Contact with cardiac tissue is a determinant of lesion efficacy during atrial fibrillation (AF) ablation. The Sensei®X Robotic Catheter System (Hansen Medical, CA) has been validated for contact force sensing. The electrical coupling index (ECI) from the EnSite Contact™ system (St. Jude Medical, MN) has been validated as an indicator of tissue contact. We aimed at analyzing ECI behavior during radiofrequency (RF) pulses maintaining a stable contact through the robotic navigation contact system.
In 15 patients (age, 59 ± 12) undergoing AF ablation, pulmonary vein (PV) isolation was guided by the Sensei®X System, employing the Contact™ catheter.
During the procedure, we assessed ECI changes associated with adequate contact based on the IntelliSense® force-sensing technology (Hansen Medical, CA. Baseline contact (27 ± 8 g/cm2) ECI value was 99 ± 13, whereas ECI values in a noncontact site (0 g/cm2) and in a light contact site (1–10 g/cm2) were respectively 66 ± 12 and 77 ± 10 (p < 0.0001). Baseline contact ECI values were not different depending on AF presentation (paroxysmal AF, 98 ± 9; persistent AF, 100 ± 9) or on cardiac rhythm (sinus rhythm, 97 ± 7; AF,101 ± 10). In all PVs, ECI was significantly reduced during and after ablation (ECI during RF, 56 ± 15; ECI after RF, 72 ± 16; p < 0.001). A mean reduction of 32.2 % during RF delivery and 25.4 % immediately after RF discontinuation compared with baseline ECI was observed.
Successful PV isolation is associated with a significant decrease in ECI of at least 20 %. This may be used as a surrogate marker of effective lesion in AF ablation.
Radiofrequency (RF) catheter ablation represents a widely accepted strategy in the treatment of atrial fibrillation (AF) [1–3]. Transmural lesion is the hallmark of an effective and long-lasting RF lesion . Different studies have shown that one of the most important determinants in creating a transmural lesion is the contact between the tip of the ablation catheter and the targeted tissue . Accordingly, size and depth of the lesion are strictly related to the force applied to the tissue [6–9]. In clinical practice, electrophysiologists currently use a combination of qualitative measures, such as tactile feedback, fluoroscopic visualization, and electrogram amplitude assessment to evaluate the quality of the catheter tip-to-tissue contact.
To optimize and to directly measure catheter tip-to-tissue contact, new technologies (e.g., remote navigation systems or sensors incorporated into catheters) have been developed recently [10, 11]. The Sensei® X Robotic Catheter System (Hansen Medical, CA) is a remote robotic manipulation system validated to both measure and monitor the amount of force affecting the ablation catheter using the IntelliSense® Fine Force technology (Hansen Medical, CA) [10, 11]. With this tool, it has been shown that the optimal contact range is between 20 and 40 g/cm2; these contact values allow creating transmural lesions minimizing collateral damage . The electrical coupling index (ECI) from the EnSite Contact™ system (St. Jude Medical, MN) is based on the calculation of the real-time complex impedance specific to the catheter tip-to-tissue interface using a three-terminal model. In animal studies, ECI has been used not only to assess electrical tissue contact but also to distinguish tissue characteristics and to describe tissue heating and lesion formation [4, 13, 14]. To date, in clinical practice, this technology has been used to evaluate catheter tip-to-tissue contact, whereas its capability to predict a stable lesion creation has not yet been clearly demonstrated systematically .
To evaluate whether ECI features during RF pulses can predict the formation of an effective lesion, in our study we have compared for the first time an impedance-based system with the robotic navigation contact system by simultaneously measuring ECI local impedence and IntelliSense® force affecting the ablation catheter. Thus, analysis of ECI characteristics during ablation has been translated into clinically significant cutoff measures.
Study population included 15 consecutive patients who underwent catheter ablation of AF with both the Medical Sensei® X Robotic Catheter System and St. Jude Medical EnSite-NavX Contact system. All the patients were enrolled in our center between September 2011 and November 2012. No patient had a history of a previous AF catheter ablation.
Noninvasive investigations for all patients were: 12-lead electrocardiogram (ECG); transthoracic echocardiography with assessment of the left atrial (LA) volume, left ventricular function, and valvular incompetence; cardiac magnetic resonance imaging (MRI) or cardiac computed axial tomography (CT) scan; and transoesophageal echocardiography for patients in persistent AF to exclude LA appendage thrombus.
