Background

Many patients undergo transesophageal echocardiography (TEE) to exclude intracardiac thrombus prior to left atrial (LA) ablation or cardioversion. Given the relatively low incidence of thrombus in this population (most of whom are already receiving chronic oral anticoagulation), the vast majority of the TEE studies performed are “negative.”^{(1, 2)} Said differently, if physicians had either a better methodology to determine high-risk patients or an alternative imaging methodology to assess for thrombus, then the majority of preoperative TEE scans currently performed would be rendered unnecessary. It is axiomatic that such a large burden of excess TEE scans in the AF ablation population incurs significant additional cost and patient morbidity.

In the December 2010 issue of Circulation: Arrhythmia and Electrophysiology, Saksena and colleagues presented the results from the Intracardiac Echocardiography Guided Cardioversion Helps Interventional Procedures (ICE-CHIP) study.⁽³⁾ The study was designed to test the hypothesis that phased-array intracardiac echocardiography (ICE) provided similar diagnostic accuracy to TEE in detecting left atrial thrombus and interatrial septal pathology.

A cohort of 95 patients, the majority undergoing left atrial catheter ablation, was imaged with both ICE and TEE using commercially available imaging platforms. Images of the interatrial septum, the LA, and the left atrial appendage (LAA) were compared. The presence of spontaneous echo contrast (SEC), intracardiac thrombus, or interatrial communication was evaluated with each modality. Two blinded investigators reviewed the images and determined whether “concordant” pathologic findings were present on both examinations for a given patient.

The main findings of the study included: 1) concordance (between ICE and TEE) of complete LA imaging in 96%; 2) concordance of complete LAA imaging in 85%; 3) concordant findings of LA and LAA SEC in 65% and 60% respectively; 4) concordant findings of interatrial aneurysm in 96%; and 5) concordant findings of LA and LAA thrombus in 97% and 92% respectively. All patients underwent imaging with both modalities without complications.

Commentary

Phased-array intracardiac echocardiography (ICE) provides a robust imaging platform that includes: variable transducer frequency, versatile imaging depth (up to 15 cm), and full color and spectral Doppler capabilities. With a multi-directional imaging transducer and up to a 110 cm working length, the phased-array ICE catheter provides versatile navigation within any cardiac chamber. Since many operators already use ICE during intracardiac ablation procedures, it seems a logical platform to compare to TEE for the detection of left atrial thrombus.

Due to the inherent impracticality of direct pathologic examination for thrombus in an AF population, TEE remains the clinical gold standard for the assessment of intracardiac thrombosis. The use of computed tomography (CT) to assess for LAA thrombosis has also been described, with one report achieving comparable sensitivity and specificity to TEE; however, the routine use of CT with its attendant contrast and radiation exposure may not be appropriate for some patients.^{(4, 5)}

Important limitations of ICE-CHIP originate from its methodology. This multi-center trial prescribed very little direct information regarding the ICE examination to be performed. In fact, despite the use of imaging “completeness” as the chief outcome variable, there was no a priori method for the operators to obtain a complete examination for either methodology; this issue is reflected in the low utilization of pulse wave Doppler velocity in both the TEE and ICE studies (26% and 5%, respectively). In fact, standard echocardiographic investigations such as color Doppler assessment for patent foramen ovale were omitted in 47% of ICE studies (despite the prescribed ICE imaging plane being located directly adjacent to the interatrial septum). Although the authors acknowledge this limitation in their discussion, it is reasonable to assume that it would have been minimized had a more specific imaging protocol been used.

The most clinically relevant finding in ICE-CHIP is the disparity between the two echo modalities in the detection of intracardiac thrombus, the most important diagnostic question posed for preoperative echo imaging. Only 1/5 (20%) patient found to have “probable” or “definite” LAA thrombus with TEE had a similar finding on ICE. Moreover 3/95 (3%) ICE studies identified mural LA thrombi not seen on TEE; these findings may well be in error given the disagreement with the corresponding TEE studies and the low incidence of non-LAA thrombus in an AF population.⁽⁶⁾ The overall incidence of thrombus was also quite high in this study (6.9% with TEE and 5.8% with ICE), possibly related to imperfect preoperative anticoagulation and/or a high proportion of persistent AF patients in this cohort.

