Differential diagnosis of narrow-QRS (≤120 ms) tachycardias

Regular narrow-QRS (≤120 ms) tachycardias

Clinical characteristics

Several clinical characteristics are useful for the appropriate diagnosis of a narrow-QRS tachycardia. Atrial fibrillation (AF) is the most commonly treated substrate, followed by atrioventricular nodal reentrant tachycardia (AVNRT), atrial flutter, and atrioventricular reentrant tachycardia (AVRT), in patients referred for catheter ablation. Thus AVNRT is the most common diagnosis in the presence of a regular narrow-QRS tachycardia. Women are more likely to be affected by AVNRT than men, whereas the converse is true for AVRT. A relationship to the menstrual cycle has been suggested, and episodes are more frequent during pregnancy in women with preexisting supraventricular tachycardia (SVT).

A sudden onset more likely points to AVNRT or AVRT, although an atrial tachycardia (AT) may also present in this way. Reentrant tachycardias tend to last longer than AT episodes, which may occur in a series of repetitive runs. Clear descriptions of pounding in the neck (the so-called frog sign) or “shirt flapping ” would point to the possible competing influences of atrial and ventricular contraction on the tricuspid valve and to AVNRT as a likely cause. ^{, ,}

Electrocardiographic differential diagnosis

In the absence of an electrocardiogram (ECG) recorded during the tachycardia, a 12-lead ECG in sinus rhythm may provide clues for the diagnosis of SVT. The presence of preexcitation in a patient with a history of regular paroxysmal palpitations is suggestive of AVRT. The absence of apparent preexcitation does not rule out the diagnosis of AVRT because it may be due to a concealed accessory pathway (AP) that conducts only retrogradely or to an atriofascicular or nodofascicular/nodoventricular bypass tract that is not apparent during sinus rhythm. An ECG taken during tachycardia is very useful in the efficient diagnosis of SVT, although it may fail to lead to a specific diagnosis.

RP interval

According to their P/QRS relationships, SVTs are classified as having short or long RP intervals. Short-RP SVTs are those with RP intervals shorter than half the tachycardia RR interval, whereas long-RP SVTs display RP greater than or equal to PR ( Fig. 10.2 ). Rarely, recording of U waves during typical AVNRT may simulate a long-RP tachycardia.

On electrophysiologic study, a very short VA interval (≤70 ms) from the onset of the QRS to the atrial depolarization in the His bundle electrogram usually indicates typical AVNRT, or less commonly focal AT, but has also been reported in AVRT. For surface ECG measurements, a cutoff interval of 90 ms has been shown to be useful and can be used if P waves are visible, but data on actual RP measurement during various types of SVT are scarce.