Increased randomness of heart rate could explain increased heart rate variability preceding onset of atrial fibrillation.

The purpose of this article is to discuss the relationship between sympathovagal dysfunction and the occurrence of paroxysmal atrial fibrillation. Onset of atrial fibrillation is subject to circadian variation; shorter episodes occur more frequently during the day, whereas longer episodes occur less frequently at night. Vagally mediated atrial fibrillation typically is preceded by bradycardia, is not triggered by stress, occurs more often at night, is more common in men, and occurs at a younger age. Sympathetically mediated atrial fibrillation is less frequent, typically occurs during the day, can be triggered by stress, and often is accompanied by increasing sinus rate and frequent supraventricular extrasystoles. Determination of heart rate variability can be used to evaluate the effects of the autonomic nervous system on sinus rate. The onset of atrial fibrillation at night may be preceded by an increase in high-frequency components of heart rate variability. This is not the case for episodes that occur during the day. The heart rate in sympathetically mediated atrial fibrillation is higher before and during the episode in comparison with the vagal type. Heart rate variability is a promising method for evaluation of the interplay of sympathetic and vagal activity before the onset of atrial fibrillation. However, the role of heart rate variability for diagnostic assessment and therapeutic decision-making in patients with paroxysmal atrial fibrillation remains to be clarified by controlled studies.