Intracardiac Echocardiography (ICE)
ICE is a type of cardiac ultrasound. It uses mechanical waves with frequencies greater than 20,000 Hz and the laws of sound reflection and refraction while crossing borders between materials of different densities. Additionally, it uses a miniature transducer to create images. The transducer is at the edge of the catheter and uses a series of crystals or a single crystal where the beam is moved by mechanical means around a circle. The images are displayed in either M or B modes with Doppler effect.
Two-dimensional echo is the modality typically used for ICE. It is used in electrophysiology laboratories for procedures such as ablations, trans-septal punctures, evaluation for intracardiac thrombus, ASD/PFO closure, and other electrophysiological procedures. ICE allows for improved anatomic visualization for areas such as the pulmonary veins and the A-V junction. ICE also enables continued radiofrequency monitoring during ablation, hemodynamic performance of the myocardium and pericardial space monitoring.
INSERTION AND STRUCTURE RECOGNITION
The typical ICE catheter is inserted via the inferior vena cava and positioned along the right atrium to the level of the mid-septum. Most often the right atrium is the area of the heart visualized most often during EP procedures. Thus, it is called the “Home View” or the “basic point of orientation”. Figure (A) shows the tricuspid valve and the right ventricle. Counterclockwise rotation of the catheter in the RA will show the terminal crest. Clockwise rotation from the inferior RA will show the Eustachian ridge with the tricuspid-caval isthmus (B).
Turning the catheter clockwise along its axis reveals the aortic valve, the right ventricular outflow tract and occasionally the pulmonary artery (figure C). Additional long axis views can be seen of both the aorta and the pulmonary trunks. When the catheter is rotated from the low right atrium, a short axis view of the coronary sinus is seen (figure D). Moving the catheter left-to-right at this same point provides a long axis view of the same structures (figure E). If the catheter is moved counterclockwise, views of the left atrium, mitral valve and left ventricle can be seen (figure F). If the depth setting of the catheter is increased with the transducer directed towards the left atrial posterior wall and placed at the level of the atrial septum, then the left and right pulmonary veins and the left atrial appendage can be visualized. The long and short axis views of the left ventricle are seen in figures G and H. See figure 1 below.
The following images (Figures 2-10) provide additional information and diagrams of anatomical landmarks seen during an ICE.
Comparison of Rotational and Phased-Array ICE
(A) Rotational or cross-sectional view of the right atrium (RA), left atrium (LA), and interatrial septum using a rotational intracardiac echocardiography (ICE) catheter. Note that the shaft of the ICE catheter (circular structure) can be seen in the RA, with the tip apposed to the interatrial septum for optimal imaging of the fossa ovalis. (B) Two-dimensional view of the interatrial septum using a phased-array ICE catheter. The catheter terminates within the body of the RA. (C) Schematic drawing of a phased-array ICE catheter in the RA in the optimal position to image the interatrial septum. Note that the ultrasound array is facing the septum but is not in apposition to it. Figure 1C provided by St. Jude Medical.
Below: The top and bottom rows demonstrate typical intra-cardiac echocardiography images obtained with the transducer in various locations within the left enclosed region. The centre row uses computed tomography images to illustrate how the intra-cardiac echocardiography images were used to integrate electrograms with anatomy. and to demarcate myocardium enclosed by encircling lesions (highlighted in blue) from myocardium which is not enclosed:
(A) whole-heart, left-lateral vantage; (B) left atrial, viewed from a posterior extra-cardiac vantage; (C) left atrial, with the left antrum viewed from an intra-cardiac vantage; (D) the two endocardial halves of (C) resulting from a cleavage plane along the red line shown in (C). On all computed tomography images, the dotted lines show the encircling lesion, and solid lines the venoatrial junctions. Numbered locations on the computed tomography images correspond to those on the intra-cardiac echocardiography images. The enclosed region was conceptualized as having four ‘walls’: 1, posterior; 2, inferior; 3, anterior; and 4, superior. The confluence of superior and inferior veins was defined as the intervenous ridge (5→6).
