Author + information
- William G. Stevenson, MD∗ ( and )
- Robert L. Abraham, MD
- Division of Cardiovascular Medicine, Arrhythmia Section, Vanderbilt University Medical Center, Nashville, Tennessee
- ↵∗Address for correspondence:
Dr. William G. Stevenson, Division of Cardiovascular Medicine, Vanderbilt University Medical Center, 1215 21st Ave South, Nashville, Tennessee 37212.
The majority of idiopathic ventricular arrhythmias arise from a focus in the outflow tract region of the right or left ventricle. The area is anatomically complex and foci that are located within the septum between the right ventricular (RV) outflow region and aorta/left ventricle (LV) can be particularly challenging to locate and ablate. When the region is thin, near the pulmonary annulus, it is possible that ablation would be successful from either right ventricular outflow tract (RVOT) or the adjacent sinus of Valsalva. Septal thickness is greater at more inferior locations and failure of ablation from 1 or both sides of the septum is common. Finding the closest site is important for success. Intramural mapping with small diameter electrode catheters deployed through the coronary vasculature has been helpful in some cases (1). Activation mapping has been the most reliable method for selecting ablation targets, is more accurate than pace-mapping, and is most commonly performed with a roving catheter (2). The conundrum with this approach is that it is difficult to know when the truly earliest point of activation has been identified. The onset of the QRS complex is the usual fiducial point for mapping, but can be difficult to precisely define, and ablation success may be achieved when activation is anywhere from 20 to 50 ms before the QRS onset (3). Thus, when initial mapping finds a point 25 ms before the QRS, should mapping continue, or is it time to ablate? And particularly for mapping in the RV, should mapping explore the pulmonary artery and great cardiac vein, or move to the aortic root and LV outflow tract for mapping? Some additional electrogram features are useful to inform this decision.
When a focus is located in the subendocardium beneath the recording electrodes, activation should move away in all directions from that site. The unipolar electrogram (with the high pass filter setting at 1 Hz or less) inscribes a QS complex (3,4). An initial R-wave is consistent with a wavefront moving toward the mapping electrode from a remote site of initial activation. The onset of the unipolar R-wave also provides a surrogate for the QRS onset, and typically precedes the first peak of the bipolar signal (4). However, a QS can be recorded over a diameter of 1 cm and may also be seen if the catheter is not in contact with the tissue. A sharp bipolar signal with the first peak coincident with the onset of the unipolar signal helps confirm likely tissue contact. Bipolar recordings from adjacent sites that show opposite initial polarities are also consistent with initial activation from a focus between the sites, provided catheter orientation remains consistent (3).
Many centers use electroanatomic mapping systems that facilitate identification of the earliest site of activation relative to adjacent sites. Even with centrifugal activation away from an earliest point, the arrhythmia focus may still be remote, with a focal-appearing endocardial breakthrough site. In this issue of JACC: Clinical Electrophysiology, Masuda et al. (5) show that a relatively large area of early activation in the RVOT is associated with failure of ablation from the RVOT, consistent with location of the focus at a remote site. They quantified the area of early activation as that encompassed by isochronal lines of increasing time from the point of earliest RVOT activation and found that a 15-ms isochrone encompassing an area of <5 cm2 had an excellent predictive accuracy for identifying a focus that could be ablated from the RVOT. These findings are consistent with those of Acosta et al. (6) and Herczku et al. (7) and who used point by point activation for mapping. In contrast to the prior study, Masuda et al. (5) used a high-resolution 64-electrode basket catheter that has an interelectrode distance of 2.5 mm along each spline.
The findings are interesting and should be relatively easily applicable, particularly with multielectrode mapping systems; there are, however, several caveats. If we consider a focus deep to the RVOT endocardium and assume uniform conduction velocity in all directions away from the focus, the depolarization wave generates a sphere with decreasing curvature as its radius increases, such that when it reaches the endocardium a relatively large area of early activation results. Conduction velocity measured along the endocardial surface would appear to be misleadingly rapid because activation of sites adjacent to the site of earliest breakthrough are activated by the wavefront coming from below, rather than centrifugal propagation away from the point of early breakthrough, as was observed in the present study.
If conduction is not uniform, results could be different. The size of the area of early activation would be determined by the relative conduction velocities in the direction from the deep focus to the endocardium versus velocities in the orthogonal directions. If conduction from the deep focus to the RV endocardium were very rapid, relative to conduction in the other directions, a small area of early activation would result and falsely suggest an endocardial focus. If the focus was close to the endocardium, but conduction to the endocardium was much slower than conduction parallel to the endocardium, a larger area of early endocardial activation may result, falsely suggesting a focus deep to the endocardium. Conduction velocity is not uniform in the septum and outflow tract region. In the RV side of the septal outflow tract, fibers are oriented longitudinal to the long axis of the outflow region. On the LV side, the fibers are circumferential in relation to the aortic annulus. Propagation from deep foci is occurring across fiber bundles, explaining the consistency of the findings between studies (6). A relatively isolated fiber connecting from a deep site to the RVOT region could produce a focal activation pattern. Whether this occurs is not certain but isolated fibers do exist around the pulmonary trunk and aorta and it is likely there are anatomic variants that have not been encountered in the small number of patients studied. Fiber orientation also likely explains the finding of Acosta et al. (6) and Herczku et al. (7) that deep foci tend to produce a more elliptical activation pattern than those closer to the subendocardium. The pattern of activation was not examined in the present study.
Examination of isochronal activation patterns provides another tool in addition to assessment of QRS morphology, pace-mapping, and other electrogram parameters to potentially distinguish deep versus superficial foci during RVOT mapping. The emergence of multielectrode mapping systems should facilitate this analysis and further improve targeting of focal arrhythmias for ablation.
↵∗ Editorials published in JACC: Clinical Electrophysiology reflect the views of the authors and do not necessarily represent the views of JACC: Clinical Electrophysiology or the American College of Cardiology.
Dr. Stevenson has received honoraria from Boston Scientific and Abbot; and has a patent for needle ablation which is coheld with Biosense Webster and cosigned to Brigham and Women’s Hospital. Dr. Abraham has reported that he has no relationships relevant to the contents of this paper to disclose.
All authors attest they are in compliance with human studies committees and animal welfare regulations of the authors’ institutions and Food and Drug Administration guidelines, including patient consent where appropriate. For more information, visit the JACC: Clinical Electrophysiology author instructions page.
- 2018 American College of Cardiology Foundation
- Yokokawa M.,
- Good E.,
- Chugh A.,
- et al.
- Delacretaz E.,
- Soejima K.,
- Gottipaty V.K.,
- Brunckhorst C.B.,
- Friedman P.L.,
- Stevenson W.G.
- Masuda J.,
- Asai M.,
- Iida O.,
- et al.
- Herczku C.,
- Berruezo A.,
- Andreu D.,
- et al.