What is the definition of sinus rhythm?

Definition of Sinus Rhythm

Sinus rhythm is defined as a cardiac rhythm that originates in the sinoatrial node, characterized by normal electrical activation of the heart 1.

Key Characteristics of Sinus Rhythm

Sinus rhythm has several defining features:

1. Origin: The electrical impulse originates in the sinoatrial (SA) node, which is located at the junction of the superior vena cava and right atrium 2

2. P wave morphology: Normal P waves that precede each QRS complex, with consistent morphology 1

   • P wave axis in the frontal plane between 0° and 90° 3
   • P waves are upright in leads II, III, and aVF

3. Conduction pattern: Normal atrial activation sequence with orderly conduction through the heart's conduction system 4

   • From SA node → atria → AV node → His-Purkinje system → ventricles

4. Rate: While often associated with a rate of 60-100 beats per minute in adults at rest, the actual definition of sinus rhythm is based on the origin of the impulse, not the rate 1

   • Sinus bradycardia: Sinus rhythm with rate <60 bpm
   • Sinus tachycardia: Sinus rhythm with rate >100 bpm

5. Regularity: Generally regular rhythm, though physiologic sinus arrhythmia (variation with respiration) may be present, especially in younger individuals 1

Anatomical Basis of Sinus Rhythm

The SA node is a specialized 3-dimensional intramural structure with unique properties:

• Located at the junction of the superior vena cava and right atrium along the crista terminalis 2
• Contains specialized pacemaker cells with intrinsic automaticity 5
• Impulse generation occurs through spontaneous diastolic depolarization of SA node cells 5
• Conduction to the atria occurs through discrete sinoatrial conduction pathways 2

Physiological Mechanisms

The spontaneous depolarization of SA node cells results from:

• Hyperpolarization-activated "pacemaker" (If) current
• L-type and T-type calcium currents
• Delayed potassium currents
• Sodium-calcium exchange currents 5

Clinical Significance

Sinus rhythm represents the normal cardiac rhythm and is essential for:

• Optimal cardiac function
• Exercise capacity
• Quality of life 5

Maintenance of sinus rhythm is a treatment goal in many arrhythmias, particularly atrial fibrillation, as it is associated with improved outcomes 6.

Distinguishing from Other Rhythms

Sinus rhythm must be distinguished from other cardiac rhythms:

• Atrial fibrillation: Characterized by irregular atrial activity, absence of distinct P waves, and irregular R-R intervals 1
• Atrial flutter: Regular atrial activity with characteristic "sawtooth" flutter waves 1
• Ectopic atrial rhythms: P wave morphology differs from sinus P waves 1
• Junctional rhythm: Absent or retrograde P waves (negative in inferior leads) 1

Common Pitfalls in Identifying Sinus Rhythm

1. Mistaking sinus tachycardia for other supraventricular tachycardias: Remember that sinus rhythm is defined by origin, not rate 3

2. Overlooking sinus node dysfunction: In patients with symptoms like syncope or dizziness, evaluate for appropriate heart rate response to exercise or other stimuli 1

3. Misinterpreting sinus arrhythmia: Normal respiratory variation in heart rate, especially in young people, should not be confused with pathological arrhythmias 1

4. Failing to recognize SA node exit sites: Surface ECG mapping may detect exit sites rather than the actual intramural pacemaker location 2

By understanding these defining characteristics, clinicians can accurately identify sinus rhythm and distinguish it from pathological cardiac rhythms that may require intervention.