Abstract
Background+Gz-time tolerance curves were developed to predict when exposure to +Gz stress exceeds human tolerance resulting in neurologic signs and symptoms. The +Gz-induced loss of consciousness (G-LOC) curves were developed to predict when +Gz stress induces G-LOC. The G-LOC curves are based on a theoretical understanding of how acceleration affects underlying physiological mechanisms affording tolerance to acceleration, their limits, and what happens when they are exceeded. The foundation of previous +Gz-time tolerance curves was based on a minimal dataset of sign and symptom endpoints.MethodsTwo G-LOC curves were established from the analysis of 888 centrifuge induced G-LOC episodes in completely healthy humans. The time from the onset of +Gz stress to the onset of unconsciousness was plotted as a function of +Gz level and the G onset rate.ResultsThe two new G-LOC curves differed significantly from previous curves in temporal characteristics and key aspects underlying neurologic response to acceleration. The new acceleration onset rate curve reveals that for onset rates ≥ 1.0 G/s, G-LOC will occur in a mean time of 9.10 s and is independent of the onset rate. The new +Gz-level curve demonstrates that G-LOC will occur in a mean time of 9.65 s for rapid onset rate exposures to +Gz levels ≥ +7 Gz. The minimum +Gz-level threshold tolerance was defined as +4.7 Gz. When +Gz onset rates are gradual, ≤ 0.2 G/s, G-LOC occurs in a mean time of 74.41 s. G-LOC did not occur earlier than 5 s for any acceleration exposure.ConclusionsThese G-LOC curves alter previous temporal predictions for loss of consciousness and advance the understanding of basic neurophysiological function during exposure to the extremes of acceleration stress. Understanding the acceleration kinetics of the loss and recovery of consciousness provides the characteristics of uncomplicated and purely ischemic causes of LOC for application in medical diagnosis of syncope, epilepsy, and other clinical causes of transient loss of consciousness. The curves are applicable to education, training, medical evaluation, and aerospace operations.
Highlights
It is well established that individuals exposed to acceleration (+Gz) stress beyond their tolerance level will lose consciousness
The resulting curves do not represent relaxed G-loss of consciousness (LOC) tolerance; all types of exposures, including those with and without performance of an anti-G straining maneuver (AGSM) and/or wearing an anti-G suits (AGS) composed the dataset. +Gz level tolerance is characterized by the contribution of all aspects of protection existing during an exposure
The +Gz-induced loss of consciousness (G-LOC) curves provide an improved standard for acceleration responses that result from exposure to profiles with gradual onset rate (GOR) of 0.05 G-LOC [22]. The onset rate (G/s) through rapid onset rate (ROR) 7.6 G/s and +Gz levels from +2.5 to +11.7 Gz
Summary
It is well established that individuals exposed to acceleration (+Gz) stress beyond their tolerance level will lose consciousness. Current understanding of the cause of the loss of consciousness (LOC) is based on acceleration producing an environment in which the cardiovascular system is unable to supply an adequate amount of oxygenated blood to the cephalic nervous system (CPNS) regions that support conscious function. Fluctuations in cardiovascular function (heart rate, blood pressure, and vascular tone) occur regularly in normal environments, but are usually not of sufficient magnitude or duration to induce neurologic functional compromise. Fluctuations occur in the acceleration (+Gz) forces humans experience in the Earth's gravitational field daily, beginning as one arises (horizontal to vertical) from sleep. It is only when acceleration stress applied in a specific direction, over a long enough duration, and at a sufficiently high magnitude, that neurologic function is embarrassed. A clear description of the human acceleration tolerance envelope is a component of understanding who we are and what environments we are able to safely enter without the threat of LOC
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