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Related Experiment Videos

Differences between syncope resulting from rapid onset acceleration and orthostatic stress

D A Self1, C D White, R M Shaffstall

  • 1Crew Technology Division, Armstrong Laboratory, Brooks AFB, TX, USA.

Aviation, Space, and Environmental Medicine
|June 1, 1996
PubMed
Summary

Loss of consciousness due to rapid acceleration (G-LOC) in aviators may be caused by hydrostatic effects, not venous pooling, as anti-G suits prevent pooling. This study compared G-LOC to orthostatic syncope, finding distinct volume changes.

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Area of Science:

  • Aerospace Medicine
  • Human Physiology
  • Cardiovascular Research

Background:

  • Orthostatic syncope involves venous pooling and vasodilation.
  • G-LOC in aviators may differ due to anti-G suits preventing venous pooling.
  • This study investigates G-LOC mechanisms compared to orthostatic stress.

Purpose of the Study:

  • To test if significant volume changes occur in dependent regions during G-LOC in anti-G suit wearers.
  • To compare venous pooling patterns in G-LOC subjects with those experiencing orthostatic syncope.

Main Methods:

  • Blood volume shifts in calf, thigh, and abdomen were measured.
  • Subjects experienced G-LOC via rapid onset centrifuge exposure (+5 Gz) wearing an inactivated anti-G suit.
  • Control subjects underwent tilt and lower body negative pressure (LBNP) to induce orthostatic presyncope.

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Main Results:

  • G-LOC subjects showed minimal net volume change in pooled regions (calf, thigh, abdomen).
  • Orthostatic syncope subjects exhibited significant venous pooling (1022 ml increase in combined segments).
  • Mean arterial pressure dropped significantly in both groups, but more severely in G-LOC subjects.

Conclusions:

  • G-LOC appears to be primarily caused by hydrostatic effects, not venous pooling.
  • Anti-G garments effectively prevent significant venous pooling during G-LOC, even when uninflated.
  • Findings differentiate G-LOC mechanisms from those of orthostatic syncope.