Inverted immersion as a novel gravitoinertial environment

BACKGROUND: Inverted immersion (II) offers a unique opportunity to swing the orientation of the gravity vector 180 degrees from its usual configuration with vestibular end organs. During II, extrathoracic fluid dynamics are identical to those of upright immersion (UI). II exposes individuals to a novel gravitoinertial environment and, therefore, should produce motion sickness (MS). HYPOTHESIS: II is more provocative of signs and symptoms of MS than UI. METHODS: Nine subjects were exposed once to II and UI. Conventional SCUBA gear was worn. In II, the subject wore a wetsuit which provided 5-7 kg force of positive buoyancy and, with no weight belt, caused him to float while inverted against the underside of a platform. An experiment with UI was identical except that a weight belt was worn which provided 5-7 kg force of negative buoyancy, and the subject stood upright against the bottom of the pool. The experiment was terminated after 3 hours or "upon the onset of the first, clear, persistent feeling of nausea", whichever came first. Throughout the experiment the subject rotated through a series of tasks: assembly of a pipe puzzle, performance of a series of head movements, and ambulation. Immediately post-dive, postural stability was assessed with tandem standing with and without eyes closed and with and without the neck extended 45 degrees. A questionnaire regarding susceptibility to motion sickness was completed pre-dive. RESULTS: No subject terminated the test because of MS during UI; seven subjects terminated the test during II (p < 0.025, McNemar's test). Posture was less stable after II than after UI (p < 0.05, sign test). MS questionnaire results did not predict susceptibility to II. CONCLUSION: II is provocative of MS and postural instability.