An Irregular Tachycardia Not Responsive to Medical Treatment: What Is the Diagnosis?

A 41-year-old woman was assessed for recurrent intermittent irregular palpitations, associated with dizziness and shortness of breath. She had a negative electrophysiology study 12 years earlier, for similar symptoms. In addition, β-blockers, calcium-channel blockers (Verapamil), and flecainide did not result in significant improvement of her arrhythmia. A recent echocardiogram disclosed a structurally normal heart. Following the electrocardiogram recorded during one of her latest emergency department visits (Figure 1), oral anticoagulation was started and a referral for electrophysiology opinion was made. What is the arrhythmia displayed in Figure 1, and what is the explanation of the irregularity in the rhythm?

Please turn the page to read the diagnosis.

This irregular arrhythmia results from a complex interplay of junctional ectopic tachycardia (JET) at 103 beats per minute, and sinus tachycardia at 108 beats per minute.

There is stable sinus rhythm throughout the trace, with some P waves hidden inside QRS complexes (P and (P) in Figure 2, respectively). All QRS complexes are narrow, with some resulting from conducted P waves, whereas others are dissociated. This finding of atrioventricular dissociation with narrow QRS complexes is suggestive of JET (* in Figure 2). The JET QRS morphology is slightly different compared with the conducted beats: they lack septal q waves in lateral leads, they are ever-so-slightly broader, and the ST segment is shifted opposite to the QRS complex. This slight difference in the QRS morphology and initial QRS forces between the JET and the normal beats suggests that the origin of the JET beats maybe infranodal and below the first ramifications of the His bundle, or that they conduct with minor degrees of aberrancy in the proximal His-Purkinje system.¹ The interval between some JET beats is twice as long as between others (Figure 2, horizontal double arrow vs single arrow, respectively), suggesting that some JET beats are not manifest on the ECG. This longer interval between the JET beats is occupied by 2 conducted QRS beats with a coupling interval which is shorter (ie, faster) than the sinus rate. This results from concealed retrograde conduction of the JET beats into the atrioventricular node (Figure 3, red arrows). The sinus beat hidden in the JET QRS is thus conducted with a longer atrioventricular delay (Figure 3, blue arrows), with subsequent shortening of the next RR interval (Figure 3, gray vs black arrows). Also, this delay of the atrioventricular conduction, with subsequent RR shortening, results in the next JET beat falling in His-Purkinje and ventricular refractoriness of the previously conducted beat, explaining why it is not manifest on the ECG even if the JET continues (Figure 3, empty star).

JET is mostly encountered in children or after cardiac surgery. It is notable for being resistant to medical therapy, and possibly incessant. If fast, it can lead to tachycardiomyopathy, heart failure, and death, especially in children.² In our case, JET is rather slow (approximately 103 beats per minute) and nonsustained, which likely explains why the cardiac function is completely normal. Symptoms, however, can be quite significant, either because of the irregular rhythm, “cannon waves” because of overlap between P waves and JET beats, or both. Catheter ablation may provide definitive cure, but is associated with high risk of iatrogenic atrioventricular block and requirement of life-long pacing, so it is usually performed as a last resort in refractory cases.³ Meanwhile, careful analysis of ECG should be performed to avoid mislabeling any irregular arrhythmia as atrial tachycardia or atrial fibrillation, and provide unnecessary anticoagulation.