Use of Intracardiac Echocardiography During Catheter Ablation for Atrial Fibrillation: Maximizing Safety and Efficacy

Success of the procedure is often dependent on mapping and ablating the pulmonary vein antrum, the area proximal to the tubular portion of the pulmonary veins that can encompass a large extent of the posterior wall of the left atrium (Figure 1). However, catheter ablation of atrial fibrillation can place the patient at risk for stroke, cardiac perforation, pulmonary vein stenosis and atrial-esophageal fistula formation. Utilization of techniques and technology to minimize or eliminate these potential risks is vital in order to provide an efficacious technique to a large number of patients safely. AcuNav (Siemens Medical Solutions, Malvern, Pennsylvania) is a 64-element phased array intracardiac echocardiographic technology (ICE) that utilizes a longitudinal side-fire imaging plane and provides very detailed images of various cardiac structures in standard echocardiographic format (Figure 2). The ICE catheter is inserted into a femoral vein and advanced into the right atrium, where 2D and Doppler imaging of the right atrium, intra-atrial septum, left atrium, and pulmonary veins can be performed. The valvular structures of the heart, right and left ventricles, pericardium, and left atrial appendage can also be visualized. ICE allows real-time observation of catheter position, catheter stability, and lesion formation during radiofrequency ablation. Direct visualization assists in the application of appropriate lesion sets, and can also help prevent complications. This article will outline the utility of ICE in catheter ablation of atrial fibrillation. Transseptal Puncture Most techniques for percutaneous catheter ablation require at least one, and some up to three, transseptal punctures. Thrombus formation on sheaths or catheters can occur quickly after transseptal puncture; therefore, aggressive anticoagulation before or immediately after transseptal puncture is critical. Improper transseptal puncture can result in cardiac perforation and tamponade or fistula formation between the right atrium and the ascending aorta. ICE provides direct visualization of the intra-atrial septum, fossa ovalis, and left atrium during transseptal puncture. Tenting of the intra-atrial septum prior to puncture and visualization of contrast/saline injected into the left atrium upon puncture can confirm proper access of the left atrium (Figure 3). In addition, transseptal puncture in the plane of the left pulmonary veins can assure a posterior placement of sheath(s) and catheter(s), allowing for a more direct approach to the pulmonary veins and posterior wall of the left atrium. Pre-Ablation Anatomical Orientation There can be significant variability of pulmonary vein and left atrial anatomy.⁵ Three-dimensional (3D) CT/MRI reconstructions of the pulmonary veins often demonstrate common ostia/pulmonary vein antra or supranumery pulmonary veins. Whereas fluoroscopy and 3D mapping systems cannot consistently reveal these abnormalities, ICE can directly follow catheters as they navigate anatomical variability (Figure 4). Pulmonary vein size and Doppler flow through the pulmonary veins can be measured with ICE. In addition, ICE can visualize the esophagus and estimate its proximity to the pulmonary veins (Figure 5). Confirmation of Catheter Positioning Understanding the orientation and size of pulmonary veins, as well as defining the extent of the pulmonary vein antra, are critical for proper placement of mapping and ablation catheters. Isolation and/or modification of the pulmonary vein antra, often including much of the posterior left atrium, may be necessary for a higher cure rate of atrial fibrillation (Figure 1).² Angiography and 3D mapping systems can misidentify the antrum of the pulmonary vein, which can lead to ablation within the tubular structure of the pulmonary veins. ICE allows for direct visualization of the pulmonary vein antrum and can guide appropriate catheter mapping and ablation (Figures 4 and 6). Direct visualization with ICE also can confirm catheter stability and avoid potential complications. This is particularly important when mapping and ablating between the anterior aspect of the left pulmonary veins and the left atrial appendage. Ablation of this ridge between the left pulmonary veins and the left atrial appendage is critical for isolation of the left pulmonary veins, and mistaking the left atrial appendage for the left superior vein can increase the risk for cardiac perforation. Assessment of Lesion Formation RF ablation causes resistive and conductive heating at the catheter tip that destroys adjacent abnormal tissue.⁶ Monitoring of lesion formation is typically based on electrogram abolition and the continuous measurement of catheter tip temperature, RF impedance and power. However, due to convective cooling (more pronounced with 8- and 10-mm, as well as cooled catheters) overheating of the tissue can still occur despite tight control of power and catheter tip temperature.^7,8 Overheating can result in excessive tissue destruction that increases the risk for pulmonary vein stenosis, thrombus formation, stroke, cardiac perforation, and fistula formation. ICE can assess lesion formation by monitoring for microbubbles (Figure 7).