Association Between Arterial Blood Gas Variation and Intraocular Pressure in Healthy Subjects Exposed to Acute Short-Term Hypobaric Hypoxia

Yuan Xie; Yiquan Yang; Ying Han; Diya Yang; Yunxiao Sun; Xinmao Wang; Anh Hong Nguyen; Yihan Chen; Jiaxin Tian; Qing Zhang; Chen Xin; Kai Cao; Huaizhou Wang; Xiaofang Liu; Guozhong Wang; Ningli Wang

doi:10.1167/tvst.8.6.22

Association Between Arterial Blood Gas Variation and Intraocular Pressure in Healthy Subjects Exposed to Acute Short-Term Hypobaric Hypoxia

Transl Vis Sci Technol. 2019 Nov 20;8(6):22. doi: 10.1167/tvst.8.6.22. eCollection 2019 Nov.

Authors

Yuan Xie^{1

2}, Yiquan Yang^{1

2}, Ying Han³, Diya Yang^{1

2}, Yunxiao Sun^{1

2}, Xinmao Wang⁴, Anh Hong Nguyen³, Yihan Chen^{1

2}, Jiaxin Tian^{1

2}, Qing Zhang², Chen Xin², Kai Cao², Huaizhou Wang^{1

2}, Xiaofang Liu⁴, Guozhong Wang⁵, Ningli Wang^{1

2}

Affiliations

¹ Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Sciences Key Laboratory, Beijing, China.
² Beijing Institute of Ophthalmology, Beijing Tongren Hospital, Capital Medical University, Beijing, China.
³ Department of Ophthalmology, University of California, San Francisco, CA, USA.
⁴ Department of Pneumology, Beijing Tongren Hospital, Capital Medical University, Beijing, China.
⁵ Department of Hyperbaric and Hypobaric Chamber, Civil Aviation General Hospital, Beijing, China.

Abstract

Purpose: To investigate the association between changes in arterial blood gases and intraocular pressure (IOP) after acute, short-term exposure to simulated elevation of 4000 m above sea level.

Methods: Twenty-five healthy young lowlanders participated in this prospective study. IOP was measured in both eyes with an Accupen tonometer. Arterial blood gas parameters (partial oxygen pressure [PaO₂], partial carbon dioxide pressure [PaCO₂], pH, and bicarbonate ion [HCO₃ ^-]) were checked using a blood gas analyzer. Measurements were taken at sea level (T1), at 15-minute (T2) and at 2-hour (T3) exposure times to simulated 4000 m above sea level in a hypobaric chamber, and upon return to sea level (T4). Associations between arterial blood gas parameters and IOP were evaluated using multivariate linear regression.

Results: PaO₂ significantly decreased at T2 and T3, resolving at T4 (P < 0.001). pH significantly increased at T2 and returned to baseline at T3 (P = 0.004). Actual and standard bicarbonate ion both dropped with IOP at T3 and T4. IOP significantly decreased from 16.4 ± 3.4 mm Hg at T1 to 15.1 ± 2.1 mm Hg (P = 0.041) at T3 and remained lower (14.9 ± 2.4 mm Hg; P = 0.029) at T4. IOP was not correlated with pH. Multivariate linear regression showed that lower IOP was associated with lower standard bicarbonate ion (beta = -1.061; 95% confidence interval, -0.049 to -2.074; P = 0.04) when adjusted for actual bicarbonate and diastolic blood pressure.

Conclusions: Hypobaric hypoxia triggers plasma bicarbonate ion reduction which, rather than pH, may decrease aqueous humor formation and subsequently cause IOP reduction. These findings may shed light on the mechanism of IOP regulation at high altitude.

Translational relevance: Hypoxia-triggered reduction in plasma bicarbonate ion may decrease aqueous humor production, leading to IOP reduction at high altitude. These findings may provide new insight into a potential mechanism of IOP regulation. Hypobaric hypoxia at high altitude is an environmental factor that can reduce IOP and, therefore, deserves further study.

Keywords: arterial blood gases; bicarbonate ion; high altitude; hypobaric chamber; hypoxia; intraocular pressure.