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J Am Coll Cardiol, 1999; 33:972-984 © 1999 by the American College of Cardiology Foundation |
* Section of Cardiology, University of Illinois at Chicago, Chicago, Illinois, USA
Boston Scientific Corporation/EP Technologies, San Jose, California, USA
Manuscript received October 28, 1997; revised manuscript received October 26, 1998, accepted December 17, 1998.
Reprint requests and correspondence: Dr. Boaz Avitall, The University of Illinois at Chicago, Department of Medicine, Section of Cardiology, 840 South Wood Street (M/C 787), Chicago, Illinois 60612
BAvitall{at}uic.edu
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Abstract |
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This article describes a catheter system designed to create linear atrial lesions and identifies electrophysiologic markers that are associated with the creation of linear lesions.
BACKGROUND
Atrial fibrillation (Afib) is the most common arrhythmia in humans and causes a significant morbidity. The success of surgical interventions has provided the impetus for the development of a catheter-based approach for the ablation of Afib.
METHODS
We tested a catheter system with 24 4-mm ring electrodes that can create loops in the atria. The electrodes can be used to record electrical activity and deliver radiofrequency power for ablation. In 33 dogs, 82 linear lesions were generated using three power titration protocols: fixed levels, manual titration guided by local electrogram activity and temperature control. Bipolar activity was recorded from the 24 electrodes before, during and after lesion generation. Data were gathered regarding lesion contiguity, transmurality and dimensions; the changes in local electrical activity amplitude; the incidence rate of rapid impedance rises and desiccation or char formation; and rhythm outcomes.
RESULTS
Catheter deployment usually requires <60 s. Linear lesions (12 to 16 cm in length and 6 ± 2 mm wide) can be generated in 24 to 48 min without moving the catheter. Effective lesion formation can be predicted by a decrease of greater than 50% in the amplitude of bipolar recordings. Splitting or fragmentation of the electrogram and increasing pacing threshold (3.1 ± 3.3 mV to 7.1 ± 3.8 mV, p < 0.01) are indicative of effective lesion formation. Impedance rises and char formation occurred at 91 ± 12°C. Linear lesion creation does not result in the initiation of Afib. However, atrial flutter was recorded after the completion of the final lesion in 3/12 hearts. When using temperature control, no char was noted in the left atrium, whereas 8% of the right atrium burns had char.
CONCLUSIONS
This adjustable loop catheter forces the atrial tissue to conform around the catheter and is capable of producing linear, contiguous lesions up to 16 cm long with minimal effort and radiation exposure. Pacing thresholds and electrogram amplitude and character are markers of effective lesion formation. Although Afib could not be induced after lesion set completion, sustained atrial flutter could be induced in 25% of the hearts.
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The goal of catheter-based ablation of Afib should be safe minimal tissue destruction allowing for the restoration of sinus rhythm under autonomic nervous system control combined with recovery of atrial mechanical transport to prevent the development of a thromboembolic event and provide hemodynamic benefits for the patient. The atrial chambers are globular, elastic and only a few millimeters thick. Therefore, we hypothesized that a catheter system that creates adjustable loops in dynamic contact with the atrial walls is most likely to be able to adapt to the atrial structure and create good contact for ablating and recording. This article summarizes our experience using a catheter technology that was designed to create linear contiguous lesions in both atria.
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Methods |
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Experimental testing of the Afib ablation catheter. All procedures were performed in compliance with the American Heart Association’s guidelines for animal research and were approved by the University of Illinois at Chicago’s Animal Care Committee. Dogs (25 to 35 kg) were premedicated with Nembutal (0.5 cc/kg) and transported to the laboratory, where anesthesia was maintained with halothane (1 to 1.5%) and O2 (2 liter/min). The femoral vein was cannulated with a self-sealed 14-F short access sheath. A precurved 70-cm 11-F guiding sheath was inserted via the access sheath into the right atrium (RA) or the left atrium (LA). For LA placement, transseptal cannulation was achieved via the Brockenbrough technique using a specially designed dilator and a long transseptal needle under transesophageal echocardiographic (TEE) guidance. In addition, the jugular and femoral veins were cannulated and 6-F quadripolar catheters were placed in the coronary sinus and the high RA. After the procedures, the hearts were excised and examined for evidence of perforation of or damage to the valves, main vessels, nerve trunks around the heart and chamber walls.
