Editor’s Perspective: In the Middle

Abstract

T he student of electrophysiology necessarily learns classic patterns and clear descriptions of anatomy and electrophysiology correlations to facilitate safe and effective invasive procedures. However, the same aid to learning serves as an obstacle when confronted with unusual arrhythmia manifestation that requires the appreciation of exceptions to the usual more simplistic constructs to succeed. In this segment of Teaching Rounds in Electrophysiology, Chen et al 1 describe 2 cases of intramural ventricular tachycardia arising from the outflow tract septum. They teach us with their precise approach for mapping and ablation how and when to break the rules we have learned and indeed use the uniqueness of the situation to solve the problem. Article see p 978What Is the Septum?The interventricular septum separates the right and left ventricles. We easily visualize the septum of the ventricular inflow portion, which is indeed central and between the right and left ventricular inflow tracts. Thus, in the left anterior oblique projection, if we have placed a catheter in the right ventricular inflow, then we know that moving the catheter leftward will allow contact on the septum. This construct, however, is not applicable to the septum of the ventricular outflow tracts. This is because the right ventricular outflow tract takes a complex course anteriorly and superficially to the left ventricular outflow tract with the pulmonic valve lying cephalad and leftward of the aortic valve.2,3 As a result, a catheter in the right ventricular outflow tract will need to be manipulated posteriorly to reach the ventricular septum, and moving leftward leads one closer to the left free wall portion of the right ventricular outflow tract near the pulmonic valve. The relationship, however, is a gradual change from the relatively midline inflow septum to the oblique and parallel-to-the-chest-wall course of the outflow tract septum. It is nearly impossible with fluoroscopy alone to identify which sites being mapped are actually septal. Intracardiac ultrasound or other imaging in real time is needed to appreciate that the myocardium being mapped or targeted for ablation is between the right and left ventricular chambers. What Is Intramural?Correlation among electrocardiography, anatomy, and a planned mapping and ablation approach for ventricular tachycardia presupposes that the right and left ventricles are distinct entities as is endocardial versus epicardial arrhythmogenic substrate. A focus or pathological tissue critical for reentrant tachycardia may reside intramurally and thus defy these clear distinctions. On the free wall of the left ventricle, intramural tissue represents the mid myocardial fibers that may be difficult to access from endovascular or subxiphoid pericardial approaches. On the septum, intramural tissue is complex, representing overlapping and intertwined myocardial fibers that can be traced to the right or left ventricular myocardium, and generally devoid of conduction tissue. Even more complex is the intramural subst...