Regular Tachycardia With Abnormal QRS Axis
A 32-year-old woman with unremarkable medical history presented with hemodynamically stable, regular tachycardia of 150 bpm (Figure 1).
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Response to ECG Challenge
The ECG (Figure 1) shows a QRS complex with extreme axis deviation (+160°) that is usually seen during ventricular arrhythmias (fascicular) with a structurally normal heart. However, a narrow QRS complex (90 ms) suggests a supraventricular tachycardia without aberrancy.
Supraventricular tachycardia with an abnormal axis can be caused by underlying heart disease, but a positive QRS in lead aVR and a negative lead I suggest limb lead reversal,1 in which case, normal R-wave progression in the precordial leads during supraventricular tachycardia is expected. The present ECG recorded a progressive decrease in R-wave amplitude from V1 to V6. A right-axis deviation with absent R-wave progression in the precordial leads is typically observed in mirror-image dextrocardia.2
During tachycardia, negative P waves in the inferior leads (Figure 2, interrupted arrows) are identified 120 ms from the beginning of the QRS complex, as is seen during either a slow-slow atypical atrioventricular nodal reentrant tachycardia or a posteroseptal bypass tract–mediated atrioventricular reentrant tachycardia.3 In both arrhythmias, the ventricular activation precedes the atrial activation that is directed from inferior to superior (hence negative P waves in inferior leads). Electric alternans (beat-to-beat variation in QRS amplitude during tachycardia, beats 1–6) in the absence of pericardial effusion is usually a result of beat-to-beat changes in calcium fluxes into the ventricular myocardium and once again can be observed in both atrioventricular nodal reentrant tachycardia and atrioventricular reentrant tachycardia. Electric alternans disappeared once the tachycardia stopped.
In summary, the ECG is suggestive of an atypical form of atrioventricular nodal reentrant tachycardia or orthodromic atrioventricular reentrant tachycardia in a patient with mirror-image dextrocardia. During cardiac electrophysiological study (gastric air bubble noted on the right side indicating the presence of both dextrocardia and situs inversus), a right-sided posteroseptal bypass tract was diagnosed and successfully ablated (Figure 3).
Although there have been reports of atrioventricular nodal reentrant tachycardia and manifest accessory pathway in a patient with mirror dextrocardia, orthodromic atrioventricular reentrant tachycardia in a patient with mirror-image dextrocardia and situs inversus totalis has rarely been reported.
The authors thank Radu Darciuc, MD, for his input during image editing.
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- © 2017 American Heart Association, Inc.