Conversion to partial isthmus-dependent flutter during cavotricuspid isthmus ablation: The utility of Advisor HD grid in identifying local fragment electrograms

Abstract

Complete block creation by radiofrequency (RF) ablation at the cavotricuspid isthmus (CTI) is a highly successful procedure for the treatment of typical atrial flutter (AFL). Occasionally, a rare type of AFL, such as lower or upper loop reentry, or partial isthmus-dependent flutter, can coexist with typical right AFL. A 73-year-old man underwent CTI ablation for a clockwise CTI-dependent typical atrial flutter. During the ablation procedure, the morphology of the flutter wave changed in the surface electrocardiogram and endocardial atrial activation sequence, suggesting that the typical AFL had converted to another AFL (AFL2). High-density mapping using the HD grid catheter could not reveal the reentrant circuit of AFL2 but detected a critical conduction gap at the boundary between the inferior vena cava and CTI. There was also an impulse collision in the remaining CTI. The RF application at the gap terminated the AFL2 and completed the block line of the CTI. Based on these findings, AFL2 was comparable with partial isthmus-dependent flutter. The present case demonstrates the utility of high-density mapping with a HD grid for the identification of small amplitude high-frequency electrograms at critical sites of the arrhythmia.

Learning objective

A rare type of atrial flutter (AFL) can coexist with typical AFL. In such cases, a high-density mapping is useful to identify the critical portion of the reentrant circuit. The Advisor HD grid multipolar catheter (Abbott, St Paul, MN, USA) is unique in that it allows bipolar recording perpendicular and parallel to the splines via 16 electrodes. In this case report, high density mapping using HD grid catheter identified small amplitude high-frequency electrograms at critical sites of the arrhythmia.

Introduction

The cavotricuspid isthmus (CTI) is a critical portion of typical atrial flutter (AFL), and creating a bidirectional block at the CTI by catheter ablation has been established as a curative therapy. Occasionally, a rare type of AFL, such as lower or upper loop reentry or partial isthmus-dependent flutter, coexists with typical AFL [1], [2], [3]. In cases where conversion to another AFL occurs, the creation of a new activation map and identification of the critical site of the reentrant circuit with a conventional bipolar catheter requires time and effort. The Advisor HD grid multipolar catheter (Abbott, St Paul, MN, USA) is useful in this issue because of its rapid high-density point acquisition [4]. Here, we report a case in which typical AFL was converted to partial isthmus-dependent AFL during CTI block. The HD grid identified small-amplitude fragment potentials at the conduction gap in the CTI and demonstrated impulse collision at the CTI.

Case report

A 73-year-old man was referred to our institution for catheter ablation for symptomatic sustained AFL. An electrocardiogram showed AFL with an upright flutter wave in leads II, III, and aVF, a negative flutter wave in lead V1, and an AFL cycle length of 236 ms, indicating a clockwise CTI-dependent typical AFL (Fig. 1A). Transthoracic echocardiography showed no structural abnormalities. An electrophysiological study was performed under mild sedation with midazolam. A decapolar electrode deflectable catheter (Inquiry, Abbott) was inserted into the coronary sinus (CS), and a duodecapolar deflectable catheter (Snake, Japan Lifeline Co. Ltd., Tokyo Japan) was placed in contact with the tricuspid annulus (TA). The right atrium (RA) was mapped during AFL using the Advisor HD Grid mapping catheter and the Ensite Velocity 3-dimensional mapping system (Abbott). The activation map revealed a typical clockwise CTI-dependent AFL propagating around the TA (AFL1) (Fig. 1B, Video 1). Therefore, CTI linear ablation was performed using a 3.5-mm irrigated-tip Tacticath ablation catheter (Abbott) (Fig. 2A). Radiofrequency (RF) at a power setting of 35 W and lesion size index (LSI) of ≥5.5 was applied in a point-by-point fashion from the TA toward the inferior vena cava (IVC). During RF application at the boundary between the IVC and CTI (Fig. 2B), both the surface electrocardiogram and endocardial activation sequence along the TA changed (Fig. 2C, D), suggesting that the typical AFL (AFL1) had converted to another AFL (AFL2). At the time of the sequence change, the intervals of the local electrocardiogram in the RA free wall were intermittently prolonged, while those at the CS remained unchanged. A collision of the wavefront in the RA free wall and double potentials were observed at the distal ablation catheter after the sequence change (Fig. 2C). A new activation map in the RA was created using the Advisor HD Grid catheter (Fig. 3A, Video 2).

Fig. 1.

(A) Twelve‑lead electrocardiogram at the beginning of the catheter ablation. Upright flutter waves in leads II, III and aVF and negative flutter wave in lead V1 indicated a clockwise cavotricuspid isthmus-dependent atrial flutter (AFL). (B) Activation map during AFL1.

CS, coronary sinus; IVC, inferior vena cava; SVC, superior vena cava; TA, tricuspid annulus.

Fig. 2.

