Accurate knowledge of the anatomy of intracardiac structures is essential for the efficiency and safety of performing interventional procedures. Although fluoroscopy is an important aid while advancing catheters in the heart, it is not sufficient to supply accurate anatomical information on the position of veins, ridges, and septa in the heart. Furthermore radiation exposure forms a risk to both the patient and the operator.

Intracardiac echocardiography (ICE) is a promising technique for imaging of intracardiac structures, and may serve as an alternative for the transoesophageal approach,¹ which is semi-invasive and requires anaesthesiology. The introduction of ICE catheters with low frequencies allows accurate visualisation of intracardiac anatomical structures with an ultrasound catheter placed exclusively in the right side of the heart. This report focuses on the use of ICE in guiding percutaneous interventional procedures.

BASICS OF INTRACARDIAC ECHOCARDIOGRAPHY

History of two dimensional intracardiac ultrasound systems

The first application of ICE was done using mechanical ultrasound systems, which were introduced in the 1980s. These systems provided high resolution imaging, but because of the high frequency of the transducers (20–40 MHz), tissue penetration was only limited and anatomic intracardiac overviews could not be obtained. The subsequent development of lower frequency transducers allowed imaging of intracardiac structures, but these systems were still limited by the low steerability and over-the-wire design of the catheters.² The clinical use was improved by the development of flexible lower frequency transducers (9 MHz), but depth control of these catheters still was not sufficient to allow visualisation of the whole heart from the right side of the heart. In the 1990s, systems modified after transoesophageal echocardiographic probes were introduced. In these systems depth was improved by the use of lower frequencies (5 MHz). However, the large size of these transducers limited the clinical use. In recent years the development of steerable phased array ultrasound catheter systems with low frequency and Doppler qualities has expanded the clinical use of ICE.

What are the technical requirements?

Mechanical ultrasound tipped catheter

The mechanical ultrasound transducer tipped catheter (Clearview, Cardiovascular Imaging Systems Inc, Fremont, California, USA) can be used for both intravascular and intracardiac imaging. For intracardiac use, a 9 MHz single element transducer is incorporated in an 8 French catheter. A piezoelectric crystal is rotated at 1800 rpm in the radial dimension perpendicular to the catheter shaft, thus providing cross sectional images in a 360° radial plane. A sheath surrounding the imaging transducer is necessary to prevent contact of the imaging transducer with the cardiac wall. The ICE catheter needs to be filled with 3–5 ml sterile water before it is connected to the ultrasound machine (Boston Scientific Corp, San Jose, California, USA). Three dimensional reconstruction of the acquired data can be created.

Phased array ultrasound tipped catheter

This system uses a 10 French ultrasound catheter (Acunav Diagnostic Ultrasound Catheter, Acuson Corporation, Mountain View, California, USA), which is positioned in the right atrium (RA) or right ventricle (RV) via a femoral approach, through a 10 French introducer. The ultrasound catheter consists of a miniaturised 64 element, phased array transducer, which is incorporated in a single use catheter. The transducer scans in the longitudinal monoplane, providing a 90° sector image with tissue penetration of approximately 15 cm. Two planes of bidirectional steering (anterior–posterior and left–right, each in a direction of 160°) are possible by using a mechanism on the handle of the catheter. The high resolution multiple frequency transducer (5–10 MHz) allows tissue penetration enhancement, thus allowing depth control. Measurements of haemodynamic and physiologic variables can be made using Doppler imaging. The catheter is connected to an ultrasound system (Acunav/Seqouia, Acuson Corp, Mountain View, California, USA).

Mechanical versus phased array ultrasound tipped catheter

Although the mechanical ultrasound systems can be used at a considerably lower cost compared to the phased array systems, there are several disadvantages as compared to the phased array transducers. Currently, no functional analysis can be performed with mechanical systems, due to the lack of colour and pulsed Doppler features. Since these systems use a single ultrasound frequency of 9 MHz and provide a limited radial depth of view (5 cm), imaging of left sided structures is not feasible with the ultrasound catheter placed in the right heart. This could increase the risk of thromboembolic complications when using the device for imaging of left sided structures. Furthermore, the design of the mechanical rotating catheter shaft, and the lack of an articulation mechanism at the handle, limits steering of the transducer and thus a dynamic view on intracardiac structures. In the remainder of the current report we mainly focus on the use of phased array ICE in guiding percutaneous interventional procedures.

Advantages and limitations of ICE during interventional procedures

In the past, if percutaneous interventions needed guiding, fluoroscopy, transthoracic echocardiography (TTE), or transoesophageal echocardiography (TOE) were used. The use of ICE has several advantages over these other techniques. No radiation is needed. In comparison to TOE, patient discomfort is less and general anaesthesia is not needed, allowing communication with the patient during the procedure. As compared to TTE, the transvenous access has the advantage that it is not necessary to position a transducer in a sterile field.

