When the ectopic atrial focus is not in close proximity to the sinus node, the sequence of atrial depolarization will be different than that associated with the sinus beat and the shapes of the sinus and ectopic P waves will differ. This ECG is from a 60 year old male, The P waves in beats 1, 3, 5 and 7 are upright in lead II, inverted in lead aVR and biphasic in lead V1. These are the sinus P waves. The P waves in beats 2, 4, 6 and 8-12 are inverted in lead II, biphasic in lead aVR and inverted in lead V1. These are ectopic P waves, arising from a focus that is not in close proximity to the sinus node. The P-P intervals are quite constant throughout the recording, suggesting that the sinus and ectopic pacemakers are isorhythmic.