Catheter positioning. Predetermined anatomical targets were chosen on the basis of the surgical Maze lesion locations, the mobility of the catheter and the ability of the catheter to adapt to the right and left atria. Various combinations of the following linear lesions were generated:
Figure 2 depicts representative fluoroscopic images of the catheter positions for each lesion. An important factor in catheter positioning is the ability to reproduce placement in specific anatomic locations in different hearts. When deflected, the catheter tends to settle in the position with the largest circumference. However, by positioning the sheath at different levels, a variety of positions can be achieved in both the right and left atria.
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Lesion formation. In the 33 dogs, a total of 82 linear lesions were generated in the RA (44 lesions) and LA (38 lesions). Radiofrequency (RF) power was delivered sequentially via each electrode using three protocols including:
Therefore, the maximum overall dwell times for ablating via all 24 electrodes were 24, 32 and 48 min for the temperature, power and amplitude protocols, respectively. In some cases the dwell times during ablation were less, if the atrium was small and required ablation with less than 24 electrodes to complete a linear lesion. One dog received lesions using two different power protocols (amplitude and temperature). Bipolar recordings from the 24 electrodes were obtained from overlapping electrode pairs (1–2, 2–3 ... 22–23, 23–24) before, during and after lesion generation. In 31 ablation procedures (80 lesions), the incidence of rapid impedance rises to >200 
was documented.
In six dogs, nine linear lesions were created, and the maximum power and maximum temperature were recorded. The amplitude of the atrial electrical activity was measured before and after lesion generation at each of the bipolar electrodes at fixed amplification and filtering (30 to 500 Hz), and the percent decrease was calculated. After excision, the hearts were soaked in tetrazolium blue stain for 8 h. The width and length of each individual lesion and the total linear lesion length were measured. The quality of each of the individual lesions with respect to contiguity, transmurality and desiccation or char formation was assessed. The lesions were defined as transmural and contiguous if no surviving muscle could be seen between the lesions and clear contiguous lesions could be seen on the epicardial surface. Char was defined as loosely adherent material that can be easily dislodged from its underlying tissue; desiccation was defined as tightly adherent, shallow, discolored material that is firmly attached to the underlying tissue. For 11 lesions created in eight dogs with amplitude or temperature power delivery protocols, each individual lesion was categorized as follows: type 0—no identifiable lesion despite power application; type 1—minor superficial lesion formation that is neither contiguous nor transmural; type 2—lesion that is either contiguous or transmural; type 3—lesion that is both contiguous and transmural (the ideal lesion type); type 4—lesion that is contiguous and transmural but has adherent desiccation, and type 5—lesion that is contiguous, transmural and covered with loosely attached char.
In seven dogs, the average heart rate and P-R interval was calculated before and after generating a set of linear lesions.
In three dogs (152 individual burns), bipolar pacing threshold values were obtained by pacing (pulse width = 2 ms, cycle length = 300 ms) between two adjacent electrodes before and after applying RF power to the cathodal electrode, without moving the catheter.
Rhythm outcomes. In 12 dogs, 50-ms cycle length burst pacing was applied for 5 s to the RAA before and after the completion of 31 linear lesions, and the resulting rhythm was recorded. The rhythm was categorized as sinus, Afib or atrial flutter. Its duration was defined as sustained (lasting >180 s) or nonsustained (lasting 30 to 180 s).
Epicardial mapping. To examine the effects of lesions on local electrical activity, the RA and LA were mapped before and after the creation of RA and LA lesions. The chest and pericardium were opened via a median sternotomy, and a handheld high density mapping plaque (112 bipolar electrodes) was placed on the LA and RA surfaces. Activation of the RA and LA was recorded during 400-ms pacing from the RA and LA appendages. The plaque was removed, and the chest was clamped closed. Lesions were created with the catheter from the endocardial surface across the areas where the plaque had been placed on the RA and LA. The chest was reopened, the plaques were repositioned in the same areas as before ablation and electrical activity was recorded during 400-ms pacing from the RA and LA appendages.