Mapping and ablation procedure
All patients signed an informed written consent before the procedure. Patients were studied under deep propofol sedation, breathing spontaneously. A temperature probe in the esophagus (FIAB Sensitherm, Italy) at the level of the left atrium tagged the esophageal location and provided intraesophageal temperature feedback during the procedure. A standard electrode catheter was placed in the coronary sinus. Afterward, a single transseptal puncture under intracardiac echocardiography (ICE) guide was performed. Unfractionated heparin was administered in bolus form immediately after the transseptal puncture and subsequently infused at a rate of 1,000 U/h adjusted to maintain an activated coagulation time of ≥300 s.
For the robotic navigation, an Artisan® X Control Catheter (Hansen Medical, CA), a 14-F robotic sheath, was inserted into the left femoral vein. The Artisan® X Control Catheter was then advanced manually to the right atrium under fluoroscopic guidance and attached to the robotic navigation arm called remote catheter manipulator (RCM) which moves the Artisan™ Catheter. The Contact Therapy Cool Path Duo (St. Jude Medical, MN, USA), a 7F unidirectional irrigated tip ablation catheter, was passed through the Artisan® X Control Catheter and the combination was then passed into the LA via the transseptal puncture site either manually or by using the robotic arm.
Three-dimensional (3D) reconstruction of LA was obtained by roving the circular mapping catheter (AFocus, St. Jude Medical, MN, USA) using the electroanatomical mapping system (NavX–EnSite Velocity; St. Jude Medical, MN, USA) and was then integrated with the Sensei® X Robotic Catheter System by the CoHesion™ 3D Visualization module (Hansen Medical, CA).
The operator remotely moved the ablation catheter with the Instinctive Motion™ Controller (Hansen Medical, CA), a 3D joystick placed on the Sensei® X System workstation, to acquire both a noncontact and a good contact position. Contact level was validated using: catheter visualization on the mapping system; intracardiac echo; evaluation of local electrograms; impedance monitoring; fluoroscopic guidance if necessary.
The catheter was held in a noncontact position, confirmed by ICE, for approximately 5 s; thus, the IntelliSense® algorithm acquired a contact value of 0 g/cm2 while, in the same position, the lowest ECI value recorded was set as the lower ECI threshold (lowest ECI value +5). To set the upper threshold the catheter was moved to a position of good contact and held in that position for 5 s. Then the lowest ECI value was set as the upper ECI threshold. According to previous studies [10, 11], the catheter was considered in good contact with the tissue when the robot displays registered values between 20 and 40 g/cm2 (see Fig. 1).
During the procedure, scaling of the technology was repeated every 30 min. Before performing ablation, alignment was carried out, and the wall contact was continuously monitored using the IntelliSense® algorithm with an upper acceptable limit set of 40 g/cm2.
For each patient, ECI values were recorded in: a noncontact site (0 g/cm2), a light contact site (force 1–10 g/cm2), and a good contact site (20–40 g/cm2).
In particular, before ablation the catheter was moved along the pulmonary veins (PVs) antra and the average of good contact ECI in at least four zones for each vein antrum was calculated. Such data were collected from the same zones during RF delivery, immediately after stopping energy delivery and 30 min after vein isolation.
Ablation was performed using the Contact Therapy Cool Path Duo (St. Jude Medical, MN, USA) catheter, RF power was set at 30–40 W with a maximum temperature limit of 42 °C with irrigation using saline infusion at a rate of 30 ml/min. Ablation was performed for at least 30 s at each location before the catheter tip was moved to a new site. The procedural endpoint was electrical PV disconnection validated by left atrium-PV entrance and exit block by stimulating respectively from coronary sinus catheter and circular mapping catheter3. In patients with persistent AF and in patients with an enlargement of the LA, even in case of paroxysmal AF, a roof-line lesion was additionally performed. In patients with AF, if sinus rhythm was not achieved during RF delivery, an electrical cardioversion was performed at the end of PVs disconnection and PVs isolation was checked in sinus rhythm. In patients with clinically documented isthmus–dependent right atrial flutter, ablation of the right atrial cavo–tricuspid isthmus was also performed.
Procedural data are presented as mean ± standard deviation if normally distributed or median (minimum/maximum range) if not. Groups were compared using Student’s t test (normally distributed data) or Mann–Whitney U test as appropriate. Categorical variables are described as count and percentage and compared using Fisher’s exact test. A p value <0.05 was considered statistically significant. SPSS (IBM, Armonk, NY, USA) was used for statistical analysis.
