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.