The field of non-coronary cardiac intervention is undergoing rapid expansion, driven by advances in technology and increasing demand for alternative, non-surgical therapies for common structural heart diseases. As a result, the volume, variety and complexity of percutaneous catheter-based procedures being performed in cardiac catheterisation laboratories are increasing. Traditionally, fluoroscopy has been used to guide such procedures, but it has important limitations, such as significant radiation exposure, a 2D perspective and limited tissue differentiation, as well as the requirement for radiographic contrast use. The greater complexity of these newer procedures, particularly those involving heart valve intervention, necessitates more sophisticated and exacting imaging techniques, both to facilitate appropriate case selection and to provide procedural guidance, thus increasing the likelihood of successful outcome. Contemporary advances in echocardiography imaging techniques ensure these modalities are well suited to the imaging requirements of this exciting and expanding field of interventional cardiology.
Realtime 3D imaging, made possible by the development of a full matrix transducer capable of acquiring pyramidal-shaped ultrasound data sets, has been a major advance in transthoracic echocardiography (TTE). Furthermore, transducers and systems capable of single-beat 3D acquisitions, thus eradicating stitching artefacts, are now available. Miniaturisation of this 3D technology has enabled coupling with a transoesophageal echocardiography (TEE) probe providing high-quality 3D TEE images. Data acquisition with 3D TEE can be performed in three different modes: narrow-angle acquisition, displaying realtime images; 3D zoom mode, generating a realtime truncated pyramidal data set; and wide-angle full-volume mode, enabling a larger pyramidal data set but requiring electrocardiographic gating and acquisition over one to seven cardiac cycles. The wide-angle full-volume mode is currently the only mode to support colour Doppler imaging. Unlike the cumbersome 3D predecessors, current data sets can be readily manipulated either online or offline to display cardiac structures or to enable quantitative analysis.
Intra-cardiac echocardiography (ICE) is a recent application of ultrasound technology that offers imaging of comparable or even superior quality to TEE. Early ICE acquired cross-sectional images using a rotating transducer similar to intra-vascular ultrasound (rotational ICE). The incorporation of miniaturised phased-array technology into an intra-cardiac catheter has now enabled sector-based imaging (phased-array ICE). Rotational ICE provides good near-field but more limited far-field imaging. However, phased-array ICE has deeper penetration and better far-field imaging than rotational ICE, in addition to being steerable and allowing Doppler imaging.
The ideal echocardiography modality would provide high-quality, realtime 3D imaging in a format easily comprehended by the interventional cardiologist, with minimal interference to the flow of the interventional procedure being performed. It would be safe, minimally invasive and widely available at low cost. Currently, no one echocardiography imaging modality fulfils all these criteria. Each has relative advantages and disadvantages compared with the others (see Table 1). Therefore, selection of the optimal imaging modality for any given interventional cardiac procedure must take into account both the specific requirements of the procedure and the relative strengths and weaknesses of the imaging modality to ensure the greatest likelihood of successful outcome.
Congenital and Acquired Septal Defects
Percutaneous Atrial Septal Defect/Patent Foramen Ovale Device Closure
Percutaneous device closure is increasingly the therapy of choice for patients with atrial septal defect (ASD) or patent foramen ovale (PFO), where clinically appropriate and in the absence of complicated cardiac anatomy or other indications for cardiac surgery.1,2 In experienced hands, it is a safe and effective procedure with a risk of serious complications, such as device embolisation, of less than 1%.3 It improves functional status in symptomatic patients and exercise capacity in both asymptomatic and symptomatic patients, although long-term follow-up safety and efficacy data are awaited.4,5
Echocardiography plays a key role before, during and after percutaneous ASD/PFO device closure. Indeed, its use for procedural guidance, in addition to primary fluoroscopy, was recently recommended by the American Society of Echocardiography (ASE).6 Typically, the diagnosis of an inter-atrial shunt is made by TTE with colour Doppler and/or agitated saline or gelofusin contrast injection. TTE also provides information about the type of defect, its haemodynamic significance and any associated anomalies. However, if percutaneous closure is clinically indicated, detailed assessment of the inter-atrial septal anatomy and surrounding structures is required by either TEE or ICE.
