Acceleration-induced loss of consciousness. A review of 500 episodes.

Unconsciousness resulting from exposure to increased levels of head-to-foot (+Gz) acceleration stress (501 unconsciousness episodes) on a centrifuge in asymptomatic, healthy human subjects was investigated. A method for quantitatively measuring the kinetics of the unconsciousness and associated phenomenon was developed. In addition, a theoretical framework for describing the central nervous system (CNS) alteration resulting from acute reduction of blood flow was formulated to allow a method for defining unconsciousness phenomenon. The length of unconsciousness and the associated incapacitation was found to be dependent on the magnitude of the CNS insult resulting from reduced blood flow. The magnitude of the insult was determined by the onset and offset rates of the +Gz-stress and the length of time at increased +Gz. The incapacitation resulting from +Gz-stress included 11.9 seconds of absolute incapacitation (unconsciousness) and 16 seconds of relative incapacitation (confusion/disorientation) for 28 seconds of total incapacitation (period of time for lack of purposeful movement). Myoclonic convulsions were observed in approximately 70% of the unconsciousness episodes. The convulsions lasted 4 seconds and occurred following the return of CNS blood flow. The convulsions occurred after 8 seconds of unconsciousness and ended coincident with the return of consciousness. They occurred when the CNS insult was of greater magnitude. Memorable dreams occurred and were considered to occur during the terminal portion of the convulsion period. The dreams occurred with exposures having longer unconsciousness. The length of unconsciousness and incapacitation was affected by the wearing of an anti-G suit, with unconsciousness and incapacitation being reduced if the suit were worn. Performance of an anti-G straining maneuver resulted in an increased length of incapacitation by allowing the subject to get to higher levels of +Gz-stress and to sustain a greater amount of acceleration exposure. The results of this 11-year study of human unconsciousness provide a quantitative kinetic description of the phenomenon in healthy humans that is completely documented on videotape. These results should be of interest to neuropsychophysiologists investigating unconsciousness, convulsive activity, and dream phenomenon. They also provide the basis for future research aimed at solving +Gz-induced loss-of-consciousness problems in fighter-aircraft aviation.