Cardiovascular effects of a sustained -Gz force in the horizontal position.

Introduction: The purpose of this investigation was to study the effect of a sustained -Gz force applied in the horizontal position on the cardiovascular system and the distribution of body fluids.

Methods: The study was conducted on four men who were submitted to two protocols. The principal difference between the two protocols was that in Protocol A the subjects remained supine, whereas in protocol B the subjects were submitted to a small -Gz force (0.38 G) for a relatively prolonged period while remaining supine. The -Gz force was generated by a short arm centrifuge (AGS or artificial gravity simulator) which has four beds radially oriented on a circular platform and allows variation of the Gz force by controlling the angular velocity. The cardiovascular and fluid distribution responses were sampled approximately every 2 min during the whole duration of the experiment (205 min). The variables recorded were thoracic impedance (Tz), stroke volume (SV), heart rate (HR), first derivative of the thoracic impedance change (dz.dt-1), and systolic (SBP) and diastolic (DBP) arterial BPs.

Results: Changes in these variables within each protocol and across protocols were analyzed statistically by group comparisons. This analysis showed that changing from the standing to the supine position causes an increase in SV and a decrease in HR and SBP and DBP. Adding a -Gz force produces the same qualitative effects but causes a greater increase in SV and a greater decrease in systolic and diastolic BPs, even though the -Gz force was only a fraction of 1 G.

Conclusion: Changing position from vertical to horizontal position and the application of a -G force produce similar cardiovascular and body fluid effects but these effects are amplified by -Gz. An attempt to find an explanation to those changes has led us to hypothesize that a combination of factors such as central venous pressure and sequestering fluids in some part of the thorax must play an important role in the characteristics of those changes. Further research on the physiological effects of prolonged -Gz may be relevant to studies in simulated microgravity.