Systematic evaluation of the inter-atrial septum and surrounding structures should be performed to determine defect type, size and PFO tunnel length, the presence of fenestrations, aneurysmal septum or prominent Eustachian valve or Chiari network, and to exclude associated abnormality of the inferior and superior vena cavae, pulmonary veins, coronary sinus and atrioventricular valves. Furthermore, consideration of the size of the defect rims and total atrial septal length are important in determining the likelihood of procedural success.7,8 This evaluation enables the interventional cardiologist to plan the procedure and to choose from a variety of ASD and PFO closure devices with differing designs and characteristics. Currently, only isolated ostium secundum type ASDs and PFOs are suitable for device closure. Possible contraindications to device closure identified by echocardiography therefore include very large (>38mm) or multiple defects, deficient (<5mm, circumferential extent >1/3) rims, close proximity of the defect to the atrioventricular valves, sinus venosus defects, anomalous pulmonary venous drainage, ASD with severe pulmonary hypertension and bi-directional or right-to-left shunting and intra-cardiac thrombus. Intra-procedural imaging (TEE or ICE) facilitates wire and catheter manipulation and positioning, balloon sizing of the defect and device deployment, as well as enabling pre- and post-closure assessment of adjacent cardiac structures and early identification of complications such as pericardial effusion.9–13 Following device closure, TTE is indicated at routine intervals to confirm device stability, evaluate the presence of any residual shunt and detect the development of late complications.11,14,15
The advent of realtime 3D TEE has enhanced the evaluation of ASDs. 2D echocardiography may underestimate both the size and complexity of an ASD; however, 3D echocardiography clearly defines inter-atrial septal anatomy and enables an en face view of the defect (see Figure 1A).16–18 Multiplanar reconstruction of the 3D data set allows accurate measurement of the minimum and maximum dimensions of the defect, facilitating selection of the optimal size and type of closure device (see Figure 1B).19 Moreover, intra-procedural realtime 3D TEE, in addition to 2D, provides superior visualisation of wires, catheters and devices and their relationships to neighbouring structures in a format that is generally more intuitively comprehended by the interventional cardiologist (see Figures 1C and 1D). The advantages of realtime 3D TEE in guiding cardiac catheter interventions are increasingly recognised.20,21
ICE is also commonly used to guide ASD/PFO device closure in many centres. It provides comparable imaging of the inter-atrial septum and surrounding structures to 2D TEE, and indeed may enable superior visualisation of the inferoposterior inter-atrial septum.11,12,22 However, it also has the distinct advantages of requiring neither general anaesthesia nor additional echocardiographic support, as it may be performed by the primary operator. As such, ICE has been shown to reduce fluoroscopy and procedure times, as well as offering comparable cost to procedures performed under general anaesthesia with TEE guidance.9,10,22,23 However, potential disadvantages of ICE include limited far-field view, lack of catheter stability, the expense of single-use ICE catheters, the need for additional training and possible provocation of atrial arrhythmias, as well as difficulty for the single operator manipulating both the ICE catheter and the device at the time of deployment.
Percutaneous Ventricular Septal Defect Closure
Successful percutaneous closure of post-myocardial infarction ventricular septal defect (VSD) has been reported.24,25 Echocardiography (often TTE) confirms the diagnosis of VSD and provides information regarding shunt size and haemodynamic significance. 3D TEE may be required to accurately define and size the defect by enabling en face visualisation, thus determining the size of closure device to be used as well as guiding the closure procedure.25
Valvular Heart Disease
Balloon Mitral Valvuloplasty
Balloon mitral valvuloplasty (BMV) is a well-established therapeutic option for patients with symptomatic mitral stenosis and appropriate anatomy. In addition to its role in the evaluation of the aetiology and severity of mitral stenosis and in case selection, TTE is used in many centres with fluoroscopy to guide BMV. TTE is a useful adjunct to fluoroscopy in transseptal puncture, provides immediate assessment of the results of valvuloplasty (visualisation of valve anatomy, determination of transvalvular gradients and mitral valve area, detection of mitral regurgitation) and allows early detection of complications.26 However, the need for TTE imaging may interfere with the execution of the procedure and may provide sub-optimal images in some patients. Alternatively, guidance may be provided with TEE or ICE.27–31 TEE is superior to TTE in excluding left atrial and left atrial appendage thrombi and in monitoring wire and balloon positioning. Newer phased-array ICE catheters now provide Doppler imaging, thus enabling comparable imaging to 2D TEE. The use of echocardiography guidance during this procedure has been shown to improve procedural success and reduce complication rates; it may also reduce procedural and fluoroscopic times.31–33 The advent of 3D TTE and TEE has enabled more accurate evaluation of mitral stenosis, and in particular of mitral valve area using multiplanar reconstruction from the 3D data set to ensure correct alignment with the often eccentric mitral valve orifice.34–37
Percutaneous Mitral Valve Repair
The role of echocardiography in assessing the aetiology and severity of mitral valve disease, as well as in guiding therapeutic strategy and prognosis, is well established. More recently, it has become clear that comprehensive characterisation of the mitral valve apparatus requires 3D TTE and TEE.38 New percutaneous mitral valve repair systems are being developed that offer a possible therapeutic option for those patients at high surgical risk. One such system, with promising early results, uses a catheter-based approach to deliver a ‘clip’ placed on the mitral valve leaflet tips, essentially creating an edge-to-edge repair.39–41 However, this approach mandates careful case selection using a combination of TTE and TEE to assess aetiology and severity of mitral regurgitation and suitability for the percutaneous clipping procedure. In particular, detailed evaluation must consider mitral valve leaflet flail gap and width, leaflet coaptation length and mitral valve area, as well as confirming the presence of secondary chordal support and the absence of leaflet cleft.
