The fentanyl pretreatment group had a higher frequency of apnea ( 94 % vs 64 % ; P = 0.0003 ) and longer duration of manual ventilation ( 3 [ interquartile range ( IQR ) , 1.5-5 ] min vs 1 [ 0-1 .5 ] min ; P < 0.0001 ) at induction .