert gas rebreathing (CI(rb)), and continuous non-invasive finger blood pressure (FBP) was repeatedly measured in 18 healthy subjects in the standing position while they were in parabolic flight at 0 and 1.8 G(z). Plasma volume (PV) and fluid regulating blood hormones were determined five times over the flight day. Eleven out of the 18 subjects were subjected to an identical test protocol in a hypobaric chamber in ambient conditions comparable to parabolic flight.

Results: CI(rb) in 0 G(z) decreased significantly during flight (early, 5.139 ± 1.326 L/min; late, 4.150 ± 1.082 L/min) because of a significant decrease in heart rate (HR) (early, 92 ± 15 min(-1); late, 78 ± 12 min(-1)), even though the stroke volume (SV) remained the same. HH produced a small decrease in the PV, both in the hypobaric chamber and in parabolic flight, indicating a dominating HH effect without a significant effect of GT on PV (-52 ± 34 and -115 ± 32 ml, respectively). Pulmonary tissue volume decreased in the HH conditions because of hypoxic pulmonary vasoconstriction (0.694 ± 0.185 and 0.560 ± 0.207 ml) but increased at 0 and 1.8 G(z) in parabolic flight (0.593 ± 0.181 and 0.885 ± 0.458 ml, respectively), indicating that cardiac output and arterial blood pressure rather than HH are the main factors affecting pulmonary vascular regulation in parabolic flight.

Conclusion: HH and GT each lead to specific responses of the cardiovascular system in parabolic flight. Whereas HH seems to be mainly responsible for the PV decrease in flight, GT overrides the hypoxic pulmonary vasoconstriction induced by HH. This finding indicates the need for careful and individual medical examination and, if necessary, health status improvement for each individual considering a parabolic flight, given the effects of the combination of HH and GT in flight.

Fig. 1

Study design: measurements were performed at regular ambient pressure before and after parabolic flight and hypobaric chamber runs (pre and post, respectively); and under low ambient pressure conditions in a standing position in a parabolic flight and in the hypobaric chamber (outbound, block 1–4, return); and in a standing position combined with gravity transitions in parabolic flight and without gravity transitions in the hypobaric chamber. Measurement blocks for cardiovascular and pulmonary data acquisition and time points of blood sampling are indicated

Fig. 2

Time course of main pulmonary parameters in parabolic flight in 18 subjects and in hypobaric chamber in 11 subjects is shown as the mean ± SE; asterisks indicate significant differences with respect to pre: *p < 0.05, **p < 0.01, ***p < 0.001; gray background indicates measurements in hypobaric hypoxia after decompression to 830 mbar

Fig. 3

Time course of heart rate and stroke index responses in the hypobaric chamber and in parabolic flight; responses in 0 G_z, solid black graph and at 1.8 G_z; dashed black graph are shown separately. Asterisks indicate significant changes with respect to pre separately for hyper- and microgravity values; gray background indicates low ambient cabin pressure

Fig. 4

Time course of arterial pressure, systemic vascular resistance and cardiac index in parabolic flight; responses at 0 G_z, solid black graph and at 1.8 G_z, dashed black graph, are shown separately. Asterisks indicate significant changes with respect to pre separately for hyper- and microgravity values; gray background indicates low ambient cabin pressure

Fig. 5

Courses of plasma volume and a subset of blood hormones are shown as a continuous black graph for parabolic flight and as dashed black graph for the hypobaric chamber results; significant differences with respect to pre are indicated as asterisk in parabolic flight (A300) and in the hypobaric chamber (chamber); the statistical significance of plasma volume and ΔAldosterone is illustrated as asterisk with respect to pre; degree symbol with respect to outbound; open diamond with respect to post 16th; and section symbol with respect to post 31st. In the Δ_proAVP and Δcortisol diagrams, the black open circle represents subject 0AP, the black cross represents subject 0AD, the black open diamond represents subject 0AT; in the renin_active diagram (Renin_A), the black open triangle represents the individual responses of subject 0AL; gray background indicates hypobaric hypoxic conditions