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Schematic representation of normal sinus rhythm showing standard waves, segments, and intervals

Sinus rhythm is a term used in medicine to describe the normal beating of the heart, as measured by an electrocardiogram (ECG). It has certain generic features that serve as hallmarks for comparison with normal ECGs.

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.

Sinus Rhythm on ECG

Sinus rhythm is characterized by a usual rate of anywhere between 60-100 bpm. Every QRS complex is preceded by a P wave and every P wave must be followed by a QRS (the opposite occurs if there is second or third degree AV block). The P wave morphology and axis must be normal and the PR interval will usually be 120 ms to 200 ms. In normal sinus rhythm, electrical impulses from the SA node travel to the AV node with successful contraction of the two atria. The electrical impulses from the AV node successfully contract the ventricles. On the ECG, there are normal PQRST elements with no evidence of arrhythmia, tachycardia, or bradycardia. Normal sinus rhythm on an ECG does not rule out a diagnosis of AMI. Documentation of the condition of ST segment and QT values must be evaluated. Simply documenting "ECG NSR" only indicates the underlying rhythm.

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