Thread by @ZiadIssaMD

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Do you know the mechanism of typical atrial flutter (AFL)?
1/
Typical AFL is a macroreentrant circuit with the activation wavefront rotating clockwise or counterclockwise around the tricuspid annulus and using the CTI as an essential part of the circuit.

2/
Conduction across the CTI is relatively slower than the rest of the atrium (likely due to the anisotropic fiber orientation), which provides the protected zone of relatively slow conduction necessary for the flutter reentry circuit.

3/
Key to the development of typical AFL is formation of a line of transverse conduction block in the RA free wall, which acts as a critical lateral boundary that prevents short-circuiting of the flutter wavefront around the IVC and, hence, extinguishes (see video).

4/
This line of block (as indicated by the presence of double potentials on the map) is usually functional, often related to the normal anisotropic and geometric properties of crista terminalis (note the normal voltage in the region of conduction block in this video).

5/
A run of transitional rhythm (e.g., AFib) of variable duration is often required to create this functional line of block and induce AFL in otherwise normal atria.

6/
The intercaval line of block line can be fixed (e.g., atriotomy scar). Then, antecedent AF may not be necessary to induce AFL. In fact, in this setting, typical AFL is more common than peri-atriotomy macroreentrant AT.

6/
The intercaval line of block line can be fixed (e.g., atriotomy scar). Then, antecedent AF may not be necessary to induce AFL. In fact, in this setting, typical AFL is more common than peri-atriotomy macroreentrant AT.

7/
Wavefront rotation in the RA free wall should not be misinterpreted as a macroreentrant circuit (especially given the inherent annotation pitfalls of some mapping systems and with incomplete maps). Entrainment mapping from the CTI helps in the DDx.

8/
It is important note that the CTI is not the “diseased” tissue that causes the AFL. We ablate the CTI just because it happens to be the easiest target to eliminate typical AFL, and not because it is arrhythmogenic substrate.