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Continuing Education
Intracardiac Echocardiography in Cardiovascular Catheter-Based Interventions: Different Devices for Different Purposes
May 2006
The use of intracardiac echocardiography (ICE) is gaining wide acceptance as one of the most powerful imaging devices in the interventional catheterization laboratory and its usefulness in different clinical settings is well known. Today, many electrophysiological, percutaneous congenital heart interventions and some peripheral vascular interventions benefit from the use of ICE.1–2 New and fascinating cardiovascular interventions such as percutaneous valve implantation and complex hybrid congenital heart disease repair are appearing on the pediatric and adult heart disease scenario, whereas others such as percutaneous valvuloplasty, atrial septal defect and patent foramen ovale transcatheter closure, stenting of baffles or conduit stenting are becoming front-line treatments for both simple and complex heart diseases. Thus, there is a potential huge field of application of intracardiac echocardiography and the use of one either piece of equipment which are obviously different as regards capabilities, image depiction and interpretation, and cost, should be properly evaluated relative to the specific catheter-based intervention requirements. The authors proposed some personal practical suggestions for conducting an ICE study.
Ultrasound Equipment. Two types of intracardiac echocardiography equipment ICE catheters are currently used in cardiovascular interventions: the AcuNav® phased-array 10 Fr, 5.5–10 Mega Hertz catheter with 90° sectorial sector image and Doppler capability (Acuson, a Siemens Corporation, Mountain View, California), and the Ultra ICE mechanical, 9 Fr, 9 Mega Hertz catheter, with 360° radial image (Boston Scientific, Natick, Massachusetts). Both systems have been fully applied in the daily clinical practice of catheter-based congenital heart disease interventions. ICE has been proven to offer significant advantages not only as effective guidance in challenging procedures, thus avoiding general anesthesia and reducing fluoroscopy time,3–4 but also economically competitive as opposed to standard transesophageal echocardiography-guided procedures.5 They also offer advantages over intravascular ultrasound, when large vessels and intracardiac structures are to be imaged. Each system has particular characteristics (Table 1).
Ultrasound Limitations and Advantages. Phased-array equipment is obviously more versatile, thanks to Doppler capability, and has a better image resolution and definition. Pulmonary veins, coronary sinus and valve apparatus and function can be optimally assessed with the electronic probe. The cost is higher than the mechanical probe but can be coupled with multiple and different consoles.
The mechanical probe doesn’t have Doppler capability and image definition is not as good, nevertheless, thanks to the 360° scan has a larger field of view and allows for a more comprehensive depiction of both atrial chambers and atrioventricular valves with their relationships and it also can be used as IVUS for great vessels. It needs a dedicated console.
Image Interpretation and Protocols
Mechanical probe. Although innumerable series of planes can be displayed by ICE, only four basic transverse sections on the horizontal plane of the body (great vessel plane, superior vena cava-right atrium junction plane, aortic valve plane, cavo-tricuspid isthmus plane) and one longitudinal section in the long axis of the heart (long-axis 4-chamber plane) are commonly used for an exhaustive evaluation of the required anatomy from the inner confines of the right atrium and great veins during catheter-based interventional procedures in case of congenital heart disease. An angulated 55° introducer is used during congenital heart disease interventions. The first four projections, the precurved introducer is withdrawn till the suprarenal portion of the inferior vena cava, whereas the long-axis view was obtained with a 55° precurved introducer sheath advanced up to the end of the catheter and turned posterior and leftward, with the transducer orientation perpendicular relative to the long-axis of the heart in order to longitudinally scan the atrial septum.
Examination is begun by navigating the ICE catheter into the superior vena cava, and subsequently withdrawing it through the body of the right atrium toward the inferior vena cava. The transducer orientation will be on the horizontal plane of the body and on the short axis of the right atrium. To appropriately display the images, they need to be electronically rotated in order to be positioned in attitudinally correct orientation, in agreement with anatomic (i.e., viewing on sections from below) or cardiac MR formats. Few specific anatomic landmarks may be used to facilitate the uninitiated beginner to orientate the image in such previously accepted fashions. The first one is the central location and precise anatomy of the ascending aorta between the pulmonary trunk on its left and superior caval vein on its right. The second one is the crista terminalis, which appears as a bright and thick structure located at the junction between the posterior smooth wall and the anterolateral trabeculated portion of the right atrium. The third one is the right atrial auricle, which appears as a large Snoopy nose-like structure. The orientation of these landmarks on the screen will be the following: ascending aorta at 2 o’clock, crista terminalis at 10 o’clock, and right atrial auricle at 12 o’clock on the screen (Figure 1A). This is an essential step when interrogating the cardiac structures by mechanical ICE, because it allows the operator to obtain a correct arrangement of the cardiac structures relative to the coordinates of the thorax: so, the left-sided structures will be displayed on the viewer’s right and the right-sided structures on the viewer’s left, whereas the anterior-sided structures will be at the top of the screen and the posterior-sided structures are at the bottom of the screen.
