This ECG demonstrates an accelerated AV nodal rhythm that is slightly faster than the sinus and a slightly irregular ventricular rate. Because of the slightly faster AV nodal rate, most of the sinus P waves are non conducted, i.e.they are dissociated from the QRS complexes, and several are not obvious because they are located within the QRS complexes. By placing calipers on the visible P waves and āmarchingā the P waves through the QRS complexes, it is possible to determine the presence of these "hidden" P waves within the QRS complexes. P waves #1, and #5-9 do not conduct through the AV node because it is refractory. When the P waves emerge just after the QRS complex, as occurs for P waves #2 and #10, they are able to enter the AV node and capture the ventricles, but with a prolonged PR interval because the AV node is still in a partially refractory state. P waves #3 and 11, and possibly P wave #4 are also conducted to the ventricles, now with a normal PR interval. Then, the slightly more rapid nodal rate dominates and the two rhythms again become dissociated. This phenomenon is referred to as āinterference dissociationā and accounts for the irregular ventricular rate. This tracing is show again with a ladder diagram on the next page.