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Figure 1 (A) Shows a picture and a fluoroscopic image of the high-density mapping catheter. Below each figure is a schematic representation indicating the orientation of the catheter spines. Note the marker band on spine-A is between electrodes 1 to 2, and on spine-B between electrodes 2 to 3. (B) Demonstrates vector mapping to identify the earliest site of activation. Shown is the catheter in four distinct locations within the left atrium (LA). Mapping is commenced along the inferior LA where the earliest activation is 54 ms ahead of the coronary sinus (CS). The catheter is moved in the direction of the spine demonstrating the earliest activation (spine-D). In the mid-posterior LA, activation precedes the CS by 62 ms. Again the catheter is moved in the earliest direction (spine-D) to a more cranial location on the LA roof. At this site, activation precedes the CS by 75 ms. At this site, spine-B, which is slightly anterior, is the earliest. Moving the catheter in the direction of spine-B to the anterior-superior LA demonstrates the site with the earliest endocardial activation (90 ms ahead of the CS, which was 40 ms ahead of the P-wave).