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Assessment of autonomic tone preceding the onset of atrial fibrillation (AF) after coronary artery bypass grafting (CABG) with heart rate variability was examined in 64 patients scheduled for elective CABG (days 2 to 5). Ninety-six-hour Holter tapes were analyzed in each patient and all events labeled by an experienced technician. The hour preceding AF was divided into 4 quarters (heart rate variability calculated per quarter) and compared with similar time episodes from the group without AF. Twenty-six of 64 patients (40%) had a total of 35 episodes. Only increased age (68 ± 5 vs 62 ± 9 years) and lower ejection fraction (66 ± 16% vs 73 ± 8%) were associated with an increased risk for AF. Before onset, a greater number of atrial premature complexes was observed. The standard deviation of all RR intervals (SDNN) showed an increase in the group with AF in the last 15 minutes (significant vs controls and within the …