Receiver operating characteristic (ROC) curves were constructed for percentage delta ECI. A statistically derived value, based on the Youden index, maximizing the sum of the sensitivity and specificity was used to define the optimal cutoff value.
The study group included 15 patients (13 males) with mean age of 59 ± 12 years, 9 (60 %) of which were affected by paroxysmal AF and 6 (40 %) by persistent AF. In two paroxysmal AF patients, a 12-lead ECG documentation of a typical atrial flutter was present. No one had history of diabetes, heart failure, renal failure, or gastro-esophageal reflux. Four patients had arterial hypertension, and in one case, a previous episode of transient ischemic attack was recorded. All of the patients were refractory to pharmacologic therapy with IC and Amiodarone. Transthoracic echocardiography showed moderate LA dilatation (mean area, 24.3 ± 3.7 cm2) and normal left ventricular systolic function (mean EF, 59 ± 9 %); no patient had valvular disease. Pre-procedural MRI or CT scan allowed to detect a left common trunk in two (13 %) patients.
Ablation procedural results
All patients underwent catheter ablation for AF. At the beginning of the procedure, six patients (40 %) were in sinus rhythm while AF was recorded in nine (60 %) patients.
Complete PV isolation could be achieved in all patients. In eight (53 %) patients, a roof line was performed, whereas in two (13 %) patients with a 12-lead ECG documentation of typical atrial flutter, a tricuspid valve-inferior caval vein linear lesion was performed.
Procedure duration was 135 ± 41 min, RF burning time was 497 ± 262 s, and total fluoroscopy exposure was 29 ± 13 min with a mean dose area product of 19,423 ± 14,725 mGy*cm2. Major complications and adverse events were not observed during the procedure and the follow-up period.
Baseline ECI values
At the beginning of the procedure, a mean of 43 ± 7 ECI values in good contact positions was acquired in each patient. Considering each vein, a mean of 10.8 ± 1.8 values were acquired in the left superior pulmonary vein (LSPV), 10.4 ± 2.3 in the LIPV, 16.5 ± 2.1 in the LCT, 10.3 ± 2.6 in the right superior pulmonary vein (RSPV), and 9.6 ± 1.9 in the right superior pulmonary vein (RIPV; p = ns). The mean ECI value during mean contact of 27 ± 8 g/cm2 with healthy atrial tissue was 99 ± 13.
The mean good contact ECI value was significantly higher than both noncontact (66 ± 12; p < 0.0001) and light contact (77 ± 10; p < 0.0001) ECI values. In addition, a significant difference was observed between noncontact and light contact ECI values (p < 0.0001).
Furthermore, baseline ECI contact values were not different depending on AF presentation (paroxysmal AF, 98 ± 9; persistent AF, 100 ± 9; p = ns) or on procedural cardiac rhythm (sinus rhythm: 97 ± 7; AF: 101 ± 10; p ns). Table 1 shows different values for each vein.
ECI trend during and after ablation
The baseline ECI value significantly decreased during effective RF application to a mean of 56 ± 15 with a significant reduction of 42.9 % (p < 0.001). The ECI immediately after discontinuing the RF delivery showed a little rise stabilizing to a steady-state mean value of 72 ± 16 with a mean reduction of 25.4 % as compared with the baseline ECI (p < 0.001) and a mean increase of 32.2 % comparing with the ECI during RF application (p < 0.001) (see Figs. 2 and 3; Movies 1, 2, and 3).
After 30 min, PV isolation was confirmed in all veins. Thirty-minute ECI values were comparable to values measured immediately at the end of RF applications (72 ± 16 post-RF vs 76 ± 19 at 30 min; p = ns). Table 2 summarizes procedural details vein by vein.
Ablation during AF: ECI measures
Nine patients underwent RF ablation during AF. At the end of PV isolation, an electrical cardioversion to restore sinus rhythm was performed in all patients. Thus, PV isolation was checked in sinus rhythm showing the presence of the left atrial–pulmonary vein (LA-PV) gap in 6/36 (17 %) veins: one LSPV, one RSPV, and four RIPVs. The presence of at least one LA-PV gap was found in 5/9 (56 %) patients.
Isolated PVs had a significantly greater reduction in the post-RF ECI value with respect to baseline in comparison to reconnected PVs (26.4 vs 19.4 %, respectively; p = 0.003). The optimal cutoff value of delta ECI for predicting PVs isolation was 20 %, which gave sensitivity of 100 % and specificity of 70 %.
