Physiologic +Gz tolerance responses over successive +Gz exposures in simulated air combat maneuvers.

Introduction: Fighter aircraft pilots are exposed to repetitive headward acceleration (+Gz) during air combat maneuvering. The objective of this study was to compare physiologic responses and the calculated +Gz tolerances of multiple successive +Gz exposures with the responses of the first +Gz exposure.
Methods: There were 13 subjects who performed simulated air combat maneuvering (SACM) profiles composed of 10 rapid-onset rate +Gz cycles with different combinations of short- or long-duration +Gz plateaus (8 or 20 s) and +Gz pauses (1 or 15 s). +Gz, plateaus were individually set at levels inducing strong physiologic responses while the +Gz pause was set at 1.4 Gz. Head-level systolic pressure, ear opacity, and vision quality were measured.
Results: The nadirs of head-level systolic pressure, ear opacity, and visual quality during each +Gz plateau were higher in subsequent cycles compared to the first +Gz cycle. There was an average increase in calculated +Gz tolerance of 0.35 +/- 0.21 Gz following the first +Gz cycle. SACMs with short +Gz pauses produced greater increases in +Gz tolerance than SACMs with long +Gz pauses.
Discussion: Cardiovascular and visual responses were improved over the course of successive +Gz cycles, indicating that the risk of +Gz-induced loss of consciousness was not increased beyond the first +Gz cycle. The increase in +Gz tolerance beginning with the second cycle is attributed to a carryover of compensatory responses, primarily vasoconstriction, with possible contribution from greater venous return and baroreflex enhancement.