Measurement of any atrial septal defect or fossa ovalis longitudinal and transverse diameters should be calculated on diastolic frames in the aortic valve and in the 4-chamber planes (Figures 1A and B). In case of transcatheter closure of ASD and PFO, the procedure can be monitored from the 4-chamber view (Figures 2 A–D).
The interpretation of ICE section of the aorta during endovascular thoracic or abdominal aortic repair, is similar to conventional intravascular ultrasound images, except for the deeper field of view that allows for depicting the aortic wall also from the caval vein side. A precurved 55° or 120° introducer is used for imaging the aortic arch or isthmus in order to obtain a more coaxial position of the transducer within the aorta and avoid the parallax errors that can occur using a conventional peripheral intravascular ultrasound catheter. A straight introducer is preferable for imaging an abdominal aneurysm or for depicting the aortic wall from the caval vein side.
Electronic probe. ICE imaging is initiated after advancing (under fluoroscopic guidance) the catheter to the mid-right atrium, also referred to as the “neutral” view or “home view”. The ICE catheter is parallel to the spine with the transducer portion facing the tricuspid valve. This is shown in Figure 3 (A1) with the corresponding image obtained by ICE in Figure 3 (A2 and A3). In this view, the tricuspid valve, right ventricular inflow and outflow, and a long axis of the pulmonary valve are seen. The aortic valve can also be seen in a transverse (short axis) view. The anterior part of the septum could be seen in this view. The ICE catheter is flexed posteriorly using the knob so that the transducer faces the interatrial septum. Fluoroscopy showing the position of the catheter, as well as a corresponding anatomic diagram is shown in Figure 3 (B1, B2, B3). The ICE image obtained shows the interatrial septum as well as the coronary sinus, pulmonary veins, depending on the exact location of the transducer. This can be referred to as the “septal view”. One can lock the tip in this position and further views can be obtained by rotating the entire handle as well as by fine-tuning the posterior/anterior or right/left knobs. The ICE catheter itself is then advanced in a cephalad direction toward the superior vena cava (SVC). This can be referred to as the SVC or “long-axis view.” A fluoroscopic image showing the position of the catheter as well as a corresponding anatomic diagram is shown in Figure 3 (C1, C2, C3). In this plane, the transducer faces the interatrial septum and the SVC can be seen as it relates to the right atrium. The interatrial septum is shown in a superior/inferior plane and corresponds to the TEE long axis view. The catheter is then in its locked position and rotated clockwise until it sits in a position with the transducer near the tricuspid valve annulus, and inferior to the aorta. A fluoroscopic image showing the catheter position and a corresponding images obtained are shown in Figure 3 (D1, D2, D3). In this view, the aortic valve can be seen in short axis as well as the interatrial septum. This corresponds to the basal short axis view obtained with TEE and is known as the “short axis view”. However, the right atrium in the near field and the left atrium is in the far field, this is opposite of what is seen with TEE. Figures 4 (E1, E2, F1, F2, G1, G2, H1, H2) and 5 (I1, I2, J1, J2, K1, K2, L1, L2) demonstrate a case of a patient with large secundum ASD that underwent successful device closure.
Present Field of Application
Interventional electrophysiology. The growth of interventional electrophysiology in recent years has placed a greater demand on imaging guidance. Currently available imaging methods to guide interventions, while useful, have significant limitations. intracardiac echocardiography has the potential to improve on these limitations and evolve into a method that could provide imaging guidance, help to prevent or instantly identify complications, help identify and quantify ablative lesions, and contribute towards the visualization of normal and abnormal conduction pathways.6 Moreover, it can be used to clearly identify the coronary sinus during left ventricle guided implantation.7 The mechanical probe is still preferred for a deep field of vision in most cases except the isolation of the pulmonary vein where an electronic Doppler probe, affords easy identification of the vein.
Transseptal puncture. Intracardiac echocardiography is useful for clearly identifying the fossa ovalis by the curtain effect of the needle against the fossa ovalis, visualized by intracardiac echocardiography. Thus, ICE makes it possible to perform a safe transseptal puncture preventing serious or even fatal complications in transseptal procedures especially when the cardiac anatomy is unusual or distorted. It also helps us to understand the possible mechanisms leading to mechanical complications in cases where fluoroscopic images are apparently normal8 especially when ICE shows a small oval fossa impinged on by an enlarged aorta anteriorly.9 Both pieces of equipment enable us to perform a safe transseptal puncture.
