This is a foundational topic in clinical cardiology. When approaching narrow complex tachycardia (NCT)—defined as a QRS duration <120 ms with a rate >100 bpm—the diagnostic goal is to systematically differentiate between supraventricular tachycardias (SVTs) that utilize the AV node versus those that do not. Here is an advanced framework for differential diagnosis, moving from the systematic analysis of the rhythm strip to specific pathophysiological mechanisms.
1. Systematic ECG Analysis Protocol
To effectively categorize an NCT, your primary task is to identify the relationship between the P-wave and the QRS complex.
The RP Interval Rule
The RP interval (time from the onset of the QRS to the onset of the P-wave) is the most powerful discriminator:
- Short RP Tachycardia (RP < PR):
- Typical AVNRT: P-waves are often “buried” in the QRS or appear as a “pseudo-r'” in V1 or “pseudo-s” in the inferior leads.
- AVRT (Orthodromic): P-waves are distinct, usually visible after the QRS, often in the ST segment.
- Long RP Tachycardia (RP > PR):
- Atypical AVNRT (Fast-Slow): P-waves are deeply inverted in inferior leads.
- Atrial Tachycardia (AT): P-waves are generally distinct.
- PJRT (Permanent Junctional Reciprocating Tachycardia): A specific, incessant form of AVRT with a very delayed retrograde P-wave, through a tortuous posteroseptal accessory pathway.
2. Advanced Diagnostic Decision Matrix
Use this logical flow to narrow your differential:
| Rhythm | P-Wave Location | Regularity | Mechanism |
| Typical AVNRT | Buried/Pseudo | Regular | Re-entry (Slow/Fast pathway) |
| Orthodromic AVRT | Post-QRS | Regular | Accessory pathway (concealed) |
| Atrial Tachycardia | Variable | Usually Regular | Focal automaticity/macro-reentry |
| Atrial Flutter | Sawtooth Waves | Usually Regular | Macro-reentry (Isthmus-dependent) |
| Atrial Fibrillation | Absent | Irregularly Irregular | Multiple re-entrant wavelets |
3. “Masterclass” Nuances
The V1 “Pseudo-r'” Sign
In typical AVNRT (slow-fast), the retrograde P-wave occurs almost simultaneously with the QRS. This creates a “pseudo-r'” deflection at the end of the QRS complex in lead V1. If you see this, you are likely looking at the slow-fast re-entry loop of the AV node.
The “S-wave” Illusion
In the inferior leads (II, III, aVF), the retrograde P-wave in typical AVNRT often manifests as a “pseudo-s” wave. This is a subtle distortion of the terminal part of the QRS. When evaluating a narrow complex tachycardia, always compare the terminal QRS morphology to a known sinus beat if available.
When SVT isn’t “Narrow”
- Antidromic AVRT: This will present as a wide complex tachycardia but is physiologically an SVT. If the patient has a known accessory pathway, always consider this even if the QRS is aberrantly wide.
- Rate-Related Bundle Branch Block (Ashman’s Phenomenon): An aberrant conduction caused by a sudden change in cycle length (e.g., atrial fibrillation) can mimic a ventricular rhythm. Aberrancy can be maintained for more cycles by concealed transseptal conduction and resemble a run of VT.
4. Next Diagnostic Steps
If the ECG remains ambiguous, clinical maneuvers or pharmacological probes are diagnostic tools:
- Vagal Maneuvers/Adenosine: These are diagnostic probes.
- Termination: If the rhythm terminates, it confirms it is AV-node dependent (AVNRT or AVRT).
- Transient Block: If AV block occurs but the tachycardia continues, you are dealing with an atrial source (Atrial Tachycardia or Flutter).
- Esophageal ECG: If the surface ECG is non-diagnostic, esophageal leads can provide high-fidelity visualization of atrial activity, allowing for precise measurement of the VA interval.