A 63-year-old man is in the cardiac surgery ICU after replacement of a myxomatous mitral valve with ruptured cordae tendineae:

• Sinus Rhythm with sinus arrhythmia and AV dissociation due to a competing accelerated junctional rhythm
• 2 capture complexes (the 5th and 6th QRS complexes)
• Moderate voltage criteria for left ventricular hypertrophy

The average sinus rate is 58/min, with mild sinus arrhythmia. The intrinsic rate of the junctional pacemaker is slightly more rapid (60/min). The failure of the P waves to conduct is due to the refractoriness of the AV node induced by the preceding junctional discharge. Notice that as the QRS complex occurs progressively earlier than the P wave, eventually the atrial stimulus (P) reaches the AV node when it has recovered and is conducted to the ventricles (AV capture).