Atrial fibrillation is the most commonly observed rhythm disorder in daily cardiology practice and corresponds to a complete disorganization of the electrical activity of the atria leading to the loss of atrial systole.
During an atrial fibrillation episode, the electrocardiogram typically shows: 1) a rapid, disorganized and irregular atrial activity (between 350 and 600 bpm); the atrial waves are difficult to analyze and produce continuous and permanent undulations of the baseline; 2) the QRS complexes are mostly narrow, irregular (justifying the name of complete arrhythmia by atrial fibrillation) with an increased rate > 100 bpm but much lower than the atrial rate. The RR intervals are of unequal duration and appear not to obey any specific rule or periodicity. This irregularity, very evocative of the diagnosis of atrial fibrillation, is secondary to the incessant bombardment of the atrioventricular node by atrial impulses.
There are, however, a number of differences in the electrocardiographic pattern depending on the characteristics of the patients. Atrial activity can be of varying amplitude. It is usually of high amplitude in patients with mitral valve disease, congenital heart disease, or with a wall enlargement (coarse AF). The amplitude on average is greater in leads in close proximity to the right atrium (right precordial leads, V1-V3). Conversely, atrial activity can sometimes be very difficult to discern due to a very small amplitude (fine AF), the diagnosis hence being suggested by indirect signs: absence of sinus P waves and irregularity of the ventricles. This often reflects the presence of an old permanent AF. The term fibrillo-flutter is inappropriate and is mistakenly used in presence of an atrial activity with high voltage which appears to be "relatively organized" (most often in V1). By definition, the flutter pattern corresponds to a perfectly regular activity, thus a "slightly irregular" atrial activity hence leading to the diagnosis of atrial fibrillation.
The ventricular rate can be regular and slow if associated with an atrioventricular conduction disorder; this is called brady-arrhythmia. A perfectly regular ventricular rhythm suggests the presence of a complete atrioventricular block with most often a junctional escape rhythm. Conversely, when conduction is excellent, the ventricles can be very rapid (> 170 bpm) with often a relatively regular pattern. The ventricular rate can be very high (> 250 bpm) and life-threatening, when the nodal filter does not function in a patient with a very permeable accessory pathway.
Given the supraventricular origin of the arrhythmia, the morphology of the QRS complexes is generally identical to that of the QRS complexes in sinus rhythm. The QRS complexes are therefore generally narrow. The occurrence of broad QRS complexes may occur under 3 circumstances: 1) in a patient with a permanent bundle branch block, 2) an intermittent broadening of the QRS may correspond to a conduction aberration occurring following very short RR intervals, 3) patients with ventricular pre-excitation.
The electrocardiogram after sinus return in this patient shows the presence of a very premature atrial complex in the T wave which frequently constitutes the triggering factor for the occurrence of an atrial arrhythmia episode.