When imaging the LA and LAA the authors obtained right atrial transseptal views at different levels. This technique places the imaging transducer at a considerable distance from the structures being imaged, often more than 4 cm based upon our experience. Overcoming this barrier often requires imaging at lower transducer frequencies, thereby limiting tissue resolution in the echo far field. Given this limitation, it is surprising that the authors were able to achieve complete imaging of the LAA with ICE in 85% of patients. Conversely, a standard mid-esophageal TEE view brings the imaging transducer in close approximation to the posterior LA. Several reports have demonstrated improved resolution of the lateral LA and LAA when imaging with ICE from more proximate structures such as the coronary sinus or pulmonary artery.^(7-9) It is reasonable to anticipate that the ICE and TEE images in ICE-CHIP may have been more similar had different ICE imaging planes been utilized.

The authors also raise the important issue of cost and safety, which have significant implications for the widespread application of ICE. The phased-array ICE catheter has a retail cost of more than $2000, and although a single ICE catheter can be resterilized at a significantly reduced cost, repeated resterilization may significantly compromise the transducer’s capability to produce quality images. Many high-volume EP laboratories have chosen to invest in dedicated ultrasound workstations, which also increase cost.

ICE requires vascular access with at least a 9 French sheath, and incurs the risks of vascular injury, thrombosis, or cardiac perforation. Although these risks are modest for experienced operators, they will no doubt limit the application of ICE as a stand-alone imaging technology. Despite these limitations, the integration of ICE into complex EP procedures, as well as contraindication to TEE imaging in many patients, provide a clear niche for ICE in the evaluation of intracardiac thrombus.

References

1. Mazarelli J, Keane MG, Wiegers S, et al. Incidence of left atrial thrombus by transesophageal echocardiography prior to pulmonary vein isolation procedures for atrial fibrillation. J Am Coll Cardiol. 2007;49:159A.
2. Scherr D, Dalal D, Chilukuri K, et al. Incidence and predictors of left atrial thrombus prior to catheter ablation of atrial fibrillation. J Cardiovasc Electrophysiol. 2009;20:379-84.
3. Saksena S, Sra J, Jordaens L, et al. A prospective comparison of cardiac imaging using intracardiac echocardiography with transesophageal echocardiography in patients with atrial fibrillation: The intracardiac echocardiography guided cardioversion helps interventional procedures study. Circ Arrhythm Electrophysiol. 2010;3:571-7.
4. Tang RB, Dong JZ, Zhang ZQ, et al. Comparison of contrast enhanced 64-slice computed tomography and transesophageal echocardiography in detection of left atrial thrombus in patients with atrial fibrillation. J Interv Card Electrophysiol. 2008;22:199-203.
5. Hur J, Kim YJ, Lee HJ, et al. Left atrial appendage thrombi in stroke patients: Detection with two-phase cardiac CT angiography versus transesophageal echocardiography. Radiology. 2009;251:683-90.
6. Manning WJ, Silverman DI, Gordon SP, Krumholz HM, Douglas PS. Cardioversion from atrial fibrillation without prolonged anticoagulation with use of transesophageal echocardiography to exclude the presence of atrial thrombi. N Engl J Med. 1993;328:750-5.
7. Reddy VY, Neuzil P, Ruskin JN. Intracardiac echocardiographic imaging of the left atrial appendage. Heart Rhythm. 2005;2:1272-3.
8. Hutchinson MD, Jacobson JT, Michele JJ, Silvestry FE, Callans DJ. A comparison of intracardiac and transesophageal echocardiography to detect left atrial appendage thrombus in a swine model. J Interv Card Electrophysiol. 2010;27:3-7.
9. Hutchinson MD, Callans DJ. Imaging the left atrial appendage with intracardiac echocardiography: Leveling the playing field. Circ Arrhythm Electrophysiol. 2010;3:564-5.

Keywords: Aneurysm, Atrial Appendage, Atrial Septum, Catheter Ablation, Coronary Sinus, Echocardiography, Electrophysiology, Pulmonary Artery, Thrombosis, Tomography

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