Characteristics of virtual unipolar electrograms for detecting isthmus block during radiofrequency ablation of typical atrial flutter

Patient characteristics

The study population consisted of 52 patients (mean age, 62 ± 12 years; males = 37, females = 15) with clinically documented AFL referred for RFA. Eleven patients had structural heart disease, including six with hypertension, two with ischemic heart disease, two with valvular heart diseases, and one with congenital heart disease.

Catheter position and electrophysiologic study

Informed, written consent was obtained from all patients. As described previously, all antiarrhythmic drugs were discontinued for at least five half-lives before the study (8,15). In all patients, a 7F, 20-pole, deflectable halo catheter with 10-mm paired spacing (Cordis-Webster Inc., Baldwin Park, California) was positioned around the tricuspid annulus to record the RA activation at the lateral wall and the low RA isthmus simultaneously. A 7F, deflectable decapolar catheter with 2-mm interelectrode distance and 5-mm space between each electrode pair was inserted into the coronary sinus (CS) via the internal jugular vein. A 9F sheath was introduced to place the noncontact mapping electrode array in the RA by the femoral vein approach. If spontaneous AFL was found at the onset of study, multicatheter mapping and noncontact mapping were performed simultaneously to investigate the re-entry circuit. If sinus rhythm was present at baseline, atrial pacing from the CS and low RA was performed to induce AFL.

Noncontact mapping system

The noncontact mapping system (Ensite 3000, Endocardial Solutions, St. Paul, Minnesota) has been described in detail previously (15). In brief, the system consists of a noncontact catheter (9F) with a multielectrode array (MEA) surrounding a 7.5-ml balloon mounted at the distal end. Raw data detected by the MEA is transferred to a silicon graphics workstation via a digitalized amplifier system. Before the MEA catheter was deployed, patients were given 3,000 U of heparin, with subsequent boluses to maintain activated clotting time between 250 and 300 s. The MEA catheter was deployed over a 0.032-inch guidewire, which had been advanced to the superior vena cava. The system locates any catheter in relation to the MEA using an impedance-based “locator signal,” that serves two purposes. First, it is used to construct a three-dimensional computer model of the virtual endocardium providing a geometry matrix for the inverse solution. To achieve this, geometric points are sampled at the beginning of the study during sinus rhythm or AFL. Second, the locator signal is also used to display and track the position of the catheter on the virtual endocardium and allows the marking of the catheter on the virtual endocardium and allows the marking of anatomic locations identified using fluoroscopy and electrogram characteristics.

Using mathematical techniques to process these potentials, the system is able to reconstruct more than 3,000 unipolar electrograms simultaneously and superimpose them onto the virtual endocardium, producing isopotential maps with a color range representing voltage amplitude. Reconstructed unipolar electrograms can also be judiciously selected from sites on the virtual endocardium and displayed individually.

RFA and follow-up

Radiofrequency energy was delivered using a quadripolar 8-mm tip electrode ablation catheter with temperature control (7F, EP Technologies Inc., Mountain View, California) to achieve a tip-tissue interface temperature of 70°C during AFL or sinus rhythm. The preset duration of each radiofrequency pulse was 120 s for each pulse. Radiofrequency energy was applied during pull-back of the ablation catheter from the right ventricle toward the inferior vena cava (IVC) to create linear lesions. After successful ablation of AFL, the electrophysiologic study was repeated to ensure complete bidirectional isthmus block using both contact (halo catheter) and noncontact mapping.

The complete clockwise CTI block was confirmed by the observation of a purely descending wavefront at the lateral wall down to the CTI during pacing from proximal CS (7,8). An incomplete clockwise CTI block was defined when a descending wavefront at the lateral wall still allowed the lateral part of the isthmus to be activated from the proximal CS, resulting in displacement of collision at the lower part of lateral RA (7,8). Positional pacing and noncontact mapping were performed to confirm complete CTI block with transcristal conduction (10).

