Confirmation of Bidirectional Block
Lateral mitral isthmus block is suspected in the presence of widely split double potentials of fixed separation recorded along the length of the ablation line during pacing from the distal CS electrode. Differential pacing is then performed with the ablation or multipolar catheter placed in the LAA. In the presence of lateral mitral isthmus block, pacing from the LAA should result in the atrial electrogram being recorded earlier on the His electrode than the CS proximal electrode followed by counter-clockwise activation around the mitral annulus with proximal to distal CS activation (see Figure 5). The activation time should be identical when pacing in the reverse direction from the CS distal electrode to the catheter in the LAA. An activation map may then be completed immediately superior to the line to confirm latest activation immediately adjacent to the line. Finally, the stimulus-to-electrogram interval in the catheter placed in the LAA should be longer with pacing from the distal CS electrode than with pacing from a more proximal CS electrode.
To confirm anterior mitral line block , differential pacing manoeuvres may be performed. However, distinguishing conduction delay from complete block can be more challenging due to the length of the line. Widely split local double potentials of a fixed separation recorded by the ablation catheter would be expected along the length of the ablation line during pacing from a multipolar mapping catheter placed lateral to the line, typically in the LAA. Testing is then performed with the ablation catheter placed septal to the line. Pacing from the ablation catheter should result in a long activation time to the LAA. This activation time should be identical when pacing in the reverse direction from the LAA and recording from the ablation catheter septal to the line. When pacing from the LAA, there should be clockwise activation around the mitral valve annulus with distal to proximal CS activation. Conversely, when pacing from the ablation catheter, there should be counter-clockwise activation around the mitral valve annulus with proximal to distal CS activation. During pacing from the ablation catheter septal to the line, an activation map can be performed using the multipolar catheter to confirm lateral to septal activation with the latest electrograms recorded immediately adjacent to the line. It is very important to pace from two sites septal to the long anterior line, typically higher up adjacent to the right superior PV and lower down nearer to the mitral annulus. Conduction block may be falsely assumed if pacing a considerable distance from the site of breakthrough in the line.