Radial Intracardiac Echocardiography Guidance in the Electrophysiology Lab

Introduction

Adjunctive imaging during electrophysiology (EP) studies continues to gain acceptance as the need for precise catheter control becomes more important. Nonfluoroscopic imaging is especially useful as we see climbing patient cumulative radiation doses.  

This article will describe the use of radial intracardiac echocardiography (ICE) during invasive EP procedures. Radial ICE (e.g., Ultra ICE^™ catheter, Boston Scientific) uses a mechanical, 9 Fr, 9 MHz catheter, with 360° radial image. The ultrasound transducer rotates every 1.4° and with full mechanical rotation of the transducer (hence, 256 stacked lines of ultrasound data), a panoramic 360° image is created that is perpendicular to the catheter shaft at the tip. Radial ICE does not offer Doppler capability and image definition is not as good as phased array ICE. The 360° scan with radial ICE is a larger field of view and allows for a more comprehensive depiction (compared to phased array) of both atrial chambers and atrioventricular valves, and it also can be used as intravascular ultrasound for great vessels. A phased-array 8 French, 4.5–11.5 MHz catheter (e.g., AcuNav^™ Ultrasound Catheter, Biosense Webster, Inc.) has a 90° sector image, Doppler capability, and is deflectable. Radial ICE’s mechanical transducer is a non-deflectable catheter; thus, a steerable sheath (e.g., Agilis, St. Jude Medical, or Zurpaz^™ 8.5F Steerable Sheath, Boston Scientific) is required for precise catheter movements beyond transseptal puncture guidance. 

Viewing Basic Cardiac Anatomy

Figure 1 depicts the basic cardiac anatomy revealed with radial ICE and the analogous fluoroscopic views. The high superior vena cava (SVC) view permits view of the ascending aorta and the pulmonary artery (PA) comes into view as one moves inferiorly toward the low SVC. The mid right atrial (RA) view tends to be the most useful view for orienting oneself in the right heart and, as discussed later, is the most useful view for transseptal access into the left atrium. The low RA view is useful for delineating complex inferior vena cava (IVC) and coronary sinus (CS) anatomy. 

Right Atrial Intraprocedural Radial ICE Guidance

AV node reentrant tachycardia requires careful catheter manipulation to modify the slow AV node pathway. Traditionally, slow AV node pathway modification is guided by fluoroscopic images and electrogram morphology. Radial ICE-guided AVNRT ablation has been well-described by Fisher et al.¹ Figure 2 depicts the radial ICE anatomy of the slow AVN pathway during an ablation for AVNRT. In this case, the patient had a persistent left SVC;  traditional EGM- and fluoroscopic-guided ablation was not successful. 

Radial ICE imaging was then used (with ICE catheter directional guidance via steerable sheath) to anatomically guide the ablation electrode to the slow AVN pathway, which is located in the region at the anterior edge of the CS os near the septal insertion of the tricuspid valve leaflet (e.g., the anterior border of the triangle of Koch). The left image shows the initial ablation catheter position and clearly demonstrates that the electrode is not in contact with the endocardium. Ablation catheter manipulation to the location depicted in the right image led to an immediately successful ablation. Radial ICE guidance during AVNRT ablations allows one to visualize and ensure catheter stability during the ablation to avoid accidental catheter migration as compared to fluoroscopy, which does not permit one to constantly monitor the electrode-endocardial interface. 

AV node ablation can generally be performed under RAO and LAO fluoroscopic guidance; however, there are times when the compact AV node cannot be ablated using traditional right atrial ablation techniques or only right bundle branch block can be obtained. Radial ICE can be used to complete AV node ablation by catheter guidance to the leftward extension of the His-Purkinje system prior to attempting a retrograde aortic approach to AV node ablation (which often requires an 8 French right femoral arterial sheath). Figure 3 depicts a typical site where complete heart block can be obtained by ablating more proximate to the leftward extension of the His bundle.

Miscellaneous (Atrial Tachycardia, Difficult CS Anatomy) 

Radial ICE can be used for detailed assessment of RA anatomy, especially during mapping of difficult atrial tachycardias. Figure 4 shows the level of RA detail that radial ICE can provide to assist EP study catheter localization.

Oftentimes, catheter access of the CS can be difficult due to anatomic variants involving both the Eustachian ridge (when using femoral venous access) and Thebesian valves. The Eustachian valve continues superiorly from the IVC as the tendon of Todaro that forms the Eustachian ridge (forming the superior aspect of the triangle of Koch).³ Additionally, prior reviews of CS anatomy⁴ revealed the presence of Thebesian valves (rudimentary valve covering the CS os) in 80% of cases. It covered one-fifth in 7%, one-third the os in 29%, one-half in 27%, two-thirds in 14%, and the entire os in 5%. Figure 5 depicts radial ICE imaging of both anatomic variants. A minimally fenestrated Thebesian valve can make CS access unfeasible, as in this case. A prominent Eustachian ridge can mandate CS access using a subclavian or jugular venous approach, as it often impedes catheter placement when using a femoral venous approach. 

