What does a retrograde P (atrial) wave look like on an electrocardiogram (ECG) in a patient with atrioventricular reentrant tachycardia (AVRT)?

Retrograde P Wave Appearance in AVRT

In orthodromic AVRT, retrograde P waves appear as distinct deflections inscribed in the ST-segment after the QRS complex, creating a short RP interval pattern that distinguishes this arrhythmia from other supraventricular tachycardias. 1

Key ECG Characteristics

Timing and Location

• The retrograde P wave is clearly visible in the early part of the ST-T segment, appearing separate from the QRS complex in 90-95% of AVRT episodes 2
• The RP interval is short (typically <100 ms but longer than in typical AVNRT), creating a "short RP" tachycardia where RP < PR 1, 2
• This contrasts with typical AVNRT where P waves are buried within or at the terminal portion of the QRS complex 1, 3

Morphology and Polarity

• The retrograde P wave polarity varies based on accessory pathway location 4, 5
• For left posterior pathways (the most common location), lead I shows a negative retrograde P wave exclusively 4
• Lead V1 differentiates right-sided (negative or isoelectric) from left-sided (solely positive) accessory pathways 4, 5
• In inferior leads (II, III, aVF), the P wave typically appears as a narrow negative deflection when the pathway is posteroseptal 2, 5
• Rarely, flutter-like sawtooth P waves can occur in AVRT, though this is uncommon 6

Distinguishing AVRT from AVNRT

Critical Differentiating Features

• The presence of a visible retrograde P wave in the ST segment with RP interval ≥100 ms predicts AVRT with 93% accuracy (sensitivity 71%, specificity 94%) 7
• Pseudo r' waves in V1 or pseudo S waves in inferior leads indicate AVNRT with 100% accuracy, effectively ruling out AVRT 5
• ST-segment elevation in lead aVR during tachycardia suggests AVRT with 83% accuracy (sensitivity 71%, specificity 83%) 7

Mechanistic Basis

• In orthodromic AVRT, the reentrant circuit involves anterograde conduction down the AV node followed by retrograde conduction up the accessory pathway 1
• This creates delayed atrial activation compared to AVNRT, where near-simultaneous atrial and ventricular activation occurs 1, 3
• The accessory pathway location determines the specific P wave morphology pattern across the 12-lead ECG 4, 5

Special Variants

Permanent Junctional Reciprocating Tachycardia (PJRT)

• PJRT represents a unique form of AVRT with a slowly conducting (decremental) accessory pathway, usually posteroseptal 1
• This creates a "long RP" tachycardia pattern where the P wave appears well before the next QRS complex 1, 2
• Deeply inverted retrograde P waves appear in leads II, III, and aVF 2
• The ECG appearance can mimic atypical AVNRT or low septal atrial tachycardia 1

Clinical Pitfalls to Avoid

• Bundle branch block during tachycardia can obscure retrograde P waves in both AVRT and AVNRT, making differentiation more difficult 2
• T-wave superimposition on retrograde P waves may require digital subtraction techniques for accurate analysis 4
• The absence of visible P waves does not exclude AVRT, as approximately 5-10% of cases may have P waves hidden within the QRS or T wave 2
• When RP interval differences between leads V1 and III exceed 20 ms, this suggests posterior-type AVNRT rather than AVRT with a posteroseptal pathway (sensitivity 71%, specificity 87%) 5