21: Atrial Fibrillation

Key Points

• Disease summary:

  • Atrial fibrillation (AF) is the most common clinical arrhythmia (affecting ~10% of those in the seventh decade of life) and is a major cause of morbidity and mortality. AF is traditionally regarded as a sporadic, nongenetic disorder, but the ability to sustain the arrhythmia has long been known to require some underlying diathesis. There is growing evidence of an important heritable basis for many forms of AF, with the recent identification of several genetic determinants.

  • In AF the normal electrical impulses that are generated by sinoatrial node are replaced by waves of disorganized electrical activity that result from a combination of very rapidly firing triggers and abnormal conduction within local atrial re-entry circuits. This prevents co-ordinated contraction of the muscle which instead fibrillates in an uncontrolled manner (this leads to the arrhythmia’s name). As a result the impulses reaching the ventricle to generate the hemodynamically important contracile activity are also highly irregular.

  • The arrhythmia involves the two upper chambers (right and left atria) of the heart, though in some instances the origins of the rapid electrical triggers may be in bands of cardiac muscle that envelop the ends of the pulmonary veins as they insert into the posterior aspect of the left atrium.

  • Clinical presentation: AF most often arises in the setting of other forms of heart disease, though there is evidence of shared mechanisms across these different contexts. AF may also arise in the absence of evidence of any associated cardiac disorder, so called lone AF. AF is often asymptomatic, but it may result in palpitations, dyspnea, chest tightness or pain, and even congestive heart failure (HF) in those predisposed. Individuals with AF have an increased risk of stroke both embolic and hemorrhagic. There may be typical precipitants for AF episodes including exercise or sleep.

• Differential diagnosis:

  • There are few other arrhythmias which exhibit the rapid irregular rhythm seen in AF. One arrhythmia that may be confused with AF is atrial flutter which results from a more organized macroreentry circuit, most commonly in the right atrium. As a result of the anatomic nature of the flutter circuit the atrial electrical activity is much more regular in nature, and AF can be distinguished from atrial flutter as the latter exhibits characteristic saw-toothed flutter waves of constant amplitude and frequency on a standard electrocardiogram. Importantly, there is no evidence of organized atrial activity on the electrocardiogram in AF, but both rhythms may result in irregular ventricular rhythm because of the nature of conduction between the atrium and the ventricle.

• Monogenic forms:

  • There are several genetic loci that have been reported in large kindreds with Mendelian AF (Table 21-1).