Left Atrial Intraprocedural Radial ICE Guidance

Transseptal punctures can be safely performed using radial ICE guidance. A suitably sized Mullins introducer sheath (10-11 French) can be used to position the radial ICE catheter along the interatrial septum, as shown in Figure 6. The Mullins sheath provides enough maneuverability to adjust the ICE catheter position in both inferior-superior and anterior-posterior directions to optimize the location of transseptal puncture in the fossa ovalis. Once ICE localization of the transseptal needle showing tenting of the septum in suitable fossa is obtained, LAO fluoroscopy is then used to guide the transseptal puncture and advancement of the left atrial sheath.

Left atrial ablations can be enhanced and accomplished by placing the radial ICE catheter directly within the left atrium (aka, intra left atrial) while utilizing intraprocedural heparinization targeting an ACT >300.^5-7 Radial ICE is a useful adjunct imaging technique for several reasons. First, direct visualization of the electrode-endocardial interface allows precise positioning of the ablation electrode to guide lesion formation. Second, radial ICE permits the delivery of “focal” left atrial ablative lesions that are generally not feasible with traditional fluoroscopic or even 3D mapping systems. Radial ICE allows the operator to visualize, in real time throughout energy application, the ablation catheter and endocardial surfaces. This real time assessment of catheter-endocardial contact allows continuous catheter position monitoring/adjustment to maintain stable endocardial contact throughout energy delivery. Third, use of continuous radial ICE during atrial fibrillation ablations allows close monitoring of catheter position and endocardial contact while minimizing dependence on fluoroscopy. Figure 7 depicts a typical view obtained when radial ICE is positioned in the left atrium using a steerable sheath.

Detailed anatomy of the pulmonary veins (PVs) can also aid in catheter positioning and stability as well as monitor for procedural complications (discussed later). Figure 8 provides views of the left and right pulmonary vestibules. The left upper (LUPV) and lower pulmonary veins (LLPV) are visualized, as are the saddles. The right intervenous saddle is not as clearly differentiated as the left in this particular example, giving the reader a better overall view of the structures surrounding the right pulmonary vestibule such as the SVC, main PA, and Waterston’s groove. Waterston’s groove is a fat-filled depression formed as the left and right atria fold into one another; it is often dissected by surgeons to expose the left atrium. Radial ICE can be carefully placed within each individual pulmonary vein to guide catheter ablation as previously described.^5-7

Radial ICE can also help guide linear ablation along the LA posterior wall for mitral annular flutter. Direct visualization of the left lower pulmonary vein, the posterior wall of the LA, the mitral annulus, and CS during ablation (both intra LA and CS) can improve catheter contact, allowing for complete linear ablation and bidirectional block (Figure 9).

Monitoring Procedural Safety

Esophageal proximity can be monitored to guide locations of ablation to help minimize risk of esophageal damage. The entire length of the esophagus that is contiguous with the left atrial posterior wall can be visualized with intra left atrial ICE to monitor ablation delivery and power titration.⁸ Figure 10A shows the typical location of the esophagus during an atrial fibrillation ablation. Ablation over the esophagus is avoided or power is titrated to minimize risk of esophageal damage. Endocardial thrombi or coagulum can be detected using radial ICE, as shown in Figure 10B. Left atrial damage can also be monitored using radial ICE. Figure 10C shows an unusual case of a tear or rent in the endocardium discovered during an atrial fibrillation ablation. However, radial ICE is not the ideal imaging modality to evaluate for pericardial effusions, given its limited far-field resolution. Figure 10D shows the pericardial space in view when an intra left ventricular ICE position is utilized. 

Electroanatomic Correlates 

Radial ICE can be used to obtain especially informative electroanatomic correlations, and has been described extensively during atrial fibrillation ablations.⁶ Aside from guiding the localization of pulmonary vein potentials during intra left atrial ICE-guided procedures (shown in Figures 7 and 8), there are several instances in which radial ICE-facilitated electroanatomic correlates can discern situations where additional ablation is not necessary during atrial fibrillation ablations; such situations include far-field EGMs and potentials derived from myocardium outside the region of ablation interest. The left atrial appendage is often located quite close to the left superior pulmonary vein, and left atrial appendage far-field electrograms can be confused with pulmonary vein potentials if this is not suspected based upon electroanatomic correlation. Figure 11A depicts the radial ICE catheter positioned in the left upper pulmonary vein (adjacent to the left atrial appendage). Figure 11B shows the intracardiac electrograms (EGMs) recorded in the left upper PV (darker, smaller amplitude) and LAA (lighter, larger amplitude); the LUPV signal appears to be a low-pass filtered version of the LAA signal.

