Intracardiac Echocardiography for Electrophysiology

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Intracardiac Echocardiography for Electrophysiology





Rationale


The increasing complexity of contemporary catheter ablation procedures has created an important niche for real-time imaging modalities in the EP laboratory. Traditional modalities such as fluoroscopy provide limited anatomical detail and incur significant potential patient and provider exposure risk. The trend toward anatomically based ablation targets and procedural end points subsumes an intimate knowledge of the specific anatomical variations of the individual patient. Integrating pre-acquired two- or three-dimensional images (e.g., tomographic reconstructions) can be quite useful for anatomical characterization, but can inherently lack real-time feedback for temporal changes that occur during the procedure.


Uniquely suited for EP procedures, intracardiac echocardiography (ICE) is capable of providing substantial real-time data with relative ease. Indeed ICE provides both physiological and anatomical data that facilitate the following important functions:



This chapter provides an introduction to and comparison of currently available ICE platforms. Core concepts regarding catheter manipulation and image optimization are reviewed. Last, the use of ICE in specific EP procedures is reviewed.



ICE Platforms


Two types of ICE transducers are commercially available: mechanical (radial) and phased array systems. The radial ICE transducer is mounted on a 9 French, nonsteerable catheter and emits an imaging beam at a 15° forward angle, perpendicular to the long axis of the catheter. The transducer is typically delivered through a curved or steerable introducer sheath into the chamber of interest. The transducer rotates at 1800 rpm and has a fixed, 9-MHz frequency; it produces a 360° imaging plane perpendicular to the axis of the catheter. Limited far-field resolution necessitates imaging proximal to the structure of interest.


The phased array ICE catheter contains a 64-element transducer with variable frequency ranging from 5 to 10 MHz, thereby providing greater flexibility to image remote structures (up to 15 cm). The phased array transducer is capable of full spectral and color Doppler measurements, greatly enhancing achievable physiological data. The transducer is mounted on a bidirectional or multidirectional 8 to 10 French catheter. The most commonly used transducer is the AcuNav™ (Siemens Medical, Mountain View, California) system, which can be deflected in four directions (anterior, posterior, right, and left) in addition to providing 360° axial rotation. Unless otherwise indicated, the remainder of this chapter describes imaging with the phased array ICE system.



Basic ICE Imaging Planes


Although imaging with ICE requires a modest learning curve, it is a logical extension for operators with basic catheter manipulation and echocardiography skills. Most new operators find the images somewhat disorienting when taken out of context. Only after an individual echo “view” is integrated mentally with the operator’s inherent knowledge of intracardiac anatomy does obtaining and interpreting ICE images become intuitive. It is of paramount importance to realize that the infinite potential orientations of the transducer within the cardiac chambers produce infinite possible imaging planes. To avoid confusion, it is useful to learn a few fiducial imaging planes from which an ICE survey is easily generated. The cardinal imaging plane or “home view” is obtained by placing the ICE catheter in a neutral, mid–right atrial position and imaging through the tricuspid valve. Other relevant structures are easily viewed with gentle clockwise (CW) rotation of the imaging catheter along its axis.



Sinus Node Modification


Among the earliest reported clinical applications for ICE was guiding sinus node modification, during which the superior lateral crista terminalis (CT) is targeted. Localizing catheters proximate to the CT with the use of fluoroscopic guidance alone is often inaccurate, with mean distances of >1 cm in more than 50% of cases.1 With ICE, the CT can be precisely identified, thereby avoiding delivery of ineffective lesions. ICE can also be used to evaluate the diameter of the junction of the superior vena cava and the right atrium, thereby avoiding excessive narrowing during ablation. Additionally, the presence of echointensity extending to the epicardial surface, often coupled with the appearance of an adjacent echodense region (representing epicardial edema), correlates strongly with the achievement of acute heart rate slowing during ablation2 (Figure 62-1).




Atrial Fibrillation Ablation



Pulmonary Vein (PV) Anatomy


The most complete integration of ICE in EP is achieved with atrial fibrillation (AF) ablation. Before the left atrium (LA) is accessed, full characterization of the PVs is easily achieved from a right atrial transseptal view. At 60° CW rotation from the home view, the left atrial appendage is visualized. Continued rotation allows characterization of the left PVs. The right PVs are directed 180° posteriorly from the home view.


Visualization of the right superior PV is often challenging because of its septal location; this can be overcome by (1) distending the interatrial septum toward the LA; (2) passing the imaging catheter into the LA through a transseptal defect; or (3) deflecting the transducer toward the tricuspid annulus.


Anatomical variations in PV anatomy are common and can be fully characterized with ICE, thereby avoiding inadvertently ignoring or damaging them during ablation. The diameter and orientation of the veins are recorded, and baseline pulse wave Doppler flow velocities are measured (Figure 62-2). If a circular mapping catheter is used to guide PV isolation, determination of the PV ostial diameter with ICE is useful in selecting an appropriate size.




Left Atrial Appendage (LAA) Visualization


Characterization of the LAA is routinely performed before LA ablation in patients with inadequate preoperative anticoagulation and/or persistent AF. The advent of LAA occlusion devices provides another potential niche for ICE imaging. The LAA can be viewed with ICE from several different imaging planes: (1) from the right atrium across the atrial septum; (2) from the left atrium; (3) from the coronary sinus; or (4) from the pulmonary artery. The recent Intra-Cardiac Echocardiography–guided Cardioversion to Help Interventional Procedures Study (ICE-CHIP) study prospectively compared LAA imaging with transesophageal echo (TEE) versus phased array ICE; the study found incomplete LAA imaging with ICE in 15% of patients, as well as a lower sensitivity to detect LAA thrombus compared with TEE.3 The comparative image quality in ICE-CHIP was potentially biased by the exclusive use of a right atrial imaging plane with ICE. Other reports have suggested that the aforementioned alternative imaging planes allow imaging more proximate to the LAA, and thus provide enhanced tissue characterization.



Three-Dimensional Image Integration


Integration of preacquired tomographic images with electroanatomical (EA) mapping systems is widely used to facilitate AF ablation. Accurate registration of these images can be challenging, in part because of the complex topography of the LA. ICE can facilitate this process by providing real-time feedback regarding registration quality by allowing visualization of the positioning of intracardiac catheters at fiducial locations (e.g., the ligament of Marshall, the PV carina) (Figure 62-3). If misalignment of the three-dimensional (3D) geometry is noted, reregistration is easily performed to improve spatial accuracy.


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Jun 5, 2016 | Posted by in CARDIAC SURGERY | Comments Off on Intracardiac Echocardiography for Electrophysiology

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