No significant difference was found in ECI reduction during RF application with respect to baseline between isolated and reconnected PVs (isolated PVs, 43.4 %; reconnected PVs, 39.4 %; p = ns).
At the mean follow-up of 11 ± 2 months, no paroxysmal AF patients had AF or atrial flutter recurrences, whereas in persistent AF group, only one patient (1/6, 17 %) had an episode of persistent AF requiring electrical cardioversion at the 3-month follow-up. At the 6-month follow-up, anthyarrhythmic drugs were discontinued in all but one paroxysmal AF patient while all persistent AF patients were maintained on 1C therapy.
In our study, ECI was used, for the first time, as a surrogate marker of atrial tissue lesion in AF ablation. Using the Sensei® X System, the catheter tip–tissue contact has been continuously measured and maintained stable; therefore, we could assume that the ECI changes during ablation were attributable to the creation of a lesion. Furthermore in our study, for the first time, the analysis of in vivo ECI reduction during and after RF pulses was performed. This analysis has allowed detecting a percentage reduction that could predict the formation of a stable lesion during a 30 -min observation period. The successful PV isolation results during AF, where atrial electrograms abolition is less predictive, confirmed the importance of ECI monitoring to achieve an effective RF lesion.
ECI as marker of effective RF lesion
During ablation procedures, the quality of contact between the ablating electrode and the endocardium profoundly affects ablation efficacy [6–9]. The traditional estimate of the angle and force of catheter tip contact had been subjectively assessed by the operator using a synthesis of different techniques such as tactile feedback, radiographic appearance, electrogram amplitude, pacing threshold, electrode temperature, and impedance, but no direct measures have been available till recently.
In recent years, new technologies have been developed, which allow to measure contact force . The Tacticath catheter (TactiCath®, Endosense, SA) uses proprietary optic fiber sensor technology within a 3.5-mm open- irrigated tip to provide real-time contact force measurements on a dedicated workstation, with a sensitivity of less than 1 g . The SmartTouch catheter (ThermoCool® SmartTouch™, Biosense-Webster, USA) can measure the force vector and force value between the catheter tip and the target myocardium via a unique sensor located at the distal tip of the irrigated RF catheter [16, 17]. IntelliSense® (Hansen Medical Inc., Mount View CA, US) is a robotic catheter navigation module that incorporates a system-based force-sensing technology and provides visual and vibration feedbacks to the operator about tissue contact [10, 12].
The EnSite Contact™ System is a proprietary device that determines electrical contact between ablation electrode and cardiac tissue by measuring complex impedance . Complex impedance refers to impedance subcomponents: resistance and reactance. The EnSite Contact System algorithm combines resistance, reactance, and impedance phase angle to create an ECI that is displayed on the navigation system. Beyond “pure” contact assessment, ECI is the only contact technology with the potential to distinguish tissue characteristics and to describe tissue heating and lesion formation [4, 13].
Recently, Holmes et al.  have studied the ECI change during transmural lesion in atria and thighs in animal models. In this work, Holmes’ group showed that an ECI reduction of at least 12 % was predictive of a lesion transmural or ≥2 mm in depth. Similarly, we described ECI change during RF lesion on human atria in vivo. Given the impossibility to directly check the depth and transmurality of the lesion, we assessed the electrical bidirectional conduction block between the atrium and PV immediately at the end of RF application and after a 30-min period. We observed that an ECI decrease of at least 20 % was necessary to obtain a stable lesion. This value was achieved with a decrease over the course of RF delivery of at least 40 % comparing with the baseline ECI. The use of the robotic system allowed us to relate the ECI change only to the tissue changing (i.e., lesion formation) as the force applied to the ablation catheter had been maintained stable and measurable in grams. Furthermore, we applied RF pulses just where the IntelliSense® fine force algorithm assured a contact force between 20 and 40 g, to avoid not only ineffective RF applications but also the risk associated with higher (i.e., perforation, steam pops, and thrombus formation) [18, 19].
