Figure 2-1: Explanation
This recording was obtained in a 25-year-old woman with a history of palpitations and dizziness. What is the mechanism of tachycardia(s)?
This figure demonstrates the transition from a wide QRS complex tachycardia to a narrow QRS complex tachycardia. The first question is whether the patient has supraventricular tachycardia with aberrancy or ventricular tachycardia with a transition to a supraventricular tachycardia. Typically, one would not expect the tachycardia rate to increase with the disappearance of aberrancy and careful measurement of the wide QRS complex tachycardia shows that it has a longer cycle length than the subsequent narrow QRS tachycardia. Does this mean that the wide complex tachycardia is ventricular tachycardia and it somehow induces a supraventricular tachycardia?
Evidence to support that this is a supraventricular tachycardia can be found in careful analysis of the T wave just preceding the onset of the arrhythmia. Note that this T wave shows a peaked contour compared with the preceding T waves and this strongly suggests that a P wave is inscribed on the T wave. This is most compatible with the onset of a supraventricular tachycardia. If the bundle branch system were used in supraventricular tachycardia, one might indeed anticipate a shorter cycle length with the disappearance of the bundle branch block. This is characteristic of AV reentry (AVRT) utilizing an accessory pathway for retrograde conduction. In such an instance, the cycle length will prolong in approximately 85% of patients who have a bundle branch block occurring on the side of the accessory pathway, in this instance a left-sided accessory pathway with left bundle branch block (LBBB) aberrancy. This is because of an increase in the circuit time reflected in the ventriculoatrial interval due to transseptal conduction time from the right to left ventricle in the presence of LBBB. The disappearance of the LBBB will shorten the reentrant circuit by allowing the left side of the heart to be activated sooner and thus shorten the tachycardia cycle length.
This patient had a concealed left free wall accessory pathway that was used in AVRT that was successfully ablated.
Figure 2-2: Explanation
This 68-year-old woman is receiving propafenone to treat atrial fibrillation. She is otherwise well. She developed near syncope and this electrocardiogram was obtained (Fig. 2–2A). What is the diagnosis?
The 12-lead electrocardiogram shows a regular tachycardia with LBBB morphology and a cycle length of approximately 220 milliseconds. While not "classic" for a typical LBBB pattern, the QS complex appears to be more typical than atypical for LBBB. Note the lack of a Q wave in ECG leads 1 and aVL and a ...