One can also see potentials derived from contiguous myocardium outside the region subtended by catheter ablation of atrial fibrillation.^6,7 These potentials may be located near the LA roof in the region of Waterston’s groove, proximal to Bachmann’s bundle and superior caval musculature. For example, distinct EGMs can be recorded from within the right superior pulmonary vein that may look like, but do not represent, latent PV potentials.

Figure 12 shows the typical radial ICE view when positioned in the right upper pulmonary vein. There are two distinct EGMs recorded; the earlier signal ~28 msec after the onset of the surface P wave, and the later signal ~70 msec after the onset of the surface P wave. The later signal represents a true pulmonary vein potential that is successfully ablated, and the earlier signal remains after the ablation; we surmise (though cannot prove) this represents Bachmann’s bundle potential. This signal can often be seen in the right upper pulmonary vein <30 msec after the onset of the surface P wave, and its presence does not reflect residual PV potentials. Bachmann’s bundle (also called the interauricular band) has a myoarchitecture that displays parallel alignment of fibers along distinct muscle bundles.⁹ Bachmann’s bundle extends from the SVC, crossing the interatrial groove, and passes leftward in the left atrium.  

Discussion

Intracardiac radial ICE can provide detailed anatomy, guide catheter ablation, enhance procedural safety, and facilitate ablative strategies; it is readily available but generally underutilized. Furthermore, ICE has utility for reducing fluoroscopy times by rendering the operator less dependent upon traditional fluoroscopic monitoring of catheter movement and position.^5,10,11 Radial intracardiac echo offers 360º views of cardiac anatomy not commonly encountered with traditional phased-array catheters or even transthoracic/transesophageal echo, though it offers comprehensive utility in guiding EP procedures.

Disclosures: The authors report no conflicts of interest regarding the content herein. Outside the submitted work, Dr. Williams reports consultancy with Advanced Cardiac Life Sciences, LLC, and reports that he provides consultancy services to the device industry including Boston Scientific. Outside the submitted work, Dr. Chandhok reports payment for a Fellows Teaching Program by Boston Scientific. 

References 

1. Fisher WG, Pelini MA, Bacon ME. Adjunctive intracardiac echocardiography to guide slow pathway ablation in human atrioventricular nodal reentrant tachycardia: anatomic insights. Circulation. 1997;96:3021-3029.
2. Morton JB, Kalman JM. Intracardiac echocardiographic anatomy for the interventional electrophysiologist. J Interv Card Electrophysiol. 2005;13:11-16.
3. Ho SY, Ernst S. Anatomy for Cardiac Electrophysiologists (2012; Cardiotext Publishing, LLV, Minneapolis, MN).
4. Pejkovic B, Krajnc I, Anderhuber F, Kosutic D. Anatomical variations of the coronary sinus ostium area of the human heart. J Int Med Res. 2008;36:314-321.
5. Schwartzman D, Nosbisch J, Housel D. Echocardiographically guided left atrial ablation: characterization of a new technique. Heart Rhythm. 2006;3:930-938.
6. Schwartzman D, Williams JL. On the electroanatomic properties of pulmonary vein antral regions enclosed by encircling ablation lesions. Europace. 2009;11:435-444.
7. Chandhok S, Williams JL, Schwartzman DS. Anatomical analysis of recurrent conduction after circumferential ablation. J Interv Card Electrophysiol. 2010;27:41-50.
8. Ren JF, Lin D, Marchlinski FE, Callans DJ, Patel V. Esophageal imaging and strategies for avoiding injury during left atrial ablation for atrial fibrillation. Heart Rhythm. 2006;3:
9. 1156-1161.
10. Ho SY, Anderson RH, Sánchez-Quintana D.  Atrial structure and fibres: morphologic bases of atrial conduction. Cardiovascular Res. 2002;54:325-336.
11. Ferguson JD, Helms A, Mangrum JM, et al. Catheter ablation of atrial fibrillation without fluoroscopy using intracardiac echocardiography and electroanatomic mapping. Circ Arrhythm Electrophysiol. 2009;2:611-619.
12. Khaykin Y, Skanes A, Whaley B, et al. Real-time integration of 2D intracardiac echocardiography and 3D electroanatomical mapping to guide ventricular tachycardia ablation. Heart Rhythm. 2008;5:1396-1402.