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Rhythms Originating from the Sinus Node

By Dominic Velasco, MD | Medically reviewed by Jennifer Bunn, RN on November 11, 2024

ACLS Certification Association videos have been peer-reviewed for medical accuracy by the ACA medical review board.

Article at a Glance

  • Four rhythms originate from the sinus node.
  • A normal sinus rhythm is the normal rhythm of a healthy heart.
  • Clinicians should know normal sinus rhythm characteristics to identify abnormalities.

The following rhythms originate from the sinus node, the pacemaker of the heart: (1) normal sinus rhythm, (2) sinus bradycardia, (3) sinus tachycardia, and (4) sinus arrhythmia.

Conduction system of the heart – sinus node.

The right atrium’s sinus node is the heart’s pacemaker.

Normal Sinus Rhythm

Normal sinus rhythm is the normal rhythmic pattern of a healthy heart.1 On a six-second strip of LEAD II, the following characteristics define normal sinus rhythm:

  • RATE: 60–100 beats per minute (bpm).
  • RHYTHM: regular R-R interval. The presence of a dropped beat or an ectopic beat means the rhythm is not a normal sinus rhythm.
  • P WAVE: the atria depolarizes first, followed by the ventricles. The impulse travels towards LEAD II, so the P wave has an upward deflection. The P wave is followed by the QRS complex.
  • PR INTERVAL: the PR interval is 0.12– 0.20 seconds and is consistent throughout the ECG strip. Varying PR intervals within the 6-second ECG strip is not a normal sinus rhythm.
  • QRS COMPLEX: the QRS complex is less than 0.12 seconds. If the QRS complex is prolonged, the rhythm is not a normal sinus rhythm.

Related Video – ECG Rhythm Review – Normal Sinus Rhythm

Read: Rhythms Originating from the Atrium

Sinus Bradycardia

Sinus bradycardia is a sinus rhythm with a rate less than 60 bpm.2

  • RATE: less than 60 bpm.
  • RHYTHM: regular R-R interval; a discrepancy of one small square is acceptable.
  • P WAVE: upward deflection followed by a QRS complex.
  • PR INTERVAL: 0.12–0.20 seconds, and consistent throughout the 6-second LEAD II strip.
  • QRS COMPLEX: less than 0.12 seconds.

Related Video – ECG Rhythm Review – Sinus Bradycardia

Sinus Tachycardia

Sinus tachycardia is a sinus rhythm with a rate more than 100 bpm.

  • RATE: more than 100 bpm.
  • RHYTHM: regular R-R interval.
  • P WAVE: upward deflection followed by a QRS complex.
  • PR INTERVAL: 0.12–0.20 seconds and consistent throughout the 6-second LEAD II strip.
  • QRS COMPLEX: less than 0.12 seconds.

Related Video – ECG Rhythm Review – Sinus Tachycardia

Normal sinus rhythm, sinus bradycardia, and sinus tachycardia ECG tracings.

Sinus bradycardia is a sinus rhythm less than 60 bpm, while sinus tachycardia is a sinus rhythm greater than 100 bpm.

Sinus Arrhythmias

Sinus arrhythmia may occur with breathing, the heart rate increasing with inspiration and decreasing with expiration. It’s more evident in infants and children than adults.3

The same characteristics are present in sinus arrhythmia as normal sinus rhythm, but there is an R-R interval irregularity.

  • RATE: 60–100 bpm.
  • RHYTHM: irregular R-R interval.
  • P WAVE: upward deflection followed by a QRS complex.
  • PR INTERVAL: 0.12–0.20 seconds, and consistent throughout the 6-second LEAD II strip.
  • QRS COMPLEX: less than 0.12 seconds.

Normal sinus rhythm versus sinus arrhythmia with R-R interval variation - ECG tracings.

Compared to a normal rhythm, sinus arrhythmia presents with a variation in the R-R interval.

Related Video – One Quick Question: What are the Differences Between ST (sinus tachycardia) and SVT?

Summary

There are four rhythms originating from the sinus node, including normal sinus rhythm, sinus bradycardia, sinus tachycardia, and sinus arrhythmia. They all have different rates, rhythms, P wave ECG placement, PR intervals, and QRS complexes. Clinicians should know the different values to effectively diagnose and treat patients.

More Free Resources to Keep You at Your Best

ACLS Certification Association (ACA) uses only high-quality medical resources and peer-reviewed studies to support the facts within our articles. Explore our editorial process to learn how our content reflects clinical accuracy and the latest best practices in medicine. As an ACA Authorized Training Center, all content is reviewed for medical accuracy by the ACA Medical Review Board.

1. Jill Seladi-Schulman, Ph.D. Understanding Sinus Rhythm. Healthline. 2021.

2. Cleveland Clinic. Sinus Bradycardia. 2022.

3. Michael P. Soos; David McComb. Sinus Arrhythmia. National Library of Medicine. 2021.

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