2.3. ICAN reentry detection algorithm

We developed an ICAN that locates a repeating‐pattern AF driver using a conventional circular catheter without the need for full mapping of the entire atria in advance. In general, the algorithm detects repeating‐pattern reentrant activity when either of two conditions is met. In one case, the time for all bipoles to be excited is prolonged and approaches the cycle length, suggesting that the catheter is on top of a reentry core. The other scenario finds anatomical macroreentry through detection of regions like scars that have low voltage or other electrogram abnormalities. When neither of these conditions is met, the algorithm suggests movement of the catheter in the direction toward the earliest activated bipole, and the process is repeated.

To carry out these general principles and locate drivers of reentry, the algorithm determines, principal wave direction (PWD), conduction delay ratio (CDR), and anatomic reentry pattern (ARP) from the bipolar electrograms at every catheter placement.

The PWD (black arrow in Figure ​Figure2A)2A) is determined as a vector starting from the catheter center and pointing to the earliest activated bipole (bipole 6 in Figure ​Figure2).2). The PWD of a 1‐second electrogram recording is then determined as a vector with an angle equal to the average PWD angles of all the cycles within the recording in reference to the catheter center.

The CDR is calculated as the average ratio of the total conduction delay to CL for all the cycles. Starting from the earliest activated bipole, the total conduction delay is determined as the sum of the time differences between the local activations of a bipole and its following counter‐clockwise bipole, excluding the bipole just clockwise from the earliest activated bipole (see Figure ​Figure22C).

The ARP is calculated as 2π minus 0.2π (the angle between a bipole pair) times the number of low‐voltage or long‐double‐potential bipole electrograms from the current and prior catheter placements that are within 1.5 catheter radius of the average location of the catheter placements.

ICAN guides a catheter from initial arbitrary placement on the atrial endocardial surface towards a reentry core. The iterative process has two major steps. Step one checks the two reentry detection criteria and determines if at the current location the catheter encircles the core of a reentry source. The two reentry criteria are based on CDR and ARP.

The CDR‐based criterion is satisfied when two close catheter placements (within 1 cm) have a CDR ≥ Th and the average location of the two placements is detected as a reentry core. This criterion will be satisfied when the catheter encompasses a reentry core (Figure ​(Figure3A)3A) with a circulatory excitation through the bipoles as the reentry wavefront completes one cycle.23 We allow a 40% variation from a perfect circulatory excitation (CDR≈1) and select a fixed value of 0.6 for Th in this study.

Simple diagram of reentry and corresponding electrograms of a circular catheter. A, Sequential activations over bipole electrograms (CDR ≈ 1) as catheter fully encompasses the reentry core. There are some deviations to the sequential activations (CRD < 1) when the catheter partially encompasses the reentry trajectory. B, There is a significant change in the sequential activations when a macroreentry anchors to an anatomical barrier such as scar. The colors are as described in Figure ​Figure2.2. CDR, conduction delay ratio

The ARP‐based criterion is satisfied when ARP≤0.2π and the average location of the catheter placements that triggered this criterion is determined as a reentry core. This criterion detects cases of a reentry where a reentry core is anchored to anatomical barriers in the form of a patchy myocardial scar or fibrosis and forms a macroreentry AF source24, 25 (Figure ​(Figure33B).

If a reentry is not detected, the algorithm proceeds to Step two, where the algorithm shifts the catheter center by catheter radius in the direction of the PWD. At the new placement, step one is repeated until a reentry source is localized or a maximum number of placements (25 in this study) is reached.

For visualization purposes, an electrogram voltage map is constructed step by step as the catheter is navigated on the tissue. We use the natural neighbor interpolation technique26 to interpolate the peak‐to‐peak voltage of the bipolar electrograms on an interpolation grid of 0.25 mm spacing and interpolation area with a radius of 15 mm (diameter of the catheter).