A sinus rhythm is a rhythm that originates in and is controlled by the sino-atrial node. This rhythm is the normal cardiac rhythm in all domestic animals. To determine if a rhythm is sinus or not based on the ECG requires the following criteria:
- There is a P wave for every QRS complex.
- The PR interval is relatively constant (especially if the R wave to R wave interval varies).
- The P wave is positive in lead II.
Since the P wave doesn’t specifically indicate sino-atrial activation but rather is the result of sino-atrial activation (indicates atrial depolarization), we can only infer the normalcy of sino-atrial activity from the routine ECG.