Introduction:

Radial ICE (UltraICE™, Boston Scientific, Natick, MA, USA) 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.  Radial ICE’s 360° scan has a larger field of view and allows for a more comprehensive depiction (compared to phased array) of both atrial chambers and atrioventricular valves with their relationships and it also can be used as intravascular ultrasound for great vessels.  Phased-array 8 French, 4.5–11.5 MHz catheter (AcuNav^TM Ultrasound Catheter, Biosense Webster Inc., Diamond Bar, CA) has a 90° sector image, Doppler capability, and is deflectable.  Radial ICE’s mechanical transducer is a non-deflectable catheter thus a steerable sheath (Zurpaz 8.5F Steerable Sheath, Boston Scientific Corporation, Natick, MA or Agilis, St. Jude Medical, St. Paul, MN) is required for precise catheter movements beyond transseptal puncture guidance.

Basic Cardiac Anatomy:

Figure 1 depicts the basic cardiac anatomy revealed with radial ICE and the analogous fluoroscopic views.  The high SVC view permits view of the SVC and ascending aorta and the PA comes into view as one moves inferiorly towards the low SVC.  The mid 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 IVC and CS anatomy.

                        

Figure 1  Basic Cardiac Anatomy with Radial Intracardiac Echocardiography (ICE).  The basic radial ICE anatomy is depicted starting superiorly in the high SVC (Level 1), low SVC (Level 2), mid RA (Level 3), and low RA (Level 4).  © 2013 Boston Scientific Corporation or its affiliates. All rights reserved. Used with permission of Boston Scientific Corporation.

 

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 superior vena cava (SVC) and ablation at attractive electrograms using fluoroscopic guidance did not yield a successful ablation.

Figure 2  Radial ICE Guidance during AVNRT Ablation of the Slow AV Node Pathway.  The left image depicts initial ablation catheter not in contact with the endocardial location of slow AVN pathway.  The right image clearly depicts adequate electrode-endocardial contact which resulted in a successful ablation.

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 triangle of Koch).  The left image shows the initial ablation catheter position and clearly demonstrates 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.

Figure 3  Completion of AV Nodal Ablation under Radial ICE Guidance.   This figure depicts radial ICE catheter location in the RVOT near the level of aortic valve.  The radial ICE allows catheter position nearer to the leftward extension of the His purkinje system in an attempt to complete AV node ablation.

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 radial ICE can provide to assist EP study catheter localization.

 Figure 4  Radial ICE Assessment of RA Anatomy.  Right atrial anatomy is nicely visualized with radial ICE and corresponding anatomic specimen.  One can see how adjunctive imaging during difficult RA ablation may help visualize catheter position and endocardial contact.  Figure taken from Springer, Journal of Interventional Cardiac Electrophysiology [2].

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 anatomy (4) revealed the presence of Thebesian valves (rudimentary valve covering 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.

Figure 5  Radial ICE Imaging of CS Anatomic Variants.  The left image shows a Thebesian valve with no obvious fenestrations covering the CS os.  The right image shows a prominent Eustachian ridge extending from the IVC and overlying the superior aspect of the CS os.

Check Back for:

Part 2 Radial Intracardiac Echocardiography in the EP Lab: Left Atrial Procedures

Part 3 Radial Intracardiac Echocardiography in the EP Lab: Monitoring Procedural Complications

Part 4 Radial Intracardiac Echocardiography in the EP Lab: Electroanatomic Correlates During Radial ICE

Note: This is adapted from work I did with Dr. Sheetal Chandhok.

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, V. 96 (1997), pp. 3021-3029.

2     Morton JB and Kalman JM, “Intracardiac Echocardiographic Anatomy for the Interventional Electrophysiologist,” J Int Cardiac Electrophysiology, V. 13 (2005), pp. 11-16.

3     Ho SY and 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 Research, V. 36 (2008), pp. 314-321.