Normal sinus rhythm is the normal rhythmic pattern of the healthy heart (see Figure 3.1); it is not an arrhythmia. The following characteristics of a normal sinus rhythm should be kept in mind when monitoring Lead II in a 6-second ECG strip:

1. The pacemaker impulse comes from the SA node and promptly travels through the normal conduction pathways.
   • The P wave pattern is uniform.
   • One P wave proceeds each QRS complex.
2. The atria depolarize first, followed by the ventricles.
   • The inherent rate of the SA node is 60–100 bpm.
   • Any rate < 60 bpm or > 100 bpm is not considered a normal sinus rhythm.
3. Normal sinus rhythm is regular.
   • The R-R interval should be regular across the rhythm strip.
   • If abnormal beats are present, but the rhythm is sinus in origin, we say that the underlying rhythm is normal sinus rhythm.

If two premature ventricular contractions (PVCs) are present on a 6-second strip and the rest of the pattern is consistent with normal sinus rhythm, we would say that the rhythm is normal sinus rhythm with PVCs.

4. The PR interval of the normal sinus rhythm is 120–200 milliseconds (0.12–0.20 seconds).
   • Any interval > 120 milliseconds or < 200 milliseconds means that the rhythm is not a normal sinus rhythm.
   • The PR interval should be consistent throughout the ECG strip.
   • It is not a normal sinus rhythm if the PR interval varies from one beat to another, even if the PR interval is consistently between 120 and 200 milliseconds.

When evaluating a 6-second ECG strip, if the first PR interval is 130 milliseconds and the subsequent PR interval is 150 milliseconds, the rhythm would not be interpreted as a normal sinus rhythm.

5. The QRS complex should be < 120 milliseconds (0.12 seconds).

Figure 3.1. The Conduction System of the Heart

Heart’s Conduction System