How to Use ICE to Aid in Catheter Ablation of Ventricular Tachycardia

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How to Use ICE to Aid in Catheter Ablation of Ventricular Tachycardia


Marc W. Deyell, MD, MSc; Mathew D. Hutchinson, MD; David J. Callans, MD


Introduction


The right and left ventricles are complex, 3-dimensional (3D) structures; integral to the ablation of ventricular arrhythmias is the ability to define this anatomy at the time of the procedure. The cornerstones of navigation in ventricular ablation are fluoroscopy and electroanatomic mapping systems; however, ICE is increasingly being used as an adjunct imaging modality. It provides invaluable complementary information that can improve both the efficacy and the safety of complex ventricular ablation.


At present, there are 2 types of commercially available ICE systems, radial and phased-array. Radial ICE (Ultra ICE™, Boston Scientific Co., San Jose, CA) has a transducer in the shaft of the catheter with a small ultrasound element at the tip, which is rotated by an external motor at 600 rpm. The ultrasound beam is emitted perpendicular to the longitudinal axis of the catheter, resulting in a circular image with the catheter at the center, similar to intravascular ultrasound imaging used in coronary interventions. The axial resolution with this catheter is excellent, but the tissue penetration is only 6 to 8 cm, and the catheter is nondeflectable, limiting its use in VT ablation.


Phased-array ICE catheters provide sector imaging, similar to transthoracic or transesophageal probes. In our laboratory we exclusively use the AcuNav™ probe (Acuson Corporation, Siemens Medical Solutions USA Inc., Malvern, PA) that is compatible with Siemens or General Electric ultrasound platforms. This 8-Fr or 10-Fr catheter has a forward-facing (perpendicular to tip), 64-element, phased-array transducer, which allows for scanning up to a 90° sector along the longitudinal axis of the catheter. It also has M-mode and pulsed or continuous-wave Doppler capabilities. The tissue penetration of the catheter is up to 16 cm, and the tip is deflectable in the anterior-posterior as well as the left-right plane, providing a flexible platform for ventricular imaging. St. Jude Medical (St. Paul, MN) produces a phased-array catheter (ViewFlex™ Xtra ICE catheter) with a stand-alone, Phillips-based ultrasound platform (ViewMate II™). This 9-Fr catheter has similar capabilities to the Siemens probe, although it cannot be integrated with a 3D mapping system (see below).


Given its advantages for guiding ventricular ablation, the remainder of this chapter will focus solely on phased-array imaging (and, specifically, the AcuNav catheter). The transseptal approach for VT ablation is commonly used in our laboratory, and ICE is invaluable for guiding punctures; however, the use of ICE in transseptal puncture is discussed in detail in another chapter.


Preprocedural Planning


Setup


Insertion of the ICE catheter requires a dedicated femoral venous sheath that is 9 Fr. A 9-Fr sheath ensures easy passage of the ICE catheter and allows for intravenous infusions through the side port during the procedure. Where possible, the ICE catheter is placed through a left femoral venous sheath with the ablation catheter on the right side to allow for ease of manipulation of each catheter independently.


Controls and Settings


The AcuNav phased-array ICE catheter is capable of transmitting ultrasound at variable frequencies of 5.5, 7.5, 8.5, and 10 MHz. We typically start with 7.5 MHz, as this is adequate for most general ICE applications. Higher frequencies (8.5 and 10 MHz) are useful for better axial resolution when the structures of interest are near to the transducer, as is the case in outflow tract tachycardias. The lower frequency (5.5 MHz) may be required to visualize distant structures such as the inferior and lateral wall of the LV in a dilated heart. As lateral resolution declines with the width of the ultrasound beam, we use the minimum sector width required when focusing on a particular area of interest, such as a papillary muscle or the aortic cusps.


For optimal images, the operator must also have a good working understanding of the variable depth-compensation control that allows the relative gain to be adjusted throughout the image depth. Decreasing the near-field gain can be useful to overcome suppression from structures near to the probe, such as a thick IAS or a pacemaker lead, thereby allowing for better visualization of far-field structures.


