ECG challenge: an incidental reset—unravelling the tachycardia

Case

A 62-year-old lady presented to the emergency department with 1 day history of palpitation, chest discomfort, and pre-syncopal attack triggered by recent onset upper respiratory tract symptoms. She admitted to having intermittent palpitation started ∼6 months prior to the current presentation. It usually lasted 10 min and resolved spontaneously, with a frequency of recurrence once a month.

ECG

Figure 1 : Electrocardiogram of the tachycardia event.

Figure 1.

Electrocardiogram of the tachycardia event.

Question 1

Which of the following event is not observed in the ECG?

1. The tachycardia has a short RP interval.

2. Atrial ectopic beats occur during the His-Purkinje refractory period.

3. The tachycardia is perturbed and reset by an ectopic beat.

4. There is presence of premature ventricular complex (PVC).

5. The tachycardia is likely of supraventricular origin.

The correct answer is D.

Discussion and explanation

This ECG has revealed narrow complex tachycardia with a R–R interval of 380 ms, which is likely of supraventricular origin. The negative deflection observed in the inferior leads at the terminal portion of each QRS complex is likely to represent a P wave. Interestingly, a few atrial ectopic beats were also observed, followed by a disappearance of previously mentioned negative deflection at the terminal portion of the subsequent QRS, which further proved to be a P wave. Hence, the tachycardia has a short RP interval of <70 ms. The close proximity of the atrial ectopic beat to the immediate QRS complex suggested that it is produced during His-Purkinje refractory period. This is further supported by the fact that it did not perturb the immediate next QRS complex—the R–R interval before and after the atrial ectopic beat remained at 380 ms. An early atrial ectopic beat occurring when His-Purkinje is not in refractoriness would have travelled down via the atrioventricular (AV) node and perturbed the immediate QRS complex. We also observed that the subsequent R–R interval following the atrial ectopic beat was shortened to 344 ms and then restored back to 380 ms, i.e. the second QRS has advanced and tachycardia reset. There is no evidence of PVC in this ECG.

Question 2

Given the above information, what is the likely diagnosis?

1. Atrioventricular nodal re-entrant tachycardia (AVNRT)

2. Junctional tachycardia (JT)

3. Atrial tachycardia (AT) with delayed AV conduction

4. Orthodromic atrioventricular reciprocating tachycardia (AVRT)

5. Atrial fibrillation (AF)

The answer is A.

Discussion and explanation

As described by Padanilam et al.,¹ any perturbation of the subsequent tachycardia beat after a premature atrial complex (PAC) during junctional refractoriness indicates anterograde conduction via slow AV nodal pathway. In our patient, atrial ectopic impulse travels down the slow pathway (shorter refractory) at slower conduction velocity and by the time it reaches the His-Purkinje system, the His refractory period has recovered and impulse can be conducted to the ventricles, hence perturbing the subsequent QRS complex. The impulse can then travel retrograde via fast pathway to atrium and the tachycardia continues at its previous cycle length of 380 ms within the AV node. This is not possible in JT or AT as impulse is not able to travel down without the presence of slow pathway. As for AVRT, the atrial ectopic beat would have terminated the tachycardia. The presence of P wave ruled out AF. For better understanding of the ECG, refer to ladder diagram drawn (see Supplementary material online, Figure S1).

Question 3

Which of the following is true regarding typical AVNRT?

1. The mechanism of tachycardia is automaticity.

2. It involves dual AV nodal pathway: antegrade conduction via fast pathway, retrograde conduction via slow pathway

3. The presence of a pseudo-R wave in lead V1 and a pseudo-S wave in inferior lead is highly suggestive of AVNRT than AVRT and AT.

4. During tachycardia event, amiodarone is recommended in haemodynamically stable patient.

5. Pharmacotherapy is a better choice compared to catheter ablation in long-term.

The answer is C.

Discussion and explanation

The mechanism of typical AVNRT involves re-entrant circuit with conduction antegrade via slow pathway and retrograde via fast pathway. In haemodynamically stable patient, the recommended first line therapy includes vagal manoeuvre and/or intravenous adenosine.² In our patient, the tachycardia terminated spontaneously. Catheter ablation is recommended for symptomatic recurrent AVNRT, and drugs therapy should be considered if ablation is not desirable or feasible.²

Supplementary material

Supplementary material is available at European Heart Journal – Case Reports online.

 

Consent: Informed consent was obtained from the patient for the preparation and publication of this manuscript, in accordance with Committee on Publication Ethics (COPE) guidelines.

Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

The data that support the findings of this study are available from the authors upon request.