The default 0.25 times setting is a slow teaching speed. The displayed animation rate is not a physiological heart rate.
Controls. Use Play or Pause, Reset, or the speed slider. The displayed animation rate describes playback only and is not a physiological heart rate.
Illustrated Mobitz I sequence. The PR interval progressively prolongs across conducted beats until one P wave is not followed by a QRS complex. The next conducted beat has a shorter PR interval and the cycle begins again. This animation shows a 3:2 conduction ratio.
Mechanism and site. The animation represents progressive AV-nodal conduction delay and refractoriness, not literal fatigue. Mobitz I most commonly occurs at the AV node. A narrow QRS supports a nodal site but does not prove it; Wenckebach periodicity can occur within or below the His bundle.
Interval relationships. P waves continue through the sequence, with the final P wave in the illustrated group remaining non-conducted. In this classic sequence, the R–R intervals shorten before the blocked P wave. With a stable atrial rhythm, the ventricular pause containing the non-conducted P wave is less than two P–P intervals. Clinical Wenckebach sequences may not show every classic interval relationship.
Clinical interpretation. Mobitz I may occur during high vagal tone or with drug effects and can be recorded sporadically in clinically healthy dogs. It can also accompany clinically important conduction disease. Interpret it in the context of clinical signs, timing, persistence, ventricular rate, pause duration, QRS morphology and associated cardiac or systemic disease.
Watch. The amber ball remains at the illustrated AV node for progressively longer intervals. The third P wave is non-conducted and no ventricular depolarisation follows. After the pause, the sequence resets.
Educational illustration only. This animation simplifies cardiac conduction and is not a diagnostic ECG, treatment recommendation, or substitute for assessment of a complete clinical tracing.