What primary characteristic differentiates atrial fibrillation from normal sinus rhythm?

Atrial fibrillation is primarily characterized by the absence of distinct P waves on the electrocardiogram (ECG). In normal sinus rhythm, P waves are present and indicate that the electrical impulses are originating from the sinoatrial (SA) node and conducting normally through the atria. Each P wave corresponds to a contraction of the atria.

In contrast, atrial fibrillation is characterized by chaotic electrical activity in the atria that results in rapid and irregular contractions. This disorganization prevents the formation of distinct P waves; instead, the ECG displays a wavy baseline without identifiable P waves, reflecting the irregular and ineffective atrial contractions. This absence of clear P waves is a hallmark feature that helps in diagnosing atrial fibrillation.

The other characteristics listed do not align with atrial fibrillation. Regular P waves, consistently normal heart rate, and a fixed PR interval are typically observed in normal sinus rhythm rather than in atrial fibrillation. This distinction is crucial for healthcare professionals in diagnosing and managing patients with arrhythmias.