INTRODUCTION: The autonomic nervous system has been proven to play a major role in the onset of atrial fibrillation (AF), along with a predisposing substrate and a specific trigger event usually consisting of a premature supraventricular ectopic beat (SVEB). By means of heart rate variability (HRV) analysis, we investigated the activity of the autonomic nervous system before SVEBs nontriggering and triggering AF. METHODS AND RESULTS: We evaluated 28 patients with at least 1 episode of sustained AF (>30 seconds) recorded during 24-hour Holter monitorings. We performed HRV analysis during the hour preceding 35 AF onsets and compared these results with the HRV values before nontriggering SVEBs with similar prematurity. According to the low frequency (LF)/high frequency (HF) ratio in the 5 minutes before the onset, AF episodes were classified as either vagal (LF/HF ratio <1.5) or adrenergic (LF/HF ratio >/=1.5).Vagal episodes (16) showed a decrease in LF (from 50.81 +/- 1.67 to 32.73 +/- 3.54) and an increase in HF (from 36.00 +/- 2.30 to 54.7 +/- 3.69) throughout the hour preceding the onset. Adrenergic episodes (19) had opposite changes in LF (from 55.4 +/- 4.95 to 67.51 +/- 5.24) and HF (from 33.78 +/- 5.82 to 27.96 +/- 3.51) during the same period. No similar trends were observed before the selected nontriggering SVEBs. CONCLUSION: Only SVEBs occurring during a phase of hyperactivity of one of the 2 branches of the autonomic nervous system are able to trigger episodes of AF.