^2,8 Bursts of microbubbles during RF ablation precede abrupt changes in catheter impedance and temperature, which are signs of overheating. Microbubbles appear to be caused by micro steam pops or boiling of tissue during RF ablation. Immediate titration of power can prevent further overheating, excessive damage and char formation on the catheter tip. Although microbubbles are not 100% sensitive or specific in predicting overheating,⁹ when it is observed during RF ablation, this should prompt the operator to decrease or discontinue power delivery in order to minimize complications. Pulmonary vein stenosis. Narrowing of the tubular structure of the pulmonary veins after catheter ablation is a well-known, but often misdiagnosed complication. When excessive energy is delivered within the tubular portion of the pulmonary veins, an aggressive healing response in these relatively narrow structures can result in pulmonary vein stenosis. The occurrence of severe stenosis has been reported to be as high as 15%.¹⁰ Real-time visualization of the tubular structure and antrum of the pulmonary veins with ICE during ablation significantly decreases the risk for pulmonary vein stenosis by avoiding inadvertent migration of the catheter into the tubular portion of the pulmonary veins. Monitoring for microbubbles also prevents overheating, and thereby decreases the risk for pulmonary vein stenosis.¹⁰Stroke. The risk for stroke during catheter ablation of atrial fibrillation has been reported to be as high as 6%.¹¹ The use of ICE with titration of power in response to microbubbles during catheter ablation of atrial fibrillation has been associated with a lower 1% risk of stroke.² ICE/microbubble-guided power titration likely reduces the risk of thrombus and char formation due to overheating. This decreased risk of stroke is supported by a study looking at transcranial Doppler imaging during catheter ablation of atrial fibrillation. Transcranial Doppler imaging can monitor for microembolic showers (MES) to the brain. This study demonstrated that MES did not correlate with RF power, impedance or catheter tip temperature (including parameters previously thought to be well within a safe range), but correlated only to concomitant observation of microbubbles by ICE. Titration of power guided by ICE resulted in significantly fewer MES versus conventional temperature- or power-controlled RF. Two strokes and one TIA occurred during conventional power/temperature-guided RF compared with one TIA during ICE-guided power titration. The number of MES during the three cerebrovascular events that occurred with conventional RF was significantly higher (Andrea Natale, personal communication). Atrial-esophageal fistula. Atrial-esophageal fistula is an extremely serious complication that is felt to occur with extensive RF ablations over the posterior wall of the left atrium.¹² Since the esophagus lies directly behind the posterior wall of the left atrium, overheating of the tissue overlying the esophagus can extend injury into the wall of the esophagus with potential for fistula formation. Although the actual incidence of this complication is not known, of the reported cases, an alarmingly high mortality has been observed. Elevation in esophageal temperatures during catheter ablation of atrial fibrillation have been found to correlate with microbubble formation, but not with power delivery, catheter tip temperature, or measured impedance (Andrea Natale, personal communication). Thus, prevention of overheating with careful ICE-guided power titration is likely to reduce the risk of atrial-esophageal fistula formation. Monitoring for Complications ICE can detect thrombus formation on sheaths, char formation on catheters, and formation of pericardial effusion during the procedure (Figures 8 and 9). Early recognition and management of these complications can prevent significant morbidity and mortality. Thrombus and char can be removed from the left atrium with retraction of the sheath and catheters into the right atrium. Early reversal of anticoagulation and immediate pericardiocentesis can stabilize an enlarging pericardial effusion. Improved Efficacy Variable success rates have been reported for catheter ablation for atrial fibrillation.^1-4 Use of ICE to confirm appropriate and complete placement of lesions in the pulmonary vein antra to isolate the pulmonary veins and microbubble-guided power titration has significantly improved the success rates of catheter ablation for atrial fibrillation (Figure 10).² The success rates can exceed 90%, even in patients with persistent and permanent atrial fibrillation as well as in those with structural heart disease.^2,13Conclusion Catheter ablation can cure patients of atrial fibrillation with good success rates. However, extensive ablations and longer times spent in the left atrium can place the patient at risk for serious complications. Careful placement of lesions in the appropriate areas while preventing overheating are critical to minimizing complications. Use of phased array ICE (AcuNav) to directly visualize left atrial and pulmonary vein anatomy during ablation not only increases safety, but can also increase the efficacy and speed of the procedure. ICE can complement any mapping technique or lesion set, and has been shown to be a vital technology in the ablation of atrial fibrillation for many high-volume centers.