Systemic hyperoxia caused a significant decrease in retinal venous diameter ( -13.0 % 4.5 % ) and arterial diameter ( -12.1 % 4.0 % ) , in retinal blood velocity ( -43.4 % 7.7 % ) , and in retinal blood flow ( -57.0 % 5.7 % ) ( P < 0.001 for all ) .