If a single P wave differs from all other P waves while the rest of the ECG remains normal, what condition might this indicate?

The identification of a single P wave that differs from the others in a normal ECG context primarily suggests the presence of a Premature Atrial Contraction (PAC). In a PAC, there is an early depolarization of the atria, leading to the generation of a P wave that appears different in morphology compared to the normal P waves. This occurs because the PAC arises from an ectopic focus in the atria rather than the sinoatrial (SA) node.

The normal rhythm continues unaffected, which is evidenced by the consistency of the rest of the ECG. In the case of PACs, one can observe the characteristic features of a PAC: the differing shape of the P wave, a normal follow-up QRS complex, and a brief pause in the subsequent heartbeat after the ectopic impulse. This generates a pattern that is distinct from other arrhythmias such as atrial fibrillation, atrial flutter, or supraventricular tachycardia, which exhibit more extensive changes in rhythm and morphology across multiple beats.

Thus, the presence of a single distinct P wave amidst a generally normal ECG supports the diagnosis of a Premature Atrial Contraction.