Sinus: atrial depolarisation starts from the sinoatrial node.
This requires:
- P wave precedes each QRS complex, with a regular PR interval.
- P wave axis is normal (0 ‐ +90) i.e. P wave upright in leads Ι and aVF .
Non‐sinus: some atrial (non‐sinus) rhythms may have P waves in front of every QRS but with an abnormal P axis.
