Background—Mechanisms favoring the occurrence of paroxysmal atrial fibrillation (PAF) are complex and poorly defined. This study was designed to analyze dynamic changes in autonomic tone preceding the onset of PAF in a large group of patients.

Methods and Results—Holter tapes from 77 unselected consecutive patients (63 men and 14 women aged 58±12 years) with PAF were analyzed. A total of 147 episodes of sustained AF (>30 minutes) were recorded and submitted to time-domain and frequency-domain heart rate variability analyses; 6 periods were studied using repeated measures ANOVA: the 24-hour period, the hour preceding PAF, and the 20 minutes before PAF divided into four 5-minute periods. In the time-domain analyses, a linear decrease in mean RR interval from 925±16 to 906±16 ms (P<0.0002) was observed before the onset of PAF, together with a significant increase in the standard deviation of NN intervals from 65±4 to 70±4 ms (P<0.02). In the frequency-domain analyses, a significant increase in high-frequency (HF, HF-NU) components was observed before PAF (P<0.001 and P<0.0001, respectively), together with a progressive decrease in low-frequency components (LF, LF-NU) (P<0.0001 and P<0.004, respectively). The low/high frequency ratio showed a linear increase until 10 minutes before PAF, followed by a sharp decrease immediately before PAF, suggesting a primary increase in adrenergic tone followed by a marked modulation toward vagal predominance. No difference was observed in these heart rate variability changes between patients with "lone" PAF and patients with structural heart disease.

Conclusions—The occurrence of PAF greatly depends on variations of the autonomic tone, with a primary increase in adrenergic tone followed by an abrupt shift toward vagal predominance.