The purpose of this study is to determine how intraocular pressure responds to changes in the levels of carbon dioxide or oxygen that a healthy individual inspires.

In response to changes in the composition of inhaled gases, blood vessels will dilate or constrict. As a result, hypercapnia or hyperoxia may affect the production and drainage of aqueous humour in the anterior chamber of the eye. The balance between the production and drainage of the aqueous humour determines the intraocular pressure. As this system is hydrodynamic, it is expected that any increase or decrease in the production of aqueous humour due to dilation or constriction of the capillaries within the ciliary body will be compensated by increased or decreased drainage at the trabecular meshwork. Therefore intraocular pressure is not expected to show a response to hypercapnia or hyperoxia, but this supposition needs to be tested in a stably controlled manner of inducing inhaled gas provocations. This study will measure the intraocular pressure at varying levels of hypercapnia and hyperoxia using a sequential rebreathing circuit and automated gas blender. This will allow the precise targeting and stable control of end-tidal partial pressure values of carbon dioxide and oxygen.

In this study, intraocular pressure will be measured at seven different inhaled gas stages. The seven stages are as follows:

1. Baseline, measured in eye A (PETCO2=38mmHg and PETO2=100mmHg)
2. 10% hypercapnic increase, measured in eye A (PETCO2=42mmHg and PETO2=100mmHg)
3. 20% hypercapnic increase, measured in eye A (PETCO2=46mmHg and PETO2=100mmHg)
4. Baseline, measured in both eyes (PETCO2=38mmHg and PETO2=100mmHg)
5. 250% hyperoxic increase, measured in eye B (PETCO2=38mmHg and PETO2=250mmHg)
6. 500% hyperoxic increase, measured in eye B (PETCO2=38mmHg and PETO2=500mmHg)
7. Baseline, measured in eye B (PETCO2=38mmHg and PETO2=100mmHg)

Inclusion Criteria:

• Age range 18-30 years old
• Visual acuity of 20/20 or better

Exclusion Criteria:

• Refractive error >±6.00 DS and/or ± 2.00 DC
• History or presence of ocular disease
• Family history of diabetes or glaucoma
• History of intraocular or refractive surgery
• Nursing or pregnant women
• History of clinically diagnosed endocrine disease
• History of vascular disease, cardiovascular disease, or any treated respiratory disorders (seasonal asthma excluded from this so long as subject not taking Rx at the time)
• History of systemic hypertension
• Habitual smoking
• Use of medications that affect blood flow

Healthy young individuals (age 18-30)