Changes in ocular flow induced by hypo- and hypercapnia relate to static visual acuity in humans

We investigated whether the change in ocular blood flow, induced by hypo- and hypercapnia, is related to static visual acuity. Eleven healthy subjects (26±5 years) underwent three treatments. A three-treatment three-period crossover design was used. In the hypocapnia treatment (HYPO), the subjects controlled their minute ventilation (VE) to a target of 25 L/min for 6 min. In the hypercapnia treatment (HYPER), the subjects inspired high-fraction CO2 gas (FICO2 = 4%) for 6 min. In the control treatment (CON), VE was not manipulated. We measured choroidal and retinal blood flow by laser speckle flowmetry as ocular blood flow, and static visual acuity using the Landolt C chart. End-tidal partial pressure of CO2 differed significantly among HYPO, HYPER and CON (21±1, 48±1, and 42±1 mmHg, respectively). Retinal blood flow decreased significantly from the baseline in HYPO (-22±5%), but increased significantly in HYPER (+3±9%) compared to CON. Decimal visual acuity was significantly lower in HYPO than in the CON (0.21±0.1 vs. 0.24±0.1 P<0.05). These results suggest that changes in ocular blood flow induced by changes in arterial CO2 partial pressure influences visual acuity.