By convention, ICE images are usually displayed with the marker on the left side of the image sector, meaning the shaft of the catheter is to the left of the image and the top of the catheter is on the right. However, in our lab, we flip the image so the marker is on the right (Figure 44.1). We find this orientation is more intuitive for the operator in the EP laboratory as the “top” of the ICE image is in the same direction (left) as the patient’s head when standing at the right side of the table. Most of the images in this chapter are shown in the conventional manner with the marker on the left and we have highlighted instances where the images are reversed.



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Figure 44.1 Long-axis view of the left ventricle from the right ventricle. The standard view is shown (left panel) with the marker on the left, denoting the shaft of the ICE catheter. In our laboratory we flip the images so the marker in on the right (right panel).


Catheter Introduction and Manipulation


The ICE catheter can be advanced to the heart without fluoroscopy. As the ICE image is essentially perpendicular to the catheter plane, a small portion of the venous lumen and wall should be visible at all times while advancing the catheter. If resistance is encountered, or if visualization of the vessel is lost, the catheter is withdrawn slightly and rotated to visualize the lumen. Once the main vessel lumen is identified, the catheter can be deflected in the direction of the lumen and re-advanced to the heart.


Manipulation of the ICE catheter during the procedure can easily be performed by the same person performing the ablation, although a second pair of hands may be optimal when ablation catheter or ICE catheter stability is an issue. An additional operator is required to manipulate the controls of the ultrasound platform and to record images. In our laboratory, this is frequently a fellow technologist or nurse. Adequate ultrasound training for this operator is therefore essential.


Procedure


Imaging of Ventricular Structures from the Right Atrium


In the next 2 sections, we review the components of a typical baseline ICE study for ventricular ablation, with emphasis on standard views for imaging of ventricular structures. However, given the maneuverability of ICE within the heart, these standard views can be modified as needed for the operator by deflecting the catheter to obtain optimal views of the structures of interest.


We always start our ICE studies from the “home” view with the ICE catheter in the neutral position in the mid-RA and oriented anteriorly to bring the tricuspid valve into view (image Video 44.1A). Here the proximal RVOT is also seen. Color Doppler imaging of the tricuspid valve is performed to quantify baseline tricuspid regurgitation (Video 44.1B) and continuous wave (CW) Doppler of the jet is used to estimate baseline pulmonary artery systolic pressure. It is especially important to establish baseline pulmonary pressures in VT ablation of patients with impaired LV function. Advancement and (often) slight clockwise rotation of the catheter brings the RVOT and the pulmonic valve into view (Figure 44.2; image Video 44.2). Further clockwise rotation reveals the aortic valve in long axis and LVOT (Figure 44.3A; image Video 44.3A). The full LVOT is often slightly out of plane in the neutral position, but adjusting the left-right tilt can improve the image. Color Doppler is placed across the aortic valve to assess for baseline stenosis or regurgitation (Figure 44.3B; Video 44.3B). The mitral valve can be viewed in long axis with further slight clockwise rotation (Figure 44.4A; image Video 44.4A). Again, some left-right tilt is usually required to bring the transducer parallel to the LV inflow. Baseline mitral regurgitation is assessed with color Doppler across the valve (Figure 44.4B; Video 44.4B).



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Figure 44.2 Advancing the ICE catheter from the home view position allows visualization of the right ventricular outflow tract (RVOT) and the pulmonary valve.



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Figure 44.3 The aortic valve and left ventricular (LV) outflow tract viewed from the right atrium (RA) (left panel). Color Doppler across the aortic valve (right panel) shows largely laminar flow across the outflow tract.



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Figure 44.4 The left ventricle (LV) inflow and mitral valve (MV) viewed from the right atrium (RA) (left panel). Color Doppler across the MV (right panel) reveals mild regurgitation (arrow).


