Atrial fibrillation is the most common rhythm disorder seen in man; prevalence in the general population is estimated at 0.4 %. There is a significant age-related distribution of the arrhythmia with 6 % of the population greater than 80 years of age suffering from atrial fibrillation. Atrial fibrillation remains an extremely rare arrhythmia in the pediatric population. Interestingly, genetic mutations in the KCNQ1 channel have been associated with atrial fibrillation as has the mutation related to the short QT syndrome (Figs. 9.3 and 9.4) has been reported (Chap. 18). Familial atrial fibrillation has involved an addition of nine different genetic defects from 13 loci; many of the genes are implicated in other cardiac arrhythmias (Chap. 19).

Atrial fibrillation commonly occurs as a comorbid condition with other cardiovascular abnormalities. It is found in patients with congestive heart failure, mitral valve stenosis and insufficiency, hypertension, hyperthyroidism, and some forms of repaired congenital heart malformations. In addition, it is well recognized in the child and young adult with complex, functionally impaired congenital heart malformation, particularly those with extensive atrial incisions or suture lines (Figs. 9.5 and 9.6). Atrial fibrillation is also associated with Wolff–Parkinson–White syndrome. Atrial fibrillation occurring in conjunction with Wolff–Parkinson–White syndrome is a potentially life-threatening situation due to the lack of decremental conduction through the accessory pathway. Some accessory pathways are capable of rapidly conducting the atrial signals to the ventricle leading to ventricular fibrillation (Chaps. 4 and 21). Following elimination of the accessory pathway either surgically or via radiofrequency ablation, atrial fibrillation resolves in a large portion of these patients. Atrial fibrillation also may be associated with several other forms of supraventricular tachycardia in young patients. Our experience and that of others has been that young patients presenting with atrial fibrillation will frequently have other underlying mechanisms of SVT such as AVNRT, AVRT, ectopic atrial tachycardia, or atrial flutter. Ablation of these other tachycardia substrates results in the resolution of atrial fibrillation. Conversion of these forms of SVT with adenosine may result in transient atrial fibrillation (Fig. 9.7) as adenosine shortens atrial refractoriness.

Another interesting association of atrial fibrillation is with obesity. This link has been demonstrated in adults as well as with pediatric patients. The exact physiological mechanism for this connection is unknown currently. We have also seen patients with atrial fibrillation that has probably resulted from enhanced vagal tone. Very strong vagal input can significantly shorten the refractory period of atrial myocardium making the atria more susceptible to microreentrant circuits which underlie atrial fibrillation (see below). This is the physiological mechanism which leads to atrial fibrillation after adenosine administration (Fig. 9.7).