Statistical analysis. Summary data are expressed as raw mean value ± SD. Since the focus of this report is on the methodology for the creation of contiguous transmural linear lesions, we analyzed the effects of different power titration techniques by assessing the local electrical activity pre and post individual lesion creation and by defining the temperature needed to create the desired lesions. The continuous data were analyzed using a nested random effects analysis of variance (ANOVA) model that treats the dogs as blocks (random) and the observations from each dog as a nested subsample. The local electrical activity (LEA) group and atrium were also included as factors in the ANOVA. The p values were considered significant if they met the level of 0.025 since two variables, namely power and temperature, were simultaneously studied. This adjusted level ensures, by Bonferroni method, an overall significance of 0.05. A McNemar’s chi-square test was used to compare arrhythmia inducibility before and after lesion generation.
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Results |
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Lesion formation: temperature, local electrogram reduction. Three protocols were used to manually titrate RF power. As shown in Table 1, power titration of 5 and 10 W resulted in 7% impedance rises. Power titration of 10 and 20 W resulted in a 14% incidence of impedance rise. Both temperature and local electrogram amplitude monitoring yielded low levels of impedance rises, 5% and 2%, respectively. Lesion character was divided into six categories. The ideal lesion (type 3) is contiguous and transmural, without any evidence of desiccation or char on either the tissue or the catheter. As shown in Figure 3, type 3 lesions were 44% of the total lesions generated with temperature-based titration and 46% of the lesions generated with 70% reduction of local electrogram amplitude. The average recorded temperature for these two protocols was 71 ± 7°C for type 3 lesions. If one combines type 3 and 4 lesions, the percent acceptable lesions increases to 70% and 57% for the temperature and the amplitude protocols, respectively. With temperature-guided power titration, in multiple cases the peak temperature exceeded the target temperature leading to overheating and impedance rises. Impedance rises and char formation (lesion type 5) were associated with temperatures of greater than 91 ± 12°C. As shown in Figure 4, inadequate lesions (type 0, 1 and 2) were associated with lower temperatures of 42 ± 3°C (p < 0.000, type 0 vs. type 3), 61 ± 11°C (p < 0.000, type 1 vs. type 3) and 68 ± 4°C (p = NS, type 2 vs. type 3). Overall, the average temperature was 74 ± 13°C and the average local electrogram reduction was 67 ± 34% for the lesions created with temperature or amplitude guidance.
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Catheter positioning. Table 4 summarizes the time required to place and remove the catheter and the duration of radiation exposure in five dogs. Fluoroscopic video recordings and electrical recordings were used to verify that the placements were in the designated positions and were consistent. In addition, after removal of the hearts, the location of radiofrequency lesions confirmed the catheter positioning. Placement of this catheter technology in either the LAH or RA Loop positions averaged less than 1 min with minimal radiation exposure.
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Discussion |
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Safety. Having multiple ablation electrodes on a single shaft allows for minimal catheter manipulation in creating long linear lesions and, therefore, may reduce both thromboembolic risk and radiation exposure. Once the catheter is in place, it remains in position for the duration of the power application to all 24 electrodes. In this study using sequential power application to each electrode, the total dwell time during ablation varied between 24 and 48 min depending on the power protocol utilized.
This ablation system was placed in 33 dogs with no mechanical injuries to the atrial or ventricular tissues, no damage to the valvular structures and no entanglement of the catheter system with tissues. However, if an additional catheter is introduced into the cardiac chamber after the deployment of the Afib ablation catheter, this second catheter should be retracted before the removal of the Afib catheter. Though it did not occur in any of our procedures, with this system there is the potential for mechanical injury to occur across the intra-atrial septum. Once the guiding sheath is placed across the septum it must be maintained at this position, particularly during catheter manipulation. If the sheath is not kept across the atrial septum, expansion of the catheter loop could result in septal tearing. In addition, if the catheter is deployed across the mitral ring it may interfere with valvular closure.