Validation of ECI values
In our work, we evaluated the main value of ECI to contact pressure in 58 PVs by acquiring at least 10 points for each vein. Up to date, the only paper showing the ECI value of atrial tissue during AF ablation is by Piorkowski et al. . The authors performed a blinded to the operator measurements of ECI values during AF ablation in 12 patients. ECI measures were performed in up to four areas of noncontact and four areas of contact showing that, as the catheter went from noncontact to contact, ECI increased from 118 ± 15 to 145 ± 24. Our ECI values, both in contact and noncontact catheter position, were lower (99 ± 13 and 66 ± 12) than those found by Piorkowski’s group. This difference could be mainly related to our method of setting the lower and upper thresholds, in fact threshold setting is crucial to determine the ECI absolute value. Once the threshold is set, a good, light, or noncontact position is made clear by the relative increase or decrease in ECI values. Of note, in our data, the mean difference between contact and noncontact values were of 36 ± 12 ECI units, thus it was very similar to the finding described previously by Piorkowski et al. . Nevertheless, we performed thresholds with the aid of an intracardiac echo and validated the calculated values with the IntelliSense® fine force module; this method allowed us to be quite sure to measure ECI in a true contact or noncontact position.
Of interest, in our study, ECI measurements were not affected by patient rhythm, in fact baseline ECI values did not differ between sinus rhythm and AF. This observation is quite obvious as ECI considers tissue impedance and, accordingly, the capacitance of cellular structure. Capacitance varies with tissue types and changes when the cell’s membrane is damaged by thermal energy but it does not change with the underlying cardiac rhythm. In his work, Piorkowski et al. observed that the ECI is not affected by the history of AF burden (paroxysmal vs persistent AF) but he did not specify the rhythm at the moment of ECI acquisition . Thus, our data should be added to overall knowledge about ECI characteristics.
Finally, more than half of our patients underwent RF catheter ablation during AF rhythm. Analyzing PV disconnection or reconnection at sinus rhythm restoration, we could further validate the cutoff of at least 20 % ECI decrease in comparison to the baseline value in predicting a stable lesion formation. Furthermore, our experience strengthens data from Gaspar et al. . In their paper, the authors showed that an ECI-guided vs an ECI-blinded ablation leaves fewer gaps in encircling lesion and, as a consequence, requires fewer touch-up RF pulses. Our data confirm the utility of monitoring ECI during RF delivery, especially when the patient is in AF and the low-amplitude intracardiac electrograms are particularly difficult to interpret. According to Piorkowski’s group findings, the integration of ECI analysis into clinical decisions on RF delivery induces a change from an ablation approach that dragged the ablating catheter over longer line segments towards a more point–by–point pattern of RF application [13, 20].
Our study has some obvious limitations. First, the relatively small sample size of the study population and the short follow-up period has not allowed us to statistically correlate the procedural ECI to a long-term success, making it difficult to draw a firm conclusion. Second, a longer observation time of 1 h or more subsequently to RF application could have been helpful. Finally, permanent PV isolation and/or reconnection was not assessed after 3 months of follow-up.
The main findings of our work is both, the evidence, in vivo, that ECI is a marker of tissue characteristics and, the identification of a cut-ff in ECI decrease able to predict the formation of a transmural and stable atrial tissue lesion. Thus, ECI monitoring during RF delivery may provide the clinician with valuable feedback regarding lesion depth. This may increase the efficacy and safety of AF catheter ablation procedures.
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The authors wish to thank Eng. Elena Romanelli, Pasquale De Iuliis, Enrico Romano, and Antonio Mininno (St. Jude Medical) for their technical support in NavX mapping and data analysis and Eng Guido Panfili (Abmedica) and Matteo Vernich (Hansen Medical) for technical support.
Antonio Dello Russo, Michela Casella, Fabrizio Bologna, Osama Al-Nono, Daniele Colombo, Viviana Biagioli, Pasquale Santangeli, Martina Zucchetti, Benedetta Majocchi, Vittoria Marino, Joseph J Gallinghouse have no relationships or conflicts to disclose. Gaetano Fassini serves as consultant for Medtronic and Biosense-Webster. Luigi Di Biase serves as consultant for Hansen Medical and Biosense-Webster. Andrea Natale has received compensation for belonging to the speakers’ bureau for St. Jude Medical, Boston Scientific, Medtronic, and Biosense-Webster. Dr. Natale is a consultant for Biosense-Webster. Claudio Tondo serves as member of the Advisory Board of Medtronic and is also a consultant for and receives lecture fees from St. Jude Medical
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Dello Russo, A., Fassini, G., Casella, M. et al. Simultaneous assessment of contact pressure and local electrical coupling index using robotic navigation. J Interv Card Electrophysiol 40, 23–31 (2014). https://doi.org/10.1007/s10840-014-9882-2
- Atrial fibrillation
- Catheter ablation
- Electrical coupling index
- Catheter tip-to-tissue contact