aAo, ascending aorta; MPA, main pulmonary artery; LAAd, distal portion of appendage complex;15 LAAp, proximal portion of appendage complex;15 LAAo, ostium of LAAp, defined as the region of transition between smooth (LAAp) and trabeculated (LAAd) endocardial contours; CS,coronary sinus; Cx, circumflex coronary artery; LV, left ventricle; RV, right ventricle; LA, body of left atrium; LPV, left pulmonary venous antrum; LAD, left anterior descending coronary artery; SCV, superior caval vein; LS, left superior pulmonary vein; LS’, branch of left superior pulmonary vein; LI, left inferior pulmonary vein; RS, right superior pulmonary vein; RI, right inferior pulmonary vein; dAo, descending aorta; LPA, left pulmonary artery; eso, oesophagus; P, pericardial recess; MA, mitral annulus; T, intra-cardiac echocardiography transducer; Tr, muscular trabeculum in appendage, tissue bridging left atrial body and contiguous left superior vein roof
Typical intra-cardiac echocardiography and computed tomography images of the right enclosed region
(A) is whole-heart, superior vantage; (B) and (C) are left atrium, extra-cardiac, posterior and right lateral vantages, respectively; (D) is left atrium, with the right antrum viewed from an intra-cardiac vantage; (E) shows the two endocardial halves resulting from the cleavage of (D) along the red line. The white circle shown in computed tomography images (A), (C), (D), and (E) as well as some intra-cardiac echocardiography images demarcates the right atrial septal ‘origin’ of the inter-atrial bundle, which is echocardiographically distinct as the region along the right septum where the anterior wall of the superior caval vein joins the atrial body.2 The enclosed region was conceptualized as having four ‘walls’: 1, posterior; 2, inferior; 3, anterior; and 4, superior. The intervenous ridge (5→6) was single in the two-vein anatomy demonstrated here, but multiple when there are supernumerary veins. The intra-cardiac echocardiography image on the top left was obtained with the intra-cardiac echocardiography transducer in the right atrium (fossa ovalis), to demonstrate the relationship between the right antrum (RPV), Waterston’s groove (w), fossa ovalis (fo), and encircling lesion. Abbreviations are as in Figure 3; in addition: RA, body of right atrium; LM, left main coronary artery; RS’, branch of right superior pulmonary vein; RPA, right pulmonary artery.
Typical electrograms obtained during mapping within enclosed regions, with data from left and right antra. The top figures show electrograms recorded from a single site (superior wall, just in enclosed region) just prior to encircling lesion deployment, immediately after the encircling lesion which did not result in isolation of the enclosed region, and again after a single secondary lesion at a distant site which resulted in isolation of the enclosed region. Surface lead V1 is also shown. Prior to ablation, overlap of non-enclosed myocardium (timing demarcated by black square) and enclosed myocardium (timing demarcated by white square) electrograms was apparent, preventing their separate assessment. After the encircling lesion, non-enclosed myocardium and enclosed myocardium electrograms have separated, attributable to delay in the latter. In this example, the amplitude of the non-enclosed myocardium electrogram was reduced after the encircling lesion, likely due to its proximity to this lesion. After the secondary lesion, only the non-enclosed myocardium electrogram remains. The bottom figure shows electrograms obtained after encircling lesions (dashed lines) which did not produce isolation of enclosed myocardium (highlighted in blue). The letters on the computed tomography images are anatomical locations from where the correspondingly lettered electrograms were recorded. At each site, activation times (in milliseconds) of both non-enclosed myocardium (first number) and enclosed myocardium (second number) electrograms were measured. In addition to electrograms recorded within the enclosed region, in each figure the electrograms from a single site outside but contiguous to the enclosed region is shown; for the right antrum example, this site was on the contiguous wall of the superior caval vein. In each case, complete isolation of the enclosed myocardium was achieved by a single secondary lesion delivered at site 5.
Figure 6: Distant (top) and close (bottom) proximity between enclosed and appendage regions:
(A) Multidimensional computed tomography image, left lateral vantage. (B) Two-dimensional computed tomography image. The space separating the enclosed and appendage regions is demarcated by the arrows. (C) Intra-cardiac echocardiography image. The lines approximate the contiguous endocardial surfaces of enclosed and appendage regions. The asterisks demarcate the sites from which the electrogram shown in (D) was recorded. (D) Electrograms recorded from comparable enclosed region locations, recorded after encircling lesions which did not achieve electrical isolation. The black and white squares demarcate timing of non-enclosed myocardium and enclosed myocardium electrograms, respectively. Patients demonstrating close proximity between enclosed and appendage regions have larger non-enclosed myocardium electrogram amplitudes and slopes.
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.
Figure 8 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 9 shows the level of RA detail radial ICE can provide to assist EP study catheter localization.
Figure 9 Radial ICE Assessment of RA Anatomy. Figure from Springer, Journal of Interventional Cardiac Electrophysiology
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
Figure 10 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 10 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.
Images and captions adapted from Schwartzman D, Williams J. Electroanatomic properties of pulmonary vein antral regions enclosed by encircling ablation lesions. Europace. 2009 Apr;11(4):435-44.
Jongbloed M R M, Schalij M J, Zeppenfeld K, Oemrawsingh P V, van der Wall E E, Bax JJ. Clinical applications of intracardiac echocardiography in interventional procedures. Heart. Jul 2005; 91(7): 981-990.