Simple congenital heart disease. Transcatheter closure of ostium secundum atrial septal defect and patent foramen ovale were performed for years using trans-esophageal echocardiography until intracardiac echocardiography arrived on the congenital heart intervention scene to increase image resolution thus avoiding general anesthesia and related morbidity and to increase patients’ comfort during this relatively new procedure.10 The electronic probe, with its Doppler capability, can assess the residual shunt immediately. Both probes can measure the fossa ovale or the atrial septal defect surrounding rims in order to select the proper device size,11 two orthogonal views were selected to measure the diameters of the fossa ovalis or of the atrial septal defect due to the elliptical shape of the defect. ICE has been proven to be equivalent to transesophageal echocardiography as regards resolution and precision.12Aortic aneurysm endovascular repair. IVUS has been proposed in order to identify patients with increased risk of major adverse complications following endovascular repair.13 Unfortunately, the field view of most peripheral intravascular ultrasound catheters is limited, and if there is a very large aneurysm some detail of the vessel may be lost. The mechanical ICE probe offers a larger field view with a radial, IVUS-like, 360° format. Thanks to its precurved long sheath it can take up a coaxial position within the vessel, especially at the aortic arch in the case of thoracic aneurysms, thus decreasing any error of parallax in the images. The intracardiac mechanical probe may be used 1) to confirm the aortic pathology and morphometric measurements; 2) to define the proximal and distal extents of the aortic lesion, documenting the position of the probe tip through fluoroscopy in relation to radiopaque ruler positioned behind the patient; 3) to detect the morphology of vascular structure (presence of calcium, distribution of intraluminal thrombus); and 4) to visualize the major side-branches (subclavian, celiac, renal and internal iliac arteries), as a distinct interruption of the aortic or common iliac artery wall. After stent graft deployment, a second pullback may be performed 1) to identify proper placement of the stent-graft across the diseased aorta; 2) to determine full and symmetrical expansion of the endoprosthesis (Figure 6); 3) to evaluate absence of any potentially flow-limiting causes adjacent to the stent-graft, and finally; 4) to verify patency of major sidebranches14 (Figures 7 A and B).
Mitral valvuloplasty. Green et al. recently15 successfully investigated the role of intracardiac echocardiography during mitral valvuloplasty in 19 patients. The electronic intracardiac ultrasound probe-guided transseptal puncture augmented the assessment of valve apparatus deformity, facilitated balloon positioning across the mitral valve and permitted postprocedural valve assessment including the identification of mitral regurgitation with color Doppler.
Investigational and Future Indications
Complex congenital heart disease. Intracardiac echocardiography has been proposed as the main imaging tool in the case of complex congenital heart disease interventions such as the creation of atrial septal defect, implantation of a fenestrated device and closure of residual shunts after complex surgery.16 It may also have a role as an intracardiac probe to determine immediate results after surgery in congenital heart disease.17 Other interventions such as repair of venous baffle stenosis or transcatheter Fontan repair of hypoplastic left heart syndrome may benefit from the use of an intracardiac probe to avoid excessive X-ray exposure and contrast volume injections in newborn babies or infants, while providing essential data to help determine the diameter of stent to be implanted or the need for angioplasty after stent positioning.
In adult congenital heart disease patients, intracardiac ultrasound may improve coarctation repair by direct imaging of the aortic wall: deep field view allowing insertion through the vein, sizing of the stent, accurate measurement of reference aortic diameter, easy identification of potential sites of dissection and procedure monitoring are the most interesting data obtained by a mechanical intracardiac probe.18 An electronic probe may be preferable when a residual shunt is to be detected whereas a mechanical probe should be used in cases of stent implantation.
Left atrial appendage transcatheter closure. Left atrial appendage closure is becoming a real choice for patients with contraindications to long-term anticoagulation therapy and atrial fibrillation.19 An intracardiac ultrasound probe can assess left atrial appendage and potential thrombi inside.20 Both electronic and mechanical probes may help the interventionist to correctly assess and size the left atrial appendage and to monitor the procedure from the venous side but the electronic probe has the potential to determinate eventual residual flow after transcatheter closure.
Percutaneous valve implantation. Implantation of percutaneous aortic or pulmonary, valve, may benefit from direct intracardiac monitoring.21 Direct sizing of valves, positioning of the stent through the valve, distance from the coronary ostia may also be fields of application for both intracardiac probes inserted from the venous side.
Conclusion
The field of percutaneous cardiovascular interventions of any kind may benefit from intracardiac ultrasound probes. Pending an ideal probe which should be 8 Fr, electronic, 360° radial scan with Doppler assessment, the endovascular specialist should be aware of the potentials of the different intracardiac probes currently available (Table 2).
Acknowledgements. The authors wish to thank Dr. Mario Zanchetta, of the Cardiovascular Department of Cittadella General Hospital, for his skill and intuition in developing mechanical intracardiac echocardiography and for Figures 6 and 7. We also wish to thank Dr. Qi-Ling Cao of the Pediatric Section at the University of Chicago.
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