Diagnosis and Mapping
In the setting of either previous ablation lines or atrial scar, the utility of the 12-lead ECG in the diagnosis of peri-mitral flutter is limited. If the patient arrives in the laboratory in sinus rhythm, we attempt to induce atrial flutter with burst atrial pacing and programmed extrastimuli from the CS catheter. Induction of flutter can be challenging and is often preceded by short periods of AF. The CS activation pattern is usually the first clue to localise the AT. Proximal to distal CS activation is expected with counter-clockwise peri-mitral flutter but also seen with cavotricuspid isthmus-dependent flutter, right ATs and tachycardias that originate from the right PVs or interatrial septum. Distal to proximal activation occurs with clockwise peri-mitral re-entry or tachycardias originating locally from the lateral LA. Entrainment is the most important diagnostic manoeuvre and is more rapid and accurate than any form of 3-dimensional (3D) mapping. A post-pacing interval minus tachycardia cycle length of less than 20 milliseconds from the proximal, mid and distal CS electrodes is consistent with peri-mitral flutter and may be confirmed with left atrial mapping (see Figure 3). If not initially performed under general anaesthetic, this is initiated prior to transseptal access. We would then proceed with double transseptal, transesophageal echocardiogram (TEE) guided access using SL1 sheaths, one of which is subsequently exchanged for a steerable Agilis sheath. Thereafter, intravenous heparin is administered to target an activated clotting time of ≥350 seconds. A high-density multipolar mapping catheter is used to map the LA. Entrainment from the LA roof as well as points on the anterior and posterior walls is needed to exclude the possibility of dual or multi-loop tachycardias. High density 3D electroanatomic mapping is then performed to confirm activation patterns but predominantly to construct a voltage map to guide the ablation strategy.