What does it mean when a P wave is superimposed on a T wave on an electrocardiogram (ECG)?

P Waves Superimposed on T Waves in ECG: Diagnostic Significance

P waves superimposed on T waves ("P-on-T" appearance) most commonly indicate focal atrial tachycardia, characterized by a long RP interval where atrial activation occurs near the end of or shortly after the T wave. 1

Diagnostic Significance of P-on-T Phenomenon

The appearance of P waves on T waves has specific diagnostic implications:

Primary Causes

• Focal Atrial Tachycardia (AT)

  • Most common cause of P-on-T appearance
  • Regular atrial activation from atrial areas with centrifugal spread
  • Atrial rates typically between 100-250 bpm
  • Creates a long RP interval pattern 1

• Atypical Atrioventricular Nodal Reentrant Tachycardia (AVNRT)

  • P wave may appear closer to the subsequent QRS complex
  • Results in a long RP interval 1

• Permanent Form of Junctional Reciprocating Tachycardia (PJRT)

  • Delayed atrial activation produces a long RP interval
  • P waves may be superimposed on T waves 1

Secondary/Less Common Causes

• Multiple Electrolyte Imbalances
  • Combined hyperkalemia, hypocalcemia, and hypomagnesemia can cause:
    • Peaked T waves
    • Prominent U waves
    • Prolongation of the descending limb of T wave that overlaps with the next P wave 2

Clinical Approach to P-on-T Identification

Examination Technique

1. Examine multiple ECG leads - particularly leads II, III, aVF, and V1 where P waves are often most visible 1
2. Assess P wave morphology - different from sinus rhythm in focal AT
3. Measure the RP interval - classify as long RP tachycardia when P-on-T is present
4. Look for AV dissociation or block - presence excludes AVRT and makes AVNRT unlikely 1

Diagnostic Challenges

• P waves superimposed on T waves may be difficult to discern, especially if:
  • Bundle-branch block is present
  • The P wave is obscured by the T wave of the preceding QRS complex 1

Clinical Significance and Differential Diagnosis

Distinguishing Features

• In orthodromic AVRT, P waves typically appear in the early part of the ST-T segment, not on the T wave 1
• In atrial flutter, there is no isoelectric baseline between P waves, unlike AT 1
• In neonates and infants, blocked atrial bigeminy can simulate sinus bradycardia with blocked P waves visible on T waves 1

Potential Pitfalls

• Misdiagnosis Risk: P-on-T pattern may be mistaken for:
  • T wave abnormalities
  • U waves
  • ST segment changes
• Hidden P Waves: Always examine T waves carefully for hidden P waves, especially in cases of apparent bradycardia 1

Management Implications

The identification of P-on-T pattern guides management by:

• Helping determine the mechanism of tachycardia
• Classifying as long RP tachycardia, which narrows treatment options
• Directing appropriate pharmacological or ablation therapy based on the specific arrhythmia mechanism 1

Understanding the P-on-T phenomenon is crucial for accurate diagnosis and appropriate management of supraventricular tachycardias and other cardiac conditions that may present with this ECG finding.