What are the criteria to diagnose a sinus rhythm (normal sinus rhythm)?

From the Guidelines

To diagnose a sinus rhythm, look for a P wave before each QRS complex, regular P waves with consistent morphology, a PR interval between 0.12-0.20 seconds, and a heart rate between 60-100 beats per minute, as outlined in the 2019 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1. The P wave should be upright in leads I and II, indicating that the electrical impulse originates from the sinoatrial (SA) node in the right atrium and travels through normal conduction pathways. Some key features to identify a sinus rhythm include:

• A P wave before each QRS complex
• Regular P waves with consistent morphology
• A PR interval between 0.12-0.20 seconds
• A heart rate between 60-100 beats per minute The QRS complex should be narrow (less than 0.12 seconds) unless there's a bundle branch block. The rhythm should be regular with consistent RR intervals, though some normal respiratory variation (sinus arrhythmia) may be present. Sinus rhythm represents normal cardiac electrical activity where the SA node acts as the primary pacemaker of the heart. If the heart rate is below 60, it's sinus bradycardia; above 100, it's sinus tachycardia—both can still be sinus rhythms with the same characteristic features but at different rates, as noted in the 2010 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care 1. When assessing any ECG, always start by confirming these sinus rhythm characteristics before evaluating for other abnormalities, and consider the criteria outlined in the 2015 ACC/AHA/HRS guideline for the management of adult patients with supraventricular tachycardia 1.

From the Research

Diagnostic Criteria for Normal Sinus Rhythm

The diagnosis of a normal sinus rhythm is based on specific electrocardiogram (ECG) criteria. The key characteristics include:

• A normal P wave axis, which is measured on the frontal plane and is between 0º and +75º 2
• Atrial depolarization conducted from the right atrium to the left atrium through Bachmann's bundle 2
• A P wave polarity that can help analyze the origin point of the rhythm 2
• The presence of a sinus rhythm with a complete right bundle branch block (RBBB) can be identified by the characteristics of the QRS complex in leads V1 to V6 2

ECG Characteristics

The ECG characteristics of a normal sinus rhythm include:

• A P wave that is upright in lead I and inverted in lead aVR
• A QRS complex that is narrow and upright in leads I, II, and V4-V6
• A T wave that is upright in leads I, II, and V4-V6
• A normal PR interval, which is the time from the beginning of the P wave to the start of the QRS complex

Differentiation from Other Rhythms

It is essential to differentiate a normal sinus rhythm from other types of sinus tachycardias, such as:

• Inappropriate sinus tachycardia
• Postural orthostatic tachycardia syndrome (POTS)
• Sinus node reentry tachycardia Each of these conditions has distinct clinical features, underlying mechanisms, and treatment strategies 3, 4, 5