Isocapnia blocks exercise-induced reductions in ocular tension

Area of Science:

• Physiology
• Ophthalmology

Background:

• Isometric exercise, specifically handgrip at 50% maximal voluntary contraction (MVC), has been anecdotally reported to lower intraocular pressure (IOP).
• The underlying physiological mechanism for this IOP reduction remains unclear, with hyperventilation being a potential contributing factor.

Purpose of the Study:

• To investigate whether the IOP-lowering effect of isometric exercise is mediated by exercise-induced hyperventilation.
• To determine if maintaining isocapnic conditions (normal CO2 levels) during exercise prevents the reduction in IOP.

Main Methods:

• 11 healthy subjects with elevated IOP (≥18 mm Hg) participated in a controlled study.
• Subjects performed 2-minute handgrip exercise at 50% MVC under three conditions: control (no exercise), exercise alone, and exercise with CO2 supplementation to maintain isocapnia.
• Intraocular pressure (IOP), minute ventilation, and end-tidal partial pressure of CO2 (PCO2) were monitored.

Main Results:

• Exercise significantly lowered IOP from 18.3 to 15.6 mm Hg, an effect that persisted for 15 minutes post-exercise.
• During exercise, minute ventilation increased, and PCO2 decreased, consistent with hyperventilation.
• When CO2 was supplemented to maintain isocapnia, the exercise-induced reduction in IOP was abolished (experimental IOP 17.8 mm Hg vs. control 18.1 mm Hg).

Conclusions:

• The fall in intraocular pressure following isometric handgrip exercise is primarily attributed to exercise-induced hypocapnia (lowered PCO2), not the exercise itself.
• These findings suggest that manipulating blood gas levels, specifically CO2, could be a therapeutic strategy for managing ocular hypertension.