Pacemaker malfunction - non-sensing

The second and fourth paced QRS complexes occur earlier than expected based upon the lower rate limit of the pacemaker. The pacemaker has failed to sense the preceding native complexes, and is therefore not inhibited by them. Thus, the pacemaker will fire and stimulate the atria or ventricles at its own predetermined rate, independently of the intrinsic rhythm. The interval between the native and paced complexes is variable while the intervals of the pacing spikes is constant.

The normal P wave in sinus rhythm is slightly notched since activation of the right atrium precedes that of the left atrium. The P wave is upright in a positive direction in leads I and II. A P wave with a uniform morphology precedes each QRS complex. The rate is between 60 and 100 beats per minute and the cycle length is uniform between sequential P waves and QRS complexes. In addition, the P wave morphology and PR intervals are identical from beat to beat.