As per Dawn - this ECG provides a nice example of atrial pacing. As always with any pacemaker — it is essential to know what parameters have been set in order to interpret the tracing. It looks like the PR interval (duration from onset of atrial spike until onset of the QRS) = 0.24 second. Depending on the patient’s underlying disorder — the programming cardiologist can set parameters to optimize the chance for spontaneous initiation of ventricular contraction if the conduction system is intact.

• It is of interest here — that in some leads (ie, I, AVL, V2,V3) — it looks like we DO see a ventricular spike in front of some complexes. This tells us that the pacemaker has indeed been set to accept a PR interval ~ 0.24 second, after which time if no spontaneous QRS complex occurs the pacemaker will stimulate the ventricles. That there is minimal difference in QRS morphology between QRS complexes with and without a ventricular spike indicates that the conduction system is indeed intact and still able to conduct with a PR interval of 0.24 second (or a slight bit more than this).