(A) Fluoroscopic images showing the location of the catheters at the beginning of cavotricuspid isthmus linear ablation. (B) Three-dimensional image showing ablation tags. During RF application at the yellow tag, the AFL1 converted into AFL2. The other tags were ablation sites before conversion. (C) Intracardiac electrograms during conversion. The intervals of the local electrocardiogram in the right atrial (RA) free wall intermittently prolonged, while those at the ostium of the coronary sinus remained unchanged. After the conversion, there was wavefront collision in the RA free wall and double potentials were observed at the distal ablation catheter. (D) Morphologic comparison of flutter waves in 12-lead electrocardiograms during AFL1 and AFL2. ABL, ablation catheter; AFL, atrial flutter; CS, coronary sinus; IVC, inferior vena cava; d, distal; LAO, left atrial oblique view; p, proximal; RAO, right atrial oblique view; TA, tricuspid annulus.

Fig. 3.

(A) Activation map in the right atrium during atrial flutter (AFL) 2. The activation slowly passed the boundary between the inferior vena cava (IVC) and cavotricuspid isthmus (CTI) from the septum to lateral wall; meanwhile, there was an impulse collision in the remaining CTI. Small-amplitude and fractionated potentials were recorded at the boundary site between the IVC and CTI. The yellow dotted line indicates conduction disturbances corresponding to linear radiofrequency lesions made before the AFL conversion. In the free wall, there was a collision of two activations: one was craniocaudal in direction in the upper part and the other was caudocranial in the lower part. (B) Intracardiac electrograms at the termination of AFL2. (C) Ablation tags at the termination of AFL2. The radiofrequency application at the green tag, indicated in white, allowed termination of AFL2.

ABL, ablation catheter; CS, coronary sinus; IVC, inferior vena cava; RAA, right atrial appendage; RF, radiofrequency; TA, tricuspid annulus.

The activation slowly passed the boundary site between the IVC and CTI from the septum to the lateral wall; meanwhile, there was an impulse collision in the remaining CTI. After passing through the CTI, the activation propagated in an ascending direction in the lateral posterior wall. In the free wall, there was a collision of two activations: one was in the craniocaudal direction in the upper part and the other was in the caudocranial direction in the lower part.

Small-amplitude fractionated potentials were recorded at the boundary between the IVC and the CTI (Fig. 3A). However, the fragment potentials were not recorded with a Tacticath ablation catheter. RF at a power setting of 35 W, targeting the fragment potentials, was delivered with reference to the location of the fragment potentials recorded by the HD grid, which resulted in the termination of AFL2 (Fig. 3B, C). The RF delivery was continued until the LSI reached 5.5. A three-dimensional map during CS ostium pacing revealed completion of the CTI block line. No further AFLs were induced by the pacing maneuvers, even with continuous isoproterenol infusion. The patient remained free from any atrial tachyarrhythmia without any-arrhythmic drugs for two years.

Discussion

In the present case, the reentrant circuit of AFL2 was not clear for the following two reasons. First, it was difficult to determine from the activation map. Second, we did not apply entrainment mapping or measure the post-pacing interval to avoid tachycardia termination or conversion [5]. Due to the lack of entrainment pacing, focal atrial tachycardia from the CTI cannot be excluded [6].

The local electrogram intervals in the RA free wall were intermittently prolonged, while those at the CS remained unchanged during conversion from AFL1 to 2. The wavefronts collided in the RA free wall after conversion. These findings excluded the participation of the RA free wall in the reentrant circuit of AFL2.

During AFL2, there was an impulse collision at the isthmus of both the orthodromic clockwise wavefront and another front returning from the lateral isthmus. RF application at the residual conduction gap on the boundary between the IVC and CTI, which likely corresponded to the subeustachian isthmus, terminated the AFL2 and no AFLs were induced after the completion of the CTI block. These findings are compatible with the characteristics of a partial isthmus-dependent flutter [3], [7]. It has been reported that partial isthmus-dependent flutters include the subeustachian isthmus in its reentrant circuit and occasionally coexists with common AFL. A previous report has suggested that an epicardial pathway can be involved in the reentrant circuit of partial isthmus-dependent flutters [7]. In addition, the involvement of the epicardial pathway might be one of the reasons why the reentrant circuit of AFL2 was not determined in this case.

The HD grid is unique in that it allows bipolar recording perpendicular and parallel to the splines via 16 electrodes [4]. Its unique electrode layout enables rapid high-density point acquisition. In the present case, the HD grid detected small-amplitude fragment potentials at the conduction gap at the boundary between the CTI and IVC, which the ablation catheter could not detect. Although the reentrant circuit of AFL2 was not clear, the HD grid detected the critical slow conduction site, leading to successful ablation.

Conclusions

A typical AFL was converted to a partial isthmus-dependent AFL during the CTI block. High-density mapping using the HD grid catheter identified the fragment potential at the boundary between the IVC and CTI and demonstrated impulse collision at the remaining CTI. In cases with conversion to reentrant tachycardia, the HD grid is considered useful for creating an activation map rapidly and detecting critical sites in the reentrant circuit.

The following are the supplementary data related to this article.

Video 1

Propagation map of atrial flutter (AFL) 1.

Video 2

Propagation map of atrial flutter (AFL) 2.

Declaration of competing interest

None.