Interesting response of narrow QRS tachycardia to premature atrial complex. What is the mechanism?

Fig. 1 shows a premature atrial complex (PAC) introduced during a narrow QRS tachycardia. What is the response and what is the interpretation?

Fig. 1

Figure represents surface electrocardiogram (I, V1, V5) and intracardiac electrograms – High right atrium (HRA), His distal (His d), coronary sinus (CS) 9,10, CS 5,6, CS 1,2 and RV distal (RV d) showing response of tachycardia to His refractory PAC.

The tachycardia is a regular RBBB tachycardia with cycle length of 440 ms and ventriculo-atrial interval of 30 ms. There is near simultaneous activation of atrium and ventricle during tachycardia. Presence of His deflection prior to the QRS makes ventricular tachycardia unlikely as the mechanism. The differentials entertained at this point were atrial tachycardia (AT) with prolonged AV conduction, junctional tachycardia (JT) and typical AV nodal reentrant tachycardia (AVNRT) with RBBB aberrancy. To distinguish among them a His (H) refractory PAC was delivered during the tachycardia. In response to the PAC (S2) there has been an increment in the H-H interval from 440 ms to 490 ms i.e., the PAC advanced the next His by 50 ms. This excludes JT since the His refractory PAC would not affect the subsequent His activation. Delay of the next tachycardia beat following the timed PAC suggests engagement of slow AV nodal pathway or decremental AV nodal conduction. In AT, the PAC can have anterograde decremental AV nodal conduction delaying the subsequent His activation. The VA interval after the reset tachycardia beat and the VA interval during the other beats are the same suggesting VA linking which makes a diagnosis of AT less likely. The delay of the next His by this specifically timed PAC with VA linking indicates typical AVNRT as the mechanism. During typical AVNRT, a His-refractory PAC can conduct antegradely over the slow pathway and can affect the next beat of SVT resulting in either advancement or delay of the next His or termination of SVT [1]. Slow pathway ablation was carried out and sustained and slow junctional rhythm was obtained. No tachycardia was induced thereafter.

Disclosures

The authors have no competing interests, funding or financial relationships to disclose.

Conflicts of interest

None.