Atrial fibrillation lasting 2.5 minutes or longer was present in 28 of 284 patients ( 9.9 % ) in the intervention group , as compared with 7 of 277 ( 2.5 % ) in the control group ( absolute difference , 7.4 percentage points ; 95 % CI , 3.4 to 11.3 ; P < 0.001 ) .