In which rhythm does the QRS complex spike before the P wave?

The rhythm in which the QRS complex appears before the P wave is characteristic of a Ventricle Pacemaker. In this rhythm, the impulses originate from the ventricles rather than from the atria or atrioventricular node. As a result, the ventricular depolarization occurs independently of atrial depolarization, leading to the observation that the QRS complex can occur without a preceding P wave, or with the P wave being significantly delayed after the QRS complex.

In contrast, the other rhythms do not feature this pattern. For instance, in Accelerated Idioventricular Rhythm, while the QRS may also be wide, it typically arises due to ectopic ventricular pacemaker activity without the presence of a P wave preceding it. However, during this rhythm, the QRS still usually follows an irregular atrial activity if present. Atrial Fibrillation is characterized by an irregularly irregular rhythm without distinct P waves, and there isn’t a consistent pattern of QRS complexes leading or following P waves. Third-degree AV Block shows a complete dissociation between atrial and ventricular activities, which can result in a variety of patterns, but usually, P waves will still be present, occurring independently of the QRS complexes.