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Atrial fibrillation is an arrhythmia characterized by chaotic atrial activity and is the most common sustained arrhythmia. Although there are multiple etiologies, it is most commonly caused by numerous atrial pacemaker sites that are initiated by a PAC. These sites are most commonly around the pulmonary veins. It is an irregularly irregular rhythm that most commonly causes a narrow complex tachycardia, though can also cause a wide complex tachycardia when there is aberrancy present. Because of the numerous atrial pacemaker sites, there is a variable RR interval and the atria fibrillate due to the chaotic electrical activity. Because all of the numerous sites are chaotically firing without any patterns, P waves are often absent. When present ('course atrial fibrillation' with 'fibrillatory waves'), they are noted to be have highly variable morphologies from the different sites.
There are different classifications of atrial fibrillation depending on its occurrence. Paroxysmal atrial fibrillation is when the rhythm is intermittent, whereas permanent atrial fibrillation is when a patient has persistent atrial fibrillation (>7 days). Permanent atrial fibrillation is when it lasts for at least a year.
Because of the chaotic atrial electrical activity, the ventricular activity is largely dependent on the response of the AV node. The RR interval is variable given the variable impulses through the AV node to the ventricles. However, many of the impulses do not travel beyond the AV node due to its upper limitations (usually no greater than 200-210bpm). Because the rates can become quite high, patients with RVR often display diffuse demand ST depressions that are not indicative of primary ischemia. Ventricular rates consistently above this limit is concerning for the presence of an accessory pathway (ie WPW). A patient is considered to have a rapid ventricular response when the rates average >110bpm. Atrial fibrillation with slow ventricular response is usually a result of AV nodal blocking effect/toxicity, digoxin toxicity, or hypothermia. Atrial fibrillation with a regular ventricular rate is indicative of complete AV block (or ventricular pacing).
Examples:
Source: LITFL
Atrial fibrillation producing an irregularly irregular rhythm that is narrow complex. There are no consistent P wave complexes throughout the EKG, though course fibrillatory waves are easily noted in V1.
Source: LITFL
Atrial fibrillation again producing the irregularly irregular rhythm. In contrast to the first example, no course fibrillatory waves are noted (though fine fibrillatory waves can be noted easily in V1).
Source: Steve Smith's blog
Atrial fibrillation with RVR with rate-related bundle branch block. Though this is a difficult EKG to identify, the RR intervals are actually variable if measured consistent with atrial fibrillation. Atrial fibrillation is a common rhythm to have intermittent rate-related bundle branch blocks due to its variable nature, with periods of faster ventricular response.
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