In our experience, the most challenging left atrial linear lesion is connecting the mitral annulus to the ostium of the left inferior PV, and it may be extremely difficult in some patients to achieve a complete and transmural lesion. The aim of the current study was to evaluate the role of ICE to guide the deployment of this linear lesion in patients with both paroxysmal and chronic AF. Different hypotheses were tested: 1) The difficulties in creating the left isthmus block were related to tissue thickness and as a consequence, ICE could be used to guide placement of the ablation line on the most vulnerable (thinnest) area; and 2) A poor contact or a specific orientation of the ablation electrode with the tissue was responsible for incomplete linear lesions. ICE could be used to see and adjust the quality of contact or catheter orientation and therefore assist in creating complete linear lesions. Patient Characteristics. The study population consisted of 13 patients referred for catheter ablation of symptomatic drug refractory paroxysmal (8 patients, or 61%) or chronic (5 patients, or 39%) AF in which an irrigated-tip catheter was used both for PV isolation and cavotricuspid isthmus ablation. Twelve patients were male (92%), and their mean age was 55 years ± 8 years. They had a history of symptomatic AF for a mean of 7.0 years ± 5.8 years, despite antiarrhythmic treatment with 4.0 ± 1.5 drugs, including amiodarone in 9 (68%). Three patients had a structural heart disease including ischemic heart disease. The hospital ethics committee approved the study and written informed consent was obtained from all patients. Patient Preparation. Patients were anticoagulated orally for at least 1 month before the procedure, and a transesophageal echocardiogram was performed in all to rule out left atrial thrombus. The procedure was performed in the fasting state under light sedation (midazolam 2 to 10 mg) and analgesia (nalbuphine 10 to 20 mg). Mapping and Ablation. Three electrode catheters were introduced percutaneously through the right femoral vein. A 6 French (Fr) quadripolar catheter (X-trem, ELA Medical, Montrouge, France) was placed in the coronary sinus (CS) and used for LA pacing and recording. A circumferential decapolar catheter (LASSO, Biosense Webster, Diamond Bar, California) and a 4 mm irrigated-tip ablation catheter (Celsius Thermo-cool, Biosense Webster) were used for mapping and ablation of PV and linear lesions. The two catheters were introduced to the LA via a single transseptal catheterization (Daig Corporation, Minnetonka, Minnesota/Preface multipurpose, Biosense Webster) or a patent foramen ovale. A bolus of 50 IU of heparin/kg of body weight was then infused. Systematic isolation of the four PV was performed at the ostia of the veins as previously described using a Cordis-Stockert generator and controlled by a temperature setting of 50 °C with a power limit of 30W. Linear ablation was performed in the LA using a lateral mitral isthmus line between the LIPV ostium and lateral mitral annulus. A flow rate of 20 to 50 ml/minute (normal saline) was used for linear ablation in order to deliver 50W to 60W while keeping the temperature of the ablation electrode lower than 50 °C. This lesion was performed at a mean of 2 cm below the ostium of the appendage, the ablation electrode being parallel to the tissue for the first eight patients. Then, based on the first results of the ICE study and on the poor contact and limited efficacy of some of the lesions, the position of the ablation electrode was changed. The linear lesion was performed higher, at the ostium of the appendage, with an ablation electrode perpendicular to tissue in order to maximize contact with the tissue and the stability of the catheter. Intracardiac Echocardiographic Study Methods and Results. An ICE* 10 Fr catheter was used with a 5.0-10.0 MHz frequency range to image the LA. (Acuson, AcuNav diagnostic ultrasound catheter, Siemens Medical Solutions). The aims and results of the ICE imaging are described below: 1. Atrial thickness. Assess the thickness of the atrial tissue of the left atrial isthmus at different levels above the CS. Three measures were recorded at 2 cm, 3 cm, and 4 cm below the ostium of the left atrial appendage (LAA). The irrigated tip ablation catheter was placed in the CS with a flow rate of 20 cc/minute to clearly define this structure and therefore the roof of the CS and the atrial thickness of the tissue. The atrial thickness measured from the superior part of the CS to the endocardial aspect of the LA was found to consistently increase from the appendage to the ostium of the CS. At 2 cm from the appendage, it was a mean of 3.4 mm ± 1 mm. At 3 and 4 cm, it was measured at 3.6 mm ± 1 mm and 4.4 mm ± 1 mm, respectively. A mean of 19 minutes ± 10 minutes of radiofrequency (RF) energy delivery were delivered to create a complete isthmus block. In addition, in 2 patients, two and six minutes of RF energy were delivered in the CS to achieve a complete block. In one patient, despite an atrial thickness of 4 mm, we failed to create a complete line of block. In addition, instability of the ablation electrode was noted with loss of tissue contact during each atrial systole. 