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ECG structure 
There are typically six distinct waves (identified by the letters P, Q, R, S, T, and U) in a single beat of the heart in sinus rhythm, and they occur in a specific order, over specific periods of time, with specific relative sizes. While there is a significant range within which variations in rhythm are considered normal, anything that deviates from sinus rhythm by more than a certain amount may be indicative of heart disease.
See also Heart rate variability.
Cardiac Conduction System and Pathway 
The cardiac conduction system is a pathway that consists of specialized cells, known as myocytes (cardiac cells), which create the natural electrical impulse that informs the heart when it needs to pump. The location within the conduction system that gives rise to electrical impulses is known as the pacemaker. The initial impulse originates in the sinoatrial (SA) node, or sinus node, which is located in the upper right atrium of the heart. The SA node is designated as the pacemaker of the heart. From the SA node, the electrical impulse spreads through interatrial tracts that spread the electrical impulse through the right and left atria and therefore cause atrial depolarization. As a result, a P-wave is observed. After the atria depolarize, the electrical impulse spreads through the internodal tracts and reach the atrioventricular node (AV node). The AV node has its own pacing rhythm that serves as a back up pacemaker in case the SA node fails to initiate an electrical impulse. Consequently, the AV node slows down the electrical impulse to allow the atria to project their blood into the ventricles. From the AV node, the impulse travels through the bundle of His, which bifurcate into the left and right bundle branches. From the branches, the impulse travels through the Purkinje fibers and allows the electrical impulse to end in the ventricles to initiate ventricular depolarization. Consequently, a QRS complex is observed.
Sinus rhythm on ECG 
Sinus rhythm, more commonly referred to as normal sinus rhythm, is designated as the normal rhythm of the heart. Several requirements must be met for an electrocardiogram to be classified as normal sinus rhythm. Criteria for a normal sinus rhythm include:
- A heart rate between 60-100 beats per minute.
- Regularity - Regular
- The SA node pacing the heart. Therefore, a "P" wave must be present for every "QRS" complex in a ratio of 1:1.
- PR interval is between .12 second and .20 second.
- QRS complex width should be less than .12 second.
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