The percutaneous mitral valve clipping procedure itself is guided primarily by TEE. This is essential, first to guide precise localisation of the transseptal puncture to the superior and posterior portion of the inter-atrial septum, facilitating approach to the mitral valve. Next, TEE ensures correct alignment of the advancing clip perpendicular to the plane of the mitral valve, and of the open arms perpendicular to the coaptation line. After leaflet grasping, leaflet approximation, severity of mitral regurgitation and transmitral gradients are assessed and then final clip deployment visualised (see Figure 2). As with other interventional procedures, TEE enables prompt detection and evaluation of any complications. Subsequent routine TTE at regular intervals enables monitoring of clip stability, severity of any residual mitral regurgitation, transmitral gradients and effects on cardiac chamber size and function.
3D TEE is essential to enable thorough assessment of mitral valve anatomy, including localisation of individual leaflet scallops and, using 3D colour, to accurately localise the origin of regurgitant jets.42 As such, 3D TEE plays a significant role in the planning of mitral valve repair, whether by a surgical or percutaneous route.43 The 3D zoom mode is unique in enabling visualisation of the mitral valve, both from the left atrial aspect in surgical orientation and from the left ventricular aspect. This latter view is particularly helpful when assessing clip orientation relative to the line of coaptation as an alternative to the 2D transgastric view, which may be difficult to achieve satisfactorily in some patients.
Transcatheter Aortic Valve Implantation
Transcatheter aortic valve implantation (TAVI) provides a percutaneous treatment option for those patients with aortic stenosis with conventional indications for aortic valve surgery but who have unacceptably high surgical risk.44–47 Currently, two devices, with differing construction and characteristics, are being implanted: the Cribier-Edwards bio-prosthesis (Edwards Lifesciences LLC, Irvine, CA) and the CoreValve aortic valve prosthesis (CoreValve Inc., Irvine, CA). The Cribier-Edwards bio-prosthesis is a balloon-expandable tri-leaflet bovine pericardial valve mounted within a tubular stainless steel stent that may be implanted via a transfemoral or a transapical route. The CoreValve is a tri-leaflet pericardial tissue valve mounted in a self-expanding nitinol stent. Echocardiography has a vital role in the evaluation of patients with aortic stenosis, in assessment of suitability for TAVI, in guiding the implantation procedure and in subsequent follow-up.48–50
Initial evaluation is performed with TTE, with more detailed imaging of the aortic valve and surrounding structures performed with TEE, either prior to (if there are concerns regarding suitability) or at the time of the TAVI procedure. Multiple echocardiographic factors should be taken into consideration when assessing suitability for TAVI, implantation route and choice of valve size, all of which are crucial in determining the likelihood of successful outcome. These factors include: aortic valve and root morphology (annulus and sinus of Valsalva size and geometry, severity and eccentricity of leaflet calcification); position of the coronary ostia relative to the aortic valve leaflets and any calcification; morphology of the left ventricular outflow tract (LVOT) and aortomitral continuity (proximal septal hypertrophy, presence of calcification); and left ventricular cavity size.
During the TAVI procedure, TEE provides realtime imaging to assist wire and catheter placement, to assess the effects of balloon valvuloplasty and ensure correct positioning of the aortic valve prosthesis, to assess for paraprosthetic regurgitation immediately following implantation and to evaluate any complications that may occur (see Figure 3).48,49 TTE is performed prior to discharge and subsequently at regular intervals to monitor function of the aortic valve prosthesis.50
Percutaneous Closure of Prosthetic Paravalvular Leaks
Prosthetic valve dehiscence is a recognised complication following valve replacement surgery and may lead to severe paravalvular regurgitation and heart failure. In those patients for whom re-do valve surgery is considered to be too high-risk, percutaneous implantation of an occlusion device in the dehisced region of the prosthetic valve has been performed.51,52 TTE and TEE are used to establish the diagnosis and TEE to guide the intervention. 3D TEE is important in exactly delineating the anatomy, allowing appropriate case selection for percutaneous device closure, and in enabling realtime imaging during the procedure.25
Atrial Fibrillation Ablation
Echocardiography, predominantly ICE, is used widely in electrophysiology laboratories to guide ablative procedures, particularly atrial fibrillation (AF) ablation. The relative advantages of ICE over TTE and TEE described earlier make it particularly suited to use in this environment.
