Fractional excretion of chloride (FECl), fractional excretion of sodium (FENa) and difference in fractional excretions of chloride and sodium (FECl-FENa). While FENa, FECl and FECl-FENa all increased significantly in response to acute respiratory acidosis, 81, 131 and 233% respectively, FECl-FENa increased the soonest after onset of acidosis and exhibited the greatest proportionate change. This data was collected from 4 subject on 8 experimental days. Data are presented as means ± SEM, * = significantly different from time 0, # = significantly different from time 120.