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Figure 1 Three-dimensional isochronal maps of the right atrium and ventricle from Patient #4 during sinus rhythm. The top left and right images show right atrial activation (right lateral projection) before and after administration of carbenoxolone, respectively. Atrial activation initiates in the region of the sinus node (red) and propagates in a craniocaudal direction with broad, fairly uniform wavefronts. The area of latest activation is shown in blue. The bottom left and right images show earliest right ventricular activation initiating from multiple sites of breakthrough around the apex (red), activating the ventricular mass with broad wavefronts and latest activation being seen around the tricuspid annulus (blue). A subjective crowding of isochrones can be seen after administration of carbenoxolone, in keeping with the slowing of conduction.