2. Atrial thickness at the ablation site. The atrial thickness was measured again after the ablation of the left atrial isthmus to look for changes following radiofrequency energy delivery. At the ablation site, the atrial thickness ranged from 2.7-6 mm, with a median of 3.4 mm. 3. Catheter position. The ICE catheter was also used to assess the characteristics of the orientation of the ablation electrode, parallel or perpendicular, and the quality of the contact with the atrial tissue along the left isthmus line. As a consequence, the ablation strategy was modified to ensure catheter stability and tissue contact in the last 5 patients in whom the ablation electrode was kept perpendicular to the tissue. While testing this second hypothesis, no failure was noted. A mean of 15 minutes ± 7 minutes of RF energy was delivered to the endocardium, and in one patient, 2.5 minutes of RF energy was also delivered in the CS to complete the block. 4. Distance of the ablation electrode to the circumflex artery. The distance of the ablation electrode to the circumflex coronary artery and the diameter of the artery were also evaluated before and after ablation in the last five patients. The distance between the circumflex artery and the ablation site at the base of the appendage ranged from 2.3-10 mm with a mean of 5 mm ± 3 mm. The diameter of the artery was 3 mm ± 0.4 mm before and 3.1 ± 0.4 mm after the completion of the linear lesion. This report outlines the utility of ICE in guiding the deployment of linear lesions for the treatment of AF using an irrigated-tip ablation catheter. Linear lesions were initially used to treat AF with the aim of replicating the Maze procedure. In the initial reports, conventional radiofrequency catheters were used and as a consequence, most of the lesions were incomplete. In addition, the procedures were extremely long with a poor safety profile. Both efficacy and safety were significantly improved by the use of irrigated tip catheters, but the ideal position for the linear lesions was still unknown. In our experience, the most attractive position was in the lateral left atrium where the activation fronts advancing via the Bachmann bundle and the CS collide. A linear lesion connecting the mitral annulus to the left inferior PV has the advantage of creating a long line of block extending to the ostium of the left superior PV either spontaneously or by RF energy exclusion of the left veins. This results in a very significant obstacle for AF maintenance, but does not disrupt the activation in sinus rhythm. The use of the ICE visualization was found to be critical to understand why this linear lesion was so difficult to achieve in some patients. The first hypothesis that the thickness of the atrial tissue was too great to allow the creation of a complete line of block was ruled out by the measurements we made. Irrigated tip catheters have been shown to achieve lesions 6-8 mm in depth, as long as good tissue contact is maintained. We observed that when the linear lesion was deployed 2-3 cm below the base of the appendage, the stability and the quality of tissue contact was not optimal in some patients. Accordingly, we modified the position of the linear lesion, which was then placed at the base of the appendage, with the catheter tip being more perpendicular than parallel with the tissue. As a consequence, the lesions were probably more consistent and coalescent resulting more frequently in complete lines of block thus supporting the second hypothesis about the usefulness of ICE to see and adjust the quality of contact or catheter orientation of the ablation electrode to the tissue surface. This study is very preliminary and describes our initial observations with ICE. A larger number of patients have to be included to confirm these initial observations and to determine the true safety of placing the linear lesion at the base of the appendage. The circumflex artery is only 5 mm from the ablation site but it is probably protected by blood flow. In conclusion, using the ICE system, we have identified the cause of incomplete left atrial isthmus ablation observed in some patients. As a consequence, the positioning of the linear lesion was modified, resulting in better efficacy.