The Effect of +Gz Acceleration on Intraocular Pressure

Chung, Ki Young; Woo, Se Joon

Abstract

BACKGROUND
The physiological change of positive acceleration is primarily focused on the hydrostatic column effects that are associated with acceleration exposures. IOP values during positive acceleration is reduced according to this theory. However, the first trial of IOP measurement during positive acceleration showed that there were no significant changes in IOP values during the +2G phase of parabolic flight. In addition, IOP study during centrifuge exposures showed that there were significant increases in IOP during +2Gz and +3Gz. Therefore, the purpose of this study is to investigate the effect of positive acceleration on IOP. METHODS: The data from 4 normal subjects (2 men and 2 women) were included in this study. The baseline IOPs of subjects were within normal range. The subjects did not wear anti-G suits and use anti-G maneuver during the acceleration exposure. Pressure phosphene tonometer was used to measure IOP. To minimize the difference between the Goldmann tonometer and the pressure phosphene tonometer, IOPs were measured in subjects for 1 week prior to this study. IOP measurements of protocol 1 were obtained with pressure phosphene tonometer at +1Gz (baseline), +2Gz, +3Gz, +4Gz, and again at +1Gz (post). IOP measurements of protocol 2 were obtained at + 1Gz (baseline), +3Gz, and again at +1Gz (post). IOP measurements of protocol 3 were obtained at + 1Gz (baseline), +4Gz, and again at +1Gz (post). A total of 55 IOP measurements for each subject were made. RESULTS: The result showed that there was a significant difference in the means of IOP in protocol 1, 2, and 3 (P<0.05). But, they did not show an increasing or decreasing trend according to increase in acceleration. In particular, subject 1 showed that there was significant decrease of IOP in protocol 1, 2, and 3 compared to the baseline IOP measurements during positive acceleration(P<0.05). Based on symptom questionnaire and VTR review, variable body contractions may affect the IOP measurements during acceleration. There were significant differences in the means of IOP among variable body contractions in subject 1 when compared to the baseline IOP measurements. CONCLUSIONS:Persistent decreases of IOP in subject 1 suggest that the mechanism of IOP changes under positive acceleration could be explained by the hydrostatic column effect. In addition, interpretation for IOP measurements under positive acceleration should be cautious due to variable muscle contraction.