General advantages of ICE are the availability of direct online information on the position of catheters and devices, and the possibility of direct monitoring of acute procedure related complications (such as thrombus formation, pericardial effusion, etc).

Several limitations of ICE currently exist. First the considerable shaft size (10 French) and the lack of additional catheter features, such as ports for guidewires, therapeutic devices and pressure, form a limitation. Second, the phased array catheters are expensive and single use only. Third, phased array ICE provides only monoplane image sections. Although this can partly be overcome by the steerability of the catheter, operators who are used to multiplane TOE transducers may have difficulty obtaining the same views. Moreover, no standard views for ICE are currently defined, as are available for TTE and TOE.

Which views can be obtained with ICE?

Using phased array ICE, views of all anatomic landmarks can be obtained with the ultrasound catheter positioned in either the RA or the RV. Although images quite similar to TOE can be obtained, the relatively “loose” position of the ultrasound probe in the heart may give the inexperienced operator a feeling of disorientation. To overcome this, structured introduction and steering/manipulation of the ultrasound tipped catheter can ultimately provide orientation, supported by recognition of the anatomical landmarks. The catheter is advanced via the femoral vein, into the middle part of the RA via the inferior caval vein, thus visualising the RA, tricuspid valve, and RV (fig 1A). This view with the catheter positioned in the middle of the RA can be used a “basic point of orientation”, from which other views can be derived. Counterclockwise rotation with the catheter positioned in the superior part of the RA provides imaging of the terminal crest, whereas clockwise rotation of the catheter from the inferior RA provides a view of the Eustachian ridge with the tricuspid–caval isthmus (fig 1B); these are important target structures in atrial flutter ablation.

By turning the catheter around its axis (clockwise fashion as seen from the operator), imaging of the aortic valve, the right ventricular outflow tract, and the pulmonary artery is feasible (fig 1C). Long axis views of both aorta and pulmonary trunks can be imaged in the same plane, which is not feasible with TOE. By rotating the catheter clockwise from the low right atrium, a short axis view of the coronary sinus can be obtained, while left-to-right movements of the ICE catheter provide a long axis view of this structure (fig 1D, E). When the catheter is further rotated, the next anatomical structure important in interventional cardiology to be recognised is the atrial septum (important for septal puncture, see below). By counterclockwise movement of the catheter from this position, imaging of the left atrium (LA), mitral valve, and left ventricle (LV) is performed (fig 1F). By increasing the depth setting of the catheter with the transducer directed at the left atrial posterior wall, imaging of the left and right pulmonary veins and the left atrial appendage (LAA) is performed from the position of the atrial septum (important for pulmonary vein (PV) ablation, see below). As is it sometimes difficult to distinguish between the LAA and the left superior PV, Doppler capacities can be used to differentiate. Finally, by advancing the catheter into the RV, detailed imaging of the LV can be obtained. Both long and short axis views of the LV can be obtained (fig 1G, H).

CLINICAL APPLICATIONS

The clinical application of ICE in interventional procedures is listed in table 1.

Detection of intracardiac thrombus

Because of the risk of systemic embolism, evaluation of the presence of intracardiac thrombus before left sided procedures is mandatory. The risk of thromboembolic events is related to the presence of LA spontaneous contrast and an LAA peak emptying velocity ⩽ 20 cm/s.³ ICE can serve as an alternative for TOE assessment of LA and LAA function⁴ (fig 2 A,B). Before left sided interventions, the LAA can be evaluated for the presence of thrombus (fig 2C).

The reported incidence of thromboembolic complications during left sided interventional procedures is approximately 2%,⁵ possibly as high as 5% in patients with a history of transient ischaemic attacks.⁶ Detection of intracardiac thrombus with ICE during left sided interventional procedures is discussed below (see PV ablation).