After discharge, each patient was regularly evaluated in the clinics with history and physical examination. If patients experienced symptoms suggestive of tachycardia, 24-h Holter monitoring or cardiac event recorder was performed to define the causes of clinical symptoms. If patients had recurrent typical AFL, RFA might be repeated.

Definitions of unipolar and bipolar electrograms

The following definitions of unipolar waveforms were used:

• RS: one positive deflection followed by a negative deflection with an R/S ratio = 1.

• rS: one small positive deflection followed by a large negative deflection with r/S <1.

• Rs: one large positive deflection followed by a small negative deflection with R/s >1.

• QS: only one large negative deflection.

• R: only one large positive deflection.

• rSR: one small positive deflection followed by a negative deflection and then positive deflection.

Statistical analysis

Quantitative values are expressed as mean ± SD. Statistical comparison of age and flutter cycle length between the three groups was performed with the Kruskal-Wallis one-way analysis of variance test. A value of p < 0.05 was considered to be statistically significant.

Electrophysiologic characteristics before RFA

There were 9 patients with spontaneous AFL and 43 patients with inducible AFL at baseline. The mean AFL cycle length was 235 ± 31 ms (range, 187 to 300 ms). Noncontact mapping and halo catheter mapping showed counterclockwise AFL in 39 patients and clockwise AFL in 13 patients. Atrial pacing terminated tachycardia in all nine patients with spontaneous AFL. During CS pacing, noncontact mapping showed that one wavefront traveled through the CTI and the other wavefront propagated upward in the septum and downward in the free wall. Both wavefronts collided in the anterolateral wall. The virtual unipolar electrograms in the CTI showed an RS (n = 9), rS (n = 41), or QS (n = 2) pattern.

Electrophysiologic characteristics after RFA

After a mean of 5 ± 3 radiofrequency pulses, 37 patients had complete bidirectional CTI block (group I), 12 patients had incomplete CTI block (group II), and 3 patients had complete bidirectional CTI block with transverse conduction in the CT mimicking CTI conduction (group III). There was no significant difference in age (p = 0.6) and AFL cycle length (p = 0.36) between the three groups.

Group I (n = 37)

After RFA, double potentials were found at the ablation line with a mean interval of 103 ± 16 ms (range, 82 to 120 ms) during CS pacing. The second component of double potentials showed an R (n = 30) or Rs (n = 7) pattern. The mean R/s ratio was 4.7 ± 0.6 (range, 4.2 to 5.9). Noncontact mapping demonstrated that one wavefront blocked in the CTI and the other wavefront propagated upward in the septal wall, over the top of the CT and downward in the anterolateral wall (Fig. 1).

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Figure 1

Complete cavotricuspid isthmus block during coronary sinus (CS) pacing after ablation. (A)After ablation, the activation wavefront originated in the CS ostium region blocked at the ablation line and propagated upward the septal wall. (B)The activation wavefront activated the roof and crossed the top of the crista terminalis. (C)The activation wavefront activated the high anterolateral wall and propagated downward. (D)The activation wavefront activated the low anterolateral wall and ended at the ablation line. The second component of virtual electrograms (virtual 6 to 10) at the ablation line showed an Rs pattern.

Group II (n = 12)

After RFA, double potentials were also found at the ablation line with a mean interval of 43 ± 7 ms (range, 34 to 55 ms) during CS pacing. The second component of double potentials showed an rS pattern in all 12 patients. The mean r/S ratio was 0.4 ± 0.2 (range, 0.2 to 0.7). Noncontact mapping demonstrated that one wavefront traveled slowly through the CTI conduction gap and the other wavefront propagated upward in the septal wall and downward in the free wall. Both wavefronts collided in the low anterolateral wall (Fig. 2).

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Figure 2

Incomplete cavotricuspid isthmus (CTI) block during coronary sinus pacing after ablation. (A)The activation wavefront originated in the coronary sinus ostium region with splitting of two wavefronts. (B)One activation wavefront propagated upward in the anterior septal wall to the roof, and the other wavefront proceeded downward in the posteroseptal wall to the CTI. (C)The upper wavefront crossed over the crista terminalis, and the lower wavefront traveled through the CTI. (D)Both wavefronts collided in the low anterolateral wall. The second component of virtual electrograms (virtual 7 to 11) showed an rS pattern.

