How does having only one abnormal P wave in an ECG reading typically manifest?

In an ECG reading, having only one abnormal P wave typically indicates a Premature Atrial Contraction (PAC). This phenomenon occurs when an ectopic focus in the atria generates an electrical impulse that triggers an early heartbeat before the next scheduled sinus node impulse. The P wave resulting from this ectopic impulse may appear different from the normal sinus P wave, often being shaped differently or occurring prematurely.

PACs are characterized by their single, isolated nature, which distinguishes them from conditions like atrial fibrillation where multiple P waves are absent or chaotic, or sinoatrial block where there would be a pause in the ECG recording due to the sinus node failing to fire. Additionally, in ventricular tachycardia, the focus of the abnormal impulse is located in the ventricles, and the P waves can be absent or dissociated from the QRS complexes, making them fundamentally different from the context of a PAC. Therefore, the presence of one abnormal P wave aligns with the characteristics of a Premature Atrial Contraction.