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

P Waves on T Waves in ECG: Clinical Significance and Interpretation

When a P wave is superimposed on a T wave on an electrocardiogram (ECG), it typically indicates an atrial tachycardia (AT) or other supraventricular tachycardia with a relatively fast heart rate where atrial activation occurs before ventricular repolarization is complete.

Mechanisms and Diagnostic Significance

Atrial Tachycardia (AT)

• In focal atrial tachycardia, P waves are typically seen near the end of or shortly after the T wave, creating a "P-on-T" appearance 1
• This occurs because the rhythm is driven by the atrium and conducts normally to the ventricles, resulting in a long RP interval (time from R wave to the next P wave) 1
• AT is characterized by regular atrial activation from atrial areas with centrifugal spread, usually with atrial rates between 100-250 bpm 1

Other Supraventricular Tachycardias

• In orthodromic atrioventricular reentrant tachycardia (AVRT), the P wave can usually be seen in the early part of the ST-T segment 1
• In atypical atrioventricular nodal reentrant tachycardia (AVNRT), the P wave may be closer to the subsequent QRS complex, providing a long RP interval 1
• In the permanent form of junctional reciprocating tachycardia (PJRT), delayed atrial activation produces a long RP interval 1

Clinical Implications

Diagnostic Value

• The presence of P waves on T waves helps differentiate various forms of supraventricular tachycardia:
  • AT typically shows P waves with morphology different from sinus rhythm 1
  • The presence of AV block during tachycardia excludes AVRT and makes AVNRT very unlikely 1
  • In AT, an isoelectric baseline is usually present between P waves, distinguishing it from atrial flutter 1

Blocked Atrial Bigeminy

• In neonates and infants, blocked atrial bigeminy can simulate sinus bradycardia, with blocked P waves visible on T waves 1
• This requires careful examination of T waves for hidden P waves to avoid misdiagnosis 1

ECG Interpretation Challenges

Distinguishing Features

• The relationship between P waves and T waves helps determine the mechanism of tachycardia:
  • Short RP tachycardias: P wave is closer to the prior QRS than to the subsequent QRS 1
  • Long RP tachycardias: P wave is closer to the subsequent QRS 1

Potential Pitfalls

• P waves superimposed on T waves may be difficult to discern, especially if bundle-branch block is present 1
• The P wave may be obscured by the T wave of the preceding QRS complex in ATs 1
• Multiple electrolyte imbalances can affect the appearance of P waves and T waves, potentially creating unusual patterns such as the "tee-pee sign" described in cases of hyperkalemia with concurrent hypocalcemia and hypomagnesemia 2

Clinical Approach to P-on-T Phenomenon

1. Confirm the presence of P waves on T waves by examining multiple leads, particularly leads II, III, aVF, and V1 where atrial activity is often best seen
2. Determine the P wave morphology to distinguish from sinus rhythm
3. Measure the RP interval to classify as short RP or long RP tachycardia
4. Look for AV dissociation or block which helps rule out certain mechanisms
5. Consider the clinical context including presence of structural heart disease, prior surgeries, or electrolyte abnormalities

Management Implications

The identification of P waves on T waves and correct diagnosis of the underlying arrhythmia mechanism guides appropriate treatment strategies, which may include:

• Acute termination with vagal maneuvers, adenosine, or other antiarrhythmic medications
• Long-term management with antiarrhythmic drugs
• Consideration of catheter ablation for definitive treatment, particularly for focal AT or accessory pathway-mediated tachycardias

Understanding the significance of P waves on T waves is essential for accurate diagnosis and appropriate management of patients with supraventricular tachycardias.