Intraocular pressure during handgrip exercise: The effect of posture and hypercapnia in young males

Purpose As part of our investigations of intraocular pressure (IOP) as a potential contributing factor to the spaceflight-associated neuro-ocular syndrome using the 6 degrees head-down tilt (6 degrees HDT) bed rest experimental model, we compared the effect of rest and isometric exercise in prone and supine 6 degrees HDT positions on IOP with that observed in the seated position. Methods Ten male volunteers (age = 22.5 +/- 3.1 yrs) participated in six interventions. All trials comprised a 10-min rest period, a 3-min isometric handgrip exercise at 30% of participant's maximum, and a 10-min recovery period. The trials were conducted under normocapnic (NCAP) or hypercapnic (FICO2 = 0.01; HCAP) conditions, the latter mimicking the ambient conditions on the International Space Station. IOP, systolic and diastolic pressures, and heart rate (HR) were measured during the trials. Results Isometric exercise-induced elevations in HR and mean arterial blood pressure. IOP in the prone 6 degrees HDT position was significantly higher (p < 0.001) compared to IOP in supine 6 degrees HDT position and seated trials at all time points. IOP increased with exercise only in a seated HCAP trial (p = 0.042). No difference was observed between trials in NCAP and HCAP. IOP in the prone 6 degrees HDT position was constantly elevated above 21 mmHg, the lower limit for clinical ocular hypertension. Conclusions IOP in the prone 6 degrees HDT position was similar to IOP reported in astronauts upon entering microgravity, potentially indicating that prone, rather than supine 6 degrees HDT position might be a more suitable experimental analog for investigating the acute ocular changes that occur in microgravity.