A Narrow QRS Tachycardia—What Is the Diagnosis?

Abstract

Case PresentationA 58-year-old man presented to the emergency department with complaints of palpitations and chest discomfort. He had multiple cardiovascular risk factors and suffered an anterior ST-segment elevation myocardial infarction 2 weeks prior, for which emergent revascularization was performed with percutaneous coronary intervention. His left ventricular ejection fraction then was 30% postinfarction and his baseline electrocardiogram (ECG) showed a left bundle branch block (LBBB) with QRS duration of 120 ms (Fig. 1).His presenting ECG at the emergency department showed a tachycardia of QRS 100 ms with right bundle branch block morphology (Fig. 2). What is the ECG diagnosis? DiscussionOur patient had a pre-existing LBBB and subsequently presented with a tachycardia with a narrower QRS duration than his baseline. There was a monophasic dominant R wave in V1 and the R/S ratio in V6 was <1. The QRS axis was deviated toward a "northwest axis" and atrioventricular dissociation was evident. These features were consistent with ventricular tachycardia (VT). 1 Narrow-QRS ECG complexes reflect rapid activation of the ventricles via the His-Purkinje system, in contrast to wide-complex tachycardia where ventricular activation is slow due to either His-Purkinje conduction abnormalities or slower myocyte-to-myocyte conduction. In the setting of a pre-existing LBBB, with the exception of bilateral bundle branch delay, the onset of supra-VT would not result in narrowing of the QRS complex. 2 The relatively narrow QRS duration could be explained by the presence of VT involving electrical capture of the His-Purkinje system. In our patient's case, the VT must have entered the His-Purkinje system below the site of LBBB, with conduction down the Purkinje fibers resulting in a narrow QRS complex. The superior axis suggests that the VT exit site was in the region of the left posterior fascicle.The likely etiology of this VT could be left posterior Purkinje fiber-mediated VT postinfarction, which has been reported to occur in acute and chronic phases of myocardial infarction. 3 Similar to idiopathic left VT, the mechanism of a Purkinje fiber-mediated VT requires substrate for a re-entrant circuit, involving the Purkinje fibers and surviving muscle bundles within areas of scarring. Ablation at the site of the presystolic or diastolic Purkinje potentials during VT has been shown to be effective in eliminating tachycardia recurrence. 4 Other etiologies include interfascicular VT and focal Purkinje VT. 5,6 In young patients without structural heart diseases, one should also consider the diagnosis of idiopathic left VT.In summary, VT should always be a consideration in the differential diagnosis of "narrow QRS" tachycardia.