Which arrhythmia is characterized by an absence of QRS complexes?

Ventricular fibrillation is characterized by an absence of QRS complexes, which is a key reason for its identification as the correct answer. In this arrhythmia, the electrical activity in the ventricles becomes chaotic, leading to ineffective contractions and an inability to pump blood. This chaotic electrical activity results in a rapid, erratic waveform on the ECG, where the recognizable QRS complexes are completely replaced by fibrillatory waves. Because the ventricles are not contracting in a coordinated manner, there is no significant electrical signal that corresponds to the normal depolarization and contraction of the ventricles.

In contrast, atrial fibrillation does present with some electrical activity, but it originates from the atria rather than the ventricles, and QRS complexes are still seen, albeit with irregularity. Sinus bradycardia involves a slower than normal heart rate but still maintains regular QRS complexes. Premature ventricular contractions (PVCs) will also show QRS complexes, though they may appear wide and bizarre when they occur. Therefore, ventricular fibrillation is distinct in that it completely lacks the QRS complexes due to disorganized electrical activity within the ventricles.