Group III (n = 3)

After RFA, double potentials were found at the ablation line with a mean interval of 41 ± 10 ms (range, 29 to 52 ms) during CS pacing. The second component of double potentials showed an rSR pattern in all three patients. Noncontact mapping demonstrated that one wavefront blocked in the CTI, the other wavefront propagated through a conduction gap in the CT, and another wavefront went upward in the septal wall and downward in the free wall. These sequential approaching wavefronts produced an rSR pattern in the second component at ablation site in three patients (Fig. 3).

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Figure 3

Complete cavotricuspid isthmus (CTI) block and transcristal conduction during coronary sinus pacing after ablation. (A)The activation wavefront originated in the coronary sinus ostium (CSO) region and split into two wavefronts. (B)One wavefront propagated upward in the septal wall to the roof, and the other wavefront crossed the gap of the crista terminalis. (C)The upper wavefront crossed over the top of crista terminalis, and the lower wavefront activated the CTI opposite the pacing site. (D)The upper wavefront propagated downward in the anterolateral wall. The second component of virtual electrograms (virtual 10 to 12) showed an rSR pattern.

Major findings

This study demonstrated the usefulness of virtual unipolar electrograms (second component of double potentials) at the ablation line for detecting CTI block after RFA of typical AFL. During CS pacing, patients with complete CTI block had an R or Rs pattern. Patients with an incomplete CTI block had an rS pattern. Patients with a complete CTI block and transcristal conduction had an rSR pattern.

Criteria of complete CTI conduction block

Because creation of complete CTI block can prevent recurrence of typical AFL, several mapping techniques have been proposed to demonstrate this criterion after RFA of the CTI (7–12). Initially, the activation mapping technique was used to show complete craniocaudal activation pattern of the opposite wall to the pacing site (7,8). Subsequently, complete CTI block was considered when a complete corridor of parallel double potentials from the right ventricle to the IVC was achieved (11). However, the presence of a very slow, persistent conduction through the ablation line can be misdiagnosed by both techniques; thus, differential pacing techniques must be used to allow determination of true CTI conduction block (16).

Recently, unipolar electrograms were demonstrated to be useful for determining the CTI block (12). Spach et al. (17,18)have shown that conduction along the longitudinal axis of the myocardial fibers produces RS or rS unipolar electrograms, and uniphasic unipolar R electrograms develop during termination of the activation wavefront. In this study, pacing from the CS before RFA resulted in RS or rS unipolar electrograms in the CTI. After RFA with complete CTI block (group I), the paced wavefront propagated around the TA to the other side of the ablation line. Therefore, the second component of double potentials at the ablation line showed an R or Rs pattern representing the wavefront termination at the ablation line. In contrast, during incomplete CTI block (group II), the wavefront can pass though the residual gap in the CTI; thus, the second component of double potentials can show an rS pattern representing an approaching and leaving activation wavefront.

CTI block with transcristal conduction

The CT has been demonstrated to be a posterior barrier in typical AFL (2–4). However, the presence of transcristal conduction during CS pacing was found by Arenal et al. (9)and Schumacher et al. (19). Using a basket catheter in the IVC orifice, Scaglione et al. (20)also found that 3 of 12 patients had a mimicking conduction through the CTI during CS pacing, which was due to conduction along the posterior part of IVC orifice. In this study, three patients showed complete CTI block with transcristal conduction during CS pacing. The second component of unipolar double potentials at the ablation line showed a characteristic rSR pattern. This unique finding has not been reported before. The rS deflection reflected the close-by activation wavefront due to transcristal conduction, and R wave represented the activation front propagating around the tricuspid annulus.

Conclusions

The study demonstrated the characteristics of virtual unipolar electrograms at the ablation line for detecting complete CTI block after RFA of typical AFL. A predominant R-wave pattern of the second component of unipolar double potentials at the ablation indicates complete CTI block, even in the presence of transcristal conduction.