What is a common characteristic of atrial fibrillation on an ECG?

Atrial fibrillation is characterized by the absence of organized electrical activity in the atria, which results in a chaotic and irregular rhythm. On an ECG, this is reflected by the presence of variable P waves—specifically, there are no distinct and identifiable P waves as seen in normal sinus rhythm. Instead, instead of P waves, small undulating waves called fibrillatory waves (or F-waves) may be present, though these can be hard to discern. The irregularity in the ventricular response, which leads to variable distances between R waves, further supports the presence of atrial fibrillation.

The irregular R-R intervals, marked by this variability, are crucial for diagnosing this arrhythmia but do not pertain to the characteristic presentation of P waves on the ECG. Identifiable F-waves would suggest a different atrial rhythm, while broad QRS complexes may indicate other underlying conduction abnormalities that are not a defining feature of atrial fibrillation itself. Therefore, the variation in how the P waves appear, or the lack thereof, is a main hallmark of atrial fibrillation on an ECG.