What is the cardiac rhythm when the P (P wave) has the same morphology but occurs irregularly?

Cardiac Rhythm with Same P Wave Morphology but Irregular Occurrence

When a cardiac rhythm shows P waves with consistent morphology but irregular timing, this is most likely sinus arrhythmia, which is a normal physiologic variant in many individuals, particularly in young and athletic people.

Characteristics of Sinus Arrhythmia

• Sinus arrhythmia is characterized by P waves with identical morphology (originating from the sinus node) but with irregular P-P intervals 1
• The P waves maintain the same shape because they all originate from the same focus (sinus node), but occur at irregular intervals 1
• This rhythm is distinct from atrial fibrillation, which shows irregular atrial activity with absence of distinct P waves 1

Differentiating from Other Arrhythmias

• Unlike multifocal atrial tachycardia (MAT), which has ≥3 distinct P-wave morphologies with irregular rhythm, sinus arrhythmia maintains consistent P-wave morphology 1
• Unlike focal atrial tachycardia, which typically shows regular P-P intervals (except during "warm-up" and "warm-down" phases), sinus arrhythmia shows persistent irregularity 1
• Sinus arrhythmia differs from sinus node reentry tachycardia, which has abrupt onset/termination and regular rhythm despite similar P-wave morphology to sinus rhythm 1

Physiologic Basis

• The irregular timing of P waves with consistent morphology is often related to respiratory variation (respiratory sinus arrhythmia) 1
• Heart rate increases during inspiration and decreases during expiration due to changes in vagal tone 1
• This is a normal physiologic finding, particularly common in:
  • Young individuals 2
  • Athletes with high vagal tone 2
  • Healthy individuals during sleep 2

Clinical Significance

• Sinus arrhythmia is generally considered a benign finding that rarely requires intervention 2
• The consistent P-wave morphology indicates a single origin of atrial activation (sinus node) 1, 3
• The irregular timing reflects normal autonomic nervous system influence on the sinus node 2

Important Distinctions

• P-wave morphological variability (changing P-wave shape) can be an indicator of paroxysmal atrial fibrillation risk 4, 5
• Beat-to-beat P-wave morphology variability has been associated with heterogeneous atrial conduction and risk for atrial fibrillation 6
• The key distinction is that in sinus arrhythmia, P-wave morphology remains consistent while only the timing varies 1

Clinical Approach

• Confirm the diagnosis by verifying that all P waves have identical morphology despite irregular timing 1, 3
• Evaluate for respiratory correlation (rate increases with inspiration, decreases with expiration) 1
• In the absence of symptoms or underlying heart disease, no specific treatment is typically required 2