Within the control group , PaCO2 was similar from ABG-1 to ABG-2 ( median [ 10th-90th quantile ] = 56.1 mm Hg [ 47.2-67 .0 ] vs 56.6 mm Hg [ 46-68 .8 ] ; P = 0.52 ; 95 % CI for the median = -3.4 to 3.4 ) .