Imaging of Ventricular Structures from the Tricuspid Valve And Right Ventricle


The ICE catheter can be advanced to the tricuspid annulus and right ventricles by deflecting it anteriorly, advancing it forward across the tricuspid valve then releasing the deflection back to the neutral position (image Video 44.5). Once the ICE catheter is in the body of the RV, it initially faces the inferior portion of the free wall and the adjacent pericardium (Figure 44.5; image Video 44.6). Rotating the catheter clockwise brings into view the LV in long axis with visualization of the postero-medial papillary muscle and the mitral valve (Figure 44.6A; image Video 44.7A). The anterolateral papillary is visualized with slight further clockwise rotation (Figure 44.6B; Video 44.7B). When imaging the left ventricle, clockwise rotation brings into view progressively more anterior structures. The ICE catheter can be advanced further into the RV to enhance visualization of the apex or withdrawn towards the tricuspid valve annulus to enhance more basal structures. Further clockwise rotation brings into view the aortic valve in short axis (Figure 44.7A; image Video 44.8A), and color Doppler here is useful for assessment of regurgitation (Figure 44.7B; Video 44.8B). This view is essential for catheter mapping and ablation of aortic cusp arrhythmias. Slight advancement of the catheter into the RV in the same plane allows visualization of the pulmonic valve in long axis and the RVOT (Figure 44.8, A and B; image Video 44.9, A and B). Continuous-wave Doppler of the pulmonic regurgitant jet can be used to assess pulmonary artery diastolic pressures. Additional clockwise rotation of the catheter then reveals the aortic valve in long axis and the ascending aorta (Figure 44.9; image Video 44.10). This view of ascending aorta may identify atheroma that would preclude a retrograde approach to mapping of the LV (Figure 44.10; image Video 44.11, A and B).



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Figure 44.5 The initial image after insertion of the ICE catheter into the right ventricle (RV) shows the inferior RV free wall (bottom left) and apex. RV trabaculation can be seen.



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Figure 44.6 Long-axis view of the left ventricle (LV) with the postero-medial papillary muscle (PM) and MV (left panel). Smooth LV endocardium is noted without trabaculation. Further clockwise rotation of the ICE catheter brings the antero-lateral PM into view (right panel). Note each PM has 2 distinct heads. RV can be seen in the right upper corner.



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Figure 44.7 The aortic valve viewed in short axis from the right ventricle (RV). All 3 cusps (L = left, R = right, N = noncoronary) are seen (left panel). Color Doppler across the aortic valve reveals trace regurgitation (arrows, right panel). The pulmonary artery (PA) is seen on the right of the images.



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Figure 44.8 The pulmonary valve and artery (PA) viewed in long axis from the right ventricle (left panel). Color Doppler across the pulmonic valve reveals 2 jets of mild regurgitation (arrows, right panel).



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Figure 44.9 The aortic valve and ascending aorta as viewed from the right ventricle.



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Figure 44.10 The ascending aorta is viewed in long axis (left panel) and obliquely (right panel) from the right ventricle. Focal atheroma at the sino-tubular junction is seen in the left panel (arrow) and more diffuse ascending aortic atheroma is seen in the right panel (arrow).


ICE to Guide Catheter Positioning and Lesion Formation


Stability of the ablation catheter tip and excellent tissue contact are the cornerstones of effective energy delivery during ablation. This is particularly important for tissue penetration in the thicker myocardium of the left ventricle. Achieving stability and contact in the ventricles can be challenging, especially in the presence of gross structural abnormalities such as scar or aneurysm.


Traditional markers of stability, including fluoroscopic appearance and EGM characteristics, are imperfect. The advent of contact-force sensing ablation catheters has greatly enhanced the ability to deliver adequate ablation lesions. However, this technology is not without limitations. In areas of complex anatomy, ICE offers direct visualization of the catheter and the anatomical structures and can help guide maneuvers to obtain optimal force. Furthermore ICE can assist in maneuvering multipolar catheters, which do not have force-sensing capabilites but can provide high resolution mapping of the myocardium.


The assessment of lesion formation during ablation can be challenging, particularly in areas of scar where EGM changes may be difficult to assess and impedance changes during ablation may not adequately reflect the true extent of myocardial damage. Increased echodensity of the myocardium, as visualized by ICE, has been previously correlated with lesion formation.1,2 Further quantification of lesion formation by ICE remains a research tool at present, but the qualitative development of increased echodensity during ablation gives the operator reassurance that adequate lesions are being produced (Figures 44.11 and 44.12; image Videos 44.12 and 44.13).



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Figure 44.11 The ablation catheter in good contact with the endocardium at the border of an anteroapical infarct (asterisks). Note the ablation lesion is demonstrated by ICE as an area of increased echodensity (arrows).

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Aug 27, 2018 | Posted by in CARDIOLOGY | Comments Off on How to Use ICE to Aid in Catheter Ablation of Ventricular Tachycardia
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