What are the criteria to identify sinus rhythm on a 12-lead electrocardiogram (ECG)?

Identifying Sinus Rhythm on a 12-Lead ECG

Sinus rhythm can be identified on a 12-lead ECG by the presence of P waves with normal morphology that are positive in leads I, II, and aVF, negative in aVR, with a frontal plane axis between 0 and 90 degrees, followed by normal QRS complexes with a consistent PR interval. 1

Key Criteria for Identifying Sinus Rhythm

P Wave Characteristics

• Morphology: P waves should have normal contour (smooth, rounded)
• Polarity:
  • Positive in leads I, II, and aVF
  • Negative in aVR
  • May be negative in leads V1 and V2
  • Positive in leads V3 to V6 1
• Axis: Frontal plane axis between 0 and 90 degrees
• Origin: P waves originate from the sinus node with centrifugal spread

Rhythm Characteristics

• Rate: Typically 60-100 beats per minute in adults at rest
• Regularity: Regular rhythm with consistent PR intervals
• PR interval: Normal (120-200 ms)
• QRS complex: Normal duration (<120 ms) unless there is a conduction abnormality

Distinguishing Sinus Rhythm from Other Rhythms

Sinus Rhythm vs. Atrial Tachycardia

• In atrial tachycardia, P waves generally occur in the second half of the tachycardia cycle and may be obscured by the T wave of the preceding QRS complex 1
• Atrial tachycardia typically has rates between 100-250 bpm, whereas sinus tachycardia is nonparoxysmal and rate-appropriate for physiological conditions 1
• In atrial tachycardia, an isoelectric baseline is usually present between P waves, which helps distinguish it from atrial flutter 1

Sinus Rhythm vs. Other SVTs

• In AVNRT, P waves are often hidden within the QRS complex or appear immediately after as a pseudo-R wave in V1 or pseudo-S wave in inferior leads 2
• In AVRT, P waves are typically visible in the ST segment, separated from the QRS by approximately 70 ms 2
• Atrial fibrillation lacks discrete P waves and has an irregularly irregular ventricular response 2

Common Pitfalls in Identifying Sinus Rhythm

1. Mistaking Sinus Tachycardia for Other SVTs:

   • Sinus tachycardia is nonparoxysmal and rate-appropriate for physiological conditions, whereas other SVTs often have abrupt onset and termination 1
   • Sinus tachycardia may be physiological (exercise, stress) or pathological (fever, hypovolemia, anemia) 1

2. Overlooking Subtle P Wave Abnormalities:

   • Changes in P wave morphology may indicate atrial pathology or ectopic atrial activation
   • Prolonged P wave duration (>120 ms) may suggest interatrial conduction delay 3

3. Confusing Sinus Arrhythmia with Pathological Rhythms:

   • Sinus arrhythmia (respiratory variation in heart rate) maintains normal P wave morphology but with slight irregularity in rhythm

Practical Approach to ECG Analysis for Sinus Rhythm

1. Assess P Wave Presence and Morphology:

   • Look for consistent P waves before each QRS complex
   • Confirm normal P wave axis and morphology (positive in I, II, aVF; negative in aVR)

2. Evaluate Rate and Regularity:

   • Calculate heart rate (normal sinus rhythm: 60-100 bpm)
   • Assess regularity of R-R intervals

3. Check PR Interval and QRS Duration:

   • PR interval should be consistent and normal (120-200 ms)
   • QRS duration should be <120 ms unless there is a bundle branch block

4. Rule Out Other Rhythms:

   • No evidence of AV dissociation
   • No sudden changes in rate or rhythm
   • P waves not hidden within QRS complexes or T waves

By systematically applying these criteria, clinicians can accurately identify sinus rhythm on a 12-lead ECG and distinguish it from pathological arrhythmias that may require intervention.