Many specific cardiac arrhythmias are dependent on the underlying anatomy, and there is therefore a need to deliver ablative therapy at precise anatomical locations; ICE facilitates this by providing excellent realtime imaging and feedback. TTE has a well-established role in assessing cardiac structure and function in patients with AF and in detecting complications before and after ablation procedures. TEE is the imaging modality of choice for exclusion of left atrial thrombus, a contraindication to ablation therapy.
ICE is useful in guiding transseptal puncture, and may reduce the risk of complications compared with fluoroscopic guidance alone, particularly where there is unusual anatomy or where a second puncture is required.53,54 Furthermore, ICE enables precise localisation of the site of transseptal puncture, facilitating access to specific sites within the left atrium.55 Subsequently, ICE is used to determine pulmonary vein anatomy and physiology prior to ablation, to assist and confirm catheter positioning, to confirm catheter tip–tissue contact and stability and to monitor for potential procedural complications such as the formation of microbubbles (indicating excessive tissue heating), pericardial effusion, thrombus formation and pulmonary vein stenosis.56–58 ICE has also been used with success in ablative procedures for other cardiac arrhythmias.59–61 Recent ASE guidance recommends the use of ICE for radiofrequency ablation for AF.6 Some centres successfully use TEE (including 3D) to guide transseptal puncture (see Figure 4) and, in addition to fluoroscopy and computed tomographical reconstruction, to guide pulmonary vein ablation procedures for patients with AF, although this practice is much less common.25
Echocardiography is often used to guide needle pericardiocentesis, in addition to fluoroscopy, and may reduce procedure-related complications.62 TTE determines the size and location of the pericardial effusion, factors important in deciding the site of approach and needle trajectory. Once the catheter is positioned in the pericardial space, its location can be confirmed by the injection of agitated saline.
While myocardial biopsy is typically performed with fluoroscopy alone, some centres use adjunctive TTE or, in selected patients, even TEE or ICE.63,64 Echocardiography guidance enables greater choice of site for the biopsy and may reduce the risk of complications.63 TEE has also been used in selected cases to guide biopsy of intra-cardiac and intra-vascular masses.65
Alcohol Septal Ablation
Alcohol septal ablation is performed in selected patients with symptomatic hypertrophic obstructive cardiomyopathy refractory to medical therapy.66 This technique involves the injection of ethanol into a septal perforator branch of the left anterior descending artery, causing localised infarction in the hypertrophied proximal ventricular septum. Echocardiography guidance is typically performed during the procedure with TTE, although TEE or ICE may be used as an alternative in some centres, with favourable results.66–68 The left ventricular outflow tract and mitral valve anatomy and function are evaluated and the correct target septal perforator identified using echocardiography contrast injection to ensure perfusion of the desired region of the septum prior to ethanol injection. The latter technique has been reported to affect interventional strategy in 15–20% of cases.69 TTE or TEE also enables immediate evaluation of results and monitoring for complications.
Left Atrial Appendage Device Occlusion
Percutaneous left atrial appendage (LAA) occlusion devices are currently under development and undergoing evaluation in clinical trials. While preliminary data have shown LAA device occlusion to be safe and feasible, it is uncertain whether it will prevent thromboembolic stroke in patients with AF.70 The LAA occlusion device is placed via a percutaneous transcatheter approach via a transseptal puncture to access the left atrium using fluoroscopic and TEE guidance. TEE is required to guide transseptal puncture, to size the ostium of the LAA to enable selection of the appropriate size ofocclusion device and to ensure optimal device placement and complete occlusion of the LAA. It also enables detection of complications. 3D TEE imaging enables excellent device visualisation with respect to the LAA.
It is likely that the exciting field of non-coronary cardiac intervention will continue to expand and evolve, necessitating advances in echocardiography technology to keep apace to support this development. Important advances will be seen in all the echocardiography modalities. Future developments that will further extend the role of ICE in the cardiac laboratory may include reduction in catheter size and improved catheter stability and handling, enhanced image quality and the development of realtime 3D imaging. Other potential advances include the integration of ICE into current electro-anatomical mapping systems and also coupling of ICE and ablative therapy into a single catheter.71 Recent development of a neonatal TEE probe small and flexible enough to allow transnasal insertion in adults has a potential application in interventional cardiology. The transnasal approach, typically better tolerated by patients, may offer the distinct advantage of enabling some interventional procedures requiring TEE guidance to be performed without general anaesthesia. However, acceptable image quality and good patient tolerance with this application are yet to be demonstrated in scientific evaluations. It is likely that a single TTE transducer with integrated 2D and 3D capability will be available soon, and this will greatly aid workflow. Increases in processing power will lead to greater temporal and spatial resolution, further improving image quality. In addition, developments in software and workflow, taking into consideration the particular demands of procedural imaging, will also aid the interventional echocardiologist.