Atrial fibrillation occurs when action potentials fire very rapidly within the pulmonary veins or atrium in a chaotic manner. The result is a very fast atrial rate — about 400 to 600 beats per minute. Because the atrial rate is so fast, and the action potentials produced are of such low amplitude, P waves will not be seen on the ECG in patients with atrial fibrillation.
At times, the P wave activity may be observed as “coarse fibrillatory waves,” and the term “coarse atrial fibrillation” is used, though there is no clinical significance to this finding.
The atrial action potentials all attempt to conduct through the atrioventricular node; however, the AV node becomes intermittently refractory and will only allow a certain number of atrial action potentials to reach the ventricles. This is the reason the ventricular rate is not also 400 to 600 bpm, but rather around 100 to 200 bpm. The degree to which action potentials can cross the AV node to the ventricles is variable and reduced by AV blocking medications.
Because the AV node is intermittently (not regularly) refractory, the QRS complexes that are produced when an atrial action potential does reach the ventricles will occur in an “irregularly irregular” manner, as there is no pattern to their frequency. This is commonly described as varying RR intervals.
The only two other rhythms that are irregularly irregular are atrial flutter with variable conduction and multifocal atrial tachycardia, or MAT. Atrial flutter has the typical “sawtooth pattern,” whereas multifocal atrial tachycardia requires three distinct P wave morphologies in one 12-lead ECG tracing. Note that there are quite a few arrhythmias that are regularly irregular, such as second-degree AV block type I (Wenkebach).
This means an ECG showing atrial fibrillation will have no visible P waves and an irregularly irregular QRS complex. The ventricular rate is frequently fast, unless the patient is on AV nodal blocking drugs such as beta-blockers or non-dihydropyridine calcium channel blockers. Fibrillatory waves may or may not be detected.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009;doi:10.1161/CIRCULATIONAHA.108.191095.