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In  normal sinus rhythm (NSR), pacemaker impulses are initiated in SA node, travel through the atrial pathways, and are delayed at the AV node. Depolarization of the atria produces a P wave. Impulse conduction proceeds unimpeded through the AV node and down the bundle branches to Purkinje fibers in the ventricles, causing ventricular depolarization and normally producing a narrow QRS complex.

Criteria:

 1. The P wave is usually upright in leads II, III, aVF and V1. Its morphology remains constant at all times.
 2. P-P and R-R intervals are equal and regular.
 3. Atrial and ventricular rates are identical and range between 60 and 100 bpm.
 4. PR interval: Normal (0.12-0.2 seconds)
 5. QRS: Normal (0.6-0.1 seconds)

Note: A normal ECG does not exclude heart disease.

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