Transseptal puncture

Potential life threatening complications of transseptal puncture include aortic puncture, pericardial puncture or tamponade, systemic arterial embolism, and perforation of the inferior caval vein.⁷ Accurate visualisation of the fossa ovalis reduces the risk of complications. Especially less experienced operators may find ICE a better imaging modality then fluoroscopy, by its ability to visualise accurately the target for transseptal puncture, the fossa ovalis.⁸ The Valsalva manoeuvre during injection of saline/contrast can be performed to reveal the presence of a patent foramen ovale (PFO) (fig 2D). In 10–20% a PFO is present, omitting the need for transseptal puncture (fig 2E). Positioning of the ultrasound catheter in the RA provides a clear vision of the interatrial septum, which consist of a thicker part, the limbus, and the remains of the primary atrial septum, the fossa ovalis, which is the safest target for the transseptal puncture. During the puncture a Brockenbrough needle (DAIG Corp) is inserted via a transseptal sheath and dilator system and directed towards the fossa ovalis. The first sign of a stable contact of the transseptal dilator at the oval fossa, is “tenting” of the septum at this site (fig 2F). Successful puncture can be confirmed by the appearance of contrast in the LA after contrast injection. After withdrawal of the needle a mapping/ ablation catheter can be positioned in the LA.

Guidance of closure of atrial septal defect and patent foramen ovale

Percutaneous transcatheter device closure of atrial septal defect (ASD) and PFO has become a safe and efficient alternative to open heart surgery. Guidance of these procedures by TOE is standard practice and in recent years several studies have reported the feasibility and safety of ICE for guiding these procedures.^9,10 The closure of ASDs guided by TOE, without use of fluoroscopy, has been described. However, although the procedure could be satisfactorily performed in most cases without prolongation of procedure times, patients needed to undergo extensive TOE, and significantly higher doses of sedation were used.¹¹ Especially for this application, when long and continuous echocardiography imaging is required, ICE reduces procedure and fluoroscopy times, does not require general anaesthesia, and is less discomforting than TOE.⁹ During these procedures, ICE is used to determine the size and location of the defect in relation to cardiac anatomy and the presence of rims and septal remnants. Clear visualisation of the ASD/PFO, RA, LA, sizing balloon and closure device is feasible with a phased array catheter positioned in the RA (fig 3). Deployment of the device can be evaluated using ICE. Particularly when the remaining part of the septum is floppy, accurate evaluation of the stability of the closure device using Doppler dynamics is necessary before releasing the device.

Guidance of interventional electrophysiology procedures

Pulmonary vein isolation procedures

The observation that ectopic foci originating in the PVs can initiate atrial fibrillation¹² has led to the development of percutaneous catheter ablation strategies aimed at ablation at the site of the PVs. Although results are promising, long procedure times and procedure related complications, such as PV stenosis, are still challenges for improvement. These issues are (in part) related to the fact that the ablation targets, the veno-atrial junctions and the PVs or their ostia, are not easily visualised using fluoroscopy. Furthermore, interindividual variations in PV anatomy occur, which may require adjustment of the ablation strategy. During PV ablation procedures, ICE can provide information on the individual PV anatomy, such as the number of PVs, the presence of common ostia and of additional PVs, which may influence the ablation strategy in anatomical based isolation procedures¹³ (fig 4A, B). Furthermore, ICE can be used to guide ablation catheters. In order to deliver optimal radiofrequency energy to the tissue, with a minimum amount of heat loss, a firm contact of the catheter tip with the myocardium must be established. It has been demonstrated that ICE can ensure adequate catheter tip–tissue contact in order to achieve transmural ablative lesions¹⁴ and that the use of ICE improves the outcome.¹⁵

Another advantage of the use of ICE during these procedures is the ability to monitor the occurrence of acute complications. Potential complications of ablation at the site of the PVs are: PV stenosis, thromboembolic complications, and perforation with pericardial tamponade. Visualisation of micro-bubble formation is an important characteristic according to which power settings of the radiofrequency energy delivery can be adjusted,¹⁵ to ensure maximal energy delivery at the lowest risk of PV stenosis (fig 4C). The occurrence of PV stenosis can be monitored by ICE by measuring the ostia of the PV and the systolic and diastolic flow velocities before and after catheter ablation. Both colour and pulse Doppler qualities can be used to demonstrate flow in the PV (fig 4D). Although usually increases in PV flow velocities do not give significant symptoms and most patients seem to return to baseline flow characteristics within three months, cases have been reported of significant late PV stenosis/occlusion requiring PV stenting.¹⁶ In general, however, mild to moderate acute changes in ostial diameters and haemodynamic parameters caused by acute oedema do not seem to have significant implications for the occurrence of late PV stenosis.¹⁷ A recent report demonstrated the utility of intracardiac Doppler assessment of LA contractile function and reservoir function to predict the outcome of PV isolation.¹⁸

Finally, another important reason to consider the use of ICE during ablation procedures in the LA is that, despite anticoagulation, left atrial thrombus may occur during left atrial interventional procedures, usually by attachment of thrombus material to the stable positioned transseptal sheet and/or mapping catheter.¹⁹

