Sinus rhythm from "head to toe"
The sinoatrial node (SAN), the pacemaker of the heart, is positioned close to the insertion of the superior vena cava at the roof of the right atrium. It is crescent shaped and spans up to 30 mm length, is 5–7 mm wide and 1–2 mm deep in adults. Anatomically it consists of a caput and a caudal part, with the caput usually showing faster spontaneous depolarization rates compared to the cauda. All parts of the SAN, however, are capable of spontaneous depolarization and sometimes the caudal part takes over for a few beats, resulting in slight alterations of the P wave axis on the ECG.
Below is an example of such a switch from the SAN caput to the caudal part and back again. The ECG was taken in a 16 years old female:
The first six beats are regular sinus beats with a P wave axis of about 65° (P waves slightly negative in aVL & positive in II, III, aVF).
The 7th, 8th & 9th beat show a slightly different P wave axis of about 0° (P waves now positive in aVL & isoelectric in aVF).
Then the axis switches back again. This finding may occasionally be seen in routine ECGs and represents a normal variant.
Further information on the SAN can be found on https://www.ncbi.nlm.nih.gov/books/NBK459238/