Activation proceeds from red to blue. *Site of pacing. (A) In this patient from Group D (aortic stenosis), activation proceeds around the end of a line of block and then activates the posterior left atrium in a superior direction. Note the electrograms at sites a to c showing fractionated electrograms along this line. (B) In this patient from Group C (mitral regurgitation), activation breaks through a line of slow conduction and then activates the posterior left atrium in both superior and inferior directions. (C) In this patient from Group B (left ventricular failure/coronary artery bypass graft), on the left side of the plaque, there is activation away from the pacing site up until the line of block. On the right side of the plaque activation is in a superior direction. Note the presence of double potentials along the line of block.

Activation proceeds from red to blue. (A) Activation pattern during sinus rhythm. Note uniform conduction occurs from septal to lateral aspect of the posterior left atrium. (B) Pacing from the septal inferior corner of the posterior left atrium. *Site of pacing. Note that activation occurs around a line of block and then superiorly up the posterior left atrium. (C) Pacing from the high posterior left atrium. (D) Pacing from the lateral inferior corner of the posterior left atrium. Note uniform conduction superiorly, however, as the wave front passes horizontally it traverses a line of slow conduction. These figures show the consistent location of the line on the posterior left atrium.

The left panel shows the electrodes on the plaque that were fractionated (black) during pacing in a Group C (mitral regurgitation) patient. The right panel shows isochronal crowding at the site of the line of slow conduction seen during pacing from the septal inferior corner of the plaque. *Site of pacing. Note the site of fractionated electrograms matches the site of slow conduction during pacing.