However , both the compression depth ( 49.7 4.2 vs 46.5 4.1 mm , P = .003 ) and proportion of chest compressions with an appropriate depth ( 47.6 % 27.8 % vs 28.0 % 23.4 % , P = .006 ) were remarkably higher when compressing the chest with the dominant hand against the sternum for those who performed suboptimal CPR .