Complex atrial flutter ablation

Electrophysiological mapping under guidance of ICE has been able to identify the cavo-tricuspid isthmus, Eustachian ridge, and the terminal crest as anatomical barriers in atrial flutter, which cannot be visualised using fluoroscopy.²⁰ Treatment of atrial flutter is performed by linear ablation at the site of the cavo-tricuspid isthmus, thus creating a line of lesions from the inferior caval vein towards the tricuspid valve annulus (fig 5A). Stable endocardial catheter contact can be confirmed by ICE.²¹ Next to the presence of an atrial flutter circuit based on re-entry around right sided anatomical structures, atrial flutter based on a macro re-entrant circuit surrounding scar tissue can occur—for example, in patients treated for congenital heart disease. In these patients, anomalous cardiac anatomy often complicates the treatment of atrial flutter. Particularly in these patients, identification of intracardiac structures using ICE is useful. Fig 5B demonstrates the cavo-tricuspid isthmus area in a patient with Ebstein’s anomaly. Due to the low insertion of the tricuspid value leaflet, part of the right ventricle is incorporated in the right atrium (length of the “atrialised” ventricle 5.68 cm).

Ablation of ventricular tachycardia

Morphologically altered myocardium—for example, the presence of scar tissue or aneurysms after myocardial infarction, and multiple aneurysms as present in arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C)—is prone to the formation of re-entrant circuits.

The majority of ventricular tachycardias (VTs), however, are secondary to ischaemic heart disease; arrhythmias originate either from extensive scars or ischaemic border zones after infarction. The regions with scar formation are frequently akinetic or dyskinetic (aneurysmatic), which can easily be visualised on ICE (fig 6).

Potential complications of VT ablations include thromboembolic events, valvar damage, perforation, and pericardial tamponade. ICE can be used to guide catheters and for continuous monitoring of complications during VT ablation procedures.^22,23 The ability of phased array ICE catheters to image left sided cardiac structures with the imaging catheter exclusively in the right heart is especially advantageous in patients with dilated ventricles, in order to evaluate the presence of intracardiac thrombus, without the need of arterial or transseptal puncture. Lamberti et al²⁴ have reported the use of ICE as an aid in establishing the position and stability of the ablation electrode in relation to the anatomic localisation of the VT in the LV outflow tract, thus avoiding the application of radiofrequency current to valvar leaflets and coronary arteries. Figure 7A–C demonstrates ablation of VT originating from the RV outflow tract. Furthermore, ICE can provide additional information to currently used imaging techniques with regard to tissue characterisation in the diagnosis of ARVD/C²⁵ (fig 7D). Thus the main value of ICE for guiding VT ablation procedures is threefold: (1) identification of the substrate; (2) monitoring of the position of catheters in relation to cardiac structures, such as valves and coronary arteries; and (3) direct monitoring of procedure related complications.

Other applications

ICE can also be used as a tool to guide diagnostic evaluation of cardiac masses. Using ICE, the extent of the process, its surface and relation to other cardiac structures can be determined. Furthermore, ICE can guide the biopsy needle and monitor acute complications of the biopsy, such as perforation or bleeding.²⁶ Figure 8 demonstrates an example of an extensive intracardiac mass. In this patient, several biopsies were obtained guided by ICE.

Furthermore, initial results on the use of ICE in balloon mitral valvuloplasty,²⁷ LAA exclusion,²⁸ and visualisation of valves in the coronary sinus²⁹ have been reported. It is in the line of expectations that the applications of ICE will expand in accordance with the expanding experience with percutaneous transcatheter procedures.

SUMMARY AND CONCLUSIONS

With the expanding development of percutaneous interventional procedures in the catheterisation laboratory, accurate online identification of intracardiac structures, catheters, and devices is mandatory. At present, the image modality most commonly used in the cath lab is fluoroscopy, which does not allow accurate evaluation of intracardiac structures. Besides TOE, ICE is at present the only imaging modality that can provide this information, combined with haemodynamic information during the procedures. ICE has several clear advantages over the use of TOE, including less patient discomfort and no anaesthesia, thus allowing communication with the patient during the procedure. Recently ICE has been implemented in a growing number of interventional procedures. Advantages of the clinical use of ICE are summarised in table 2.

In conclusion, ICE is a valuable tool for guiding percutaneous interventional procedures, such as transseptal puncture and placement of closure devices. Efficiency is improved in electrophysiological interventional procedures by the ability to identify anatomical structures and integrate this information with electrophysiological information. Furthermore, ICE can be used as a diagnostic tool. Direct detection of intraprocedural complications, such as pericardial effusion and cardiac tamponade, is possible with ICE, allowing immediate intervention. Integration of ICE in procedures is likely to result in reduction of fluoroscopy and procedure times and improved outcome.