Left: The top two traces are routine surface ECG leads (II and aVF). The bottom trace (V4) is actually a recording from an intracardiac electrode catheter where the tip is placed in the right ventricle, touching the right ventricular myocardium. So the bottom trace is a series of ventricular electrograms and is normal. Right: Pathology specimen from a dog with atrial standstill. The atrial myocardium has been completely replaced by fibrous tissue.

As above but now the electrode catheter is in the right atrium with the electrode placed in contact with atrial myocardium. No atrial electrical activity is present.

As before but instead of recording electrical activity, a temporary pacing generator is now attached to the electrode catheter and the right ventricle is being electrically stimulated periodically (i.e., paced). This results in ventricular depolarizations (i.e., QRS complexes), as expected.

Again, as before but now the right atrium is being electrically stimulated. This results in sharp deflections that occur every 20 mm. In a normal atrium atrial depolarization would be evident as P waves following each stimulation. In this case of atrial standstill, there is no response.