Catheter positioning. Placement of this system in the atria usually required less than 1 min in this animal model. Although this system naturally creates loops at the points of maximal diameter, it can be positioned in various other anatomic locations by manipulating the guiding sheath. The catheter naturally adapts to the RA Loop and LAH positions, whereas the LA vertical positions usually require sheath manipulation. Poor contact can be improved without significantly changing the location of the lesion by simply expanding the size of the loop.
Lesion formation. The most important determinant in the effective creation of a RF lesion is the electrode–tissue contact. Using the variable loop concept, the globular-shaped atrial chambers will adapt around the catheter providing continuous contact. When using temperature control with this technology, over 90% of the lesions created in both atria were both contiguous and transmural; the incidence of impedance rises was minimized. The catheter can be used to create linear lesions 6 mm wide and up to 16 cm long with minimal manipulation. In this study, linear lesions were made by ablating individual electrodes. The time required to create an entire linear lesion could be greatly reduced by using a closed-loop system to simultaneously deliver RF power to multiple electrodes to maintain a set temperature.
Recordings. Using 4-mm ring electrodes separated at 4-mm intervals allows for discrete localized recording of electrical activity before, during and after ablation. Such recordings provide online monitoring of the lesion formation. A reduction of greater than 50% in the recorded electrogram is a good indication of effective lesion formation. When lesions are created during sinus rhythm or flutter, the formation of double potentials and fragmented electrical activity signifies transmural lesion creation. The mechanism of this observation is likely related to the formation of a split atrial depolarization wavefront around the catheter as a result of the lesion. The recording of double potentials around an area of conduction block has been previously described by Feld and Shahandeh-Rad (4). Because these recordings are made from adjacent electrodes, the destruction of the tissue underneath and between these electrodes results in marked diminution of the recorded electrical activity. Further confirmation of tissue destruction is noted by the significant increase in the pacing threshold. The ability to retrieve discrete localized recordings from multiple electrodes provides atrial mapping capability and allows for safeguarding of the His bundle.
Rhythm outcome. After 75% of the ablation procedures, no arrhythmia was inducible. The most common arrhythmia induced during the ablation procedures was sustained atrial flutter, which was present after 42% of the individual linear lesions and at the conclusion of 3/12 ablation procedures. At baseline, these three hearts exhibited sinus rhythm, sustained Afib and sustained atrial flutter, respectively.
Atrial flutter circuits were created and terminated by linear lesion generation. In four dogs, the generation of a linear lesion in the LA resulted in the inducibility of sustained atrial flutter that was not inducible preablation. In three of these four dogs, atrial flutter was noninducible after creating a single additional linear lesion (1 RAI, 2 LAV). In an additional dog, sustained atrial flutter was inducible at baseline, but only sinus rhythm was recorded after creating a LAH lesion. In this study, Afib was never a consequence of creating a linear lesion. In fact, in one dog with inducible sustained Afib at baseline, the creation of linear lesions eliminated the Afib. Additional studies are needed to show that this technology and lesion set will be effective in ablating Afib in a chronic model before evaluating its efficacy in humans.
Atrial fibrillation ablation in humans. Recently several reports were presented supporting the notion that chronic Afib can be terminated in humans (5–7). Perhaps the most important conclusion to be drawn from these studies is that, despite the technological limitations and the uncertainty of the character of the lesions that were created, the investigators were still able to terminate Afib. Clearly, as we have learned from Cox et al., to achieve a high degree of success in most patients with chronic Afib, a careful Maze-type procedure may be necessary. A procedure that requires a thoracotomy may not allow for repeat surgery if the Afib recurs. Thus, an extensive and meticulous complete Maze procedure is required in every surgical patient to ensure the desired outcome. If catheter-based ablation can be safely developed, it would allow for extensive investigation into the mechanisms of Afib in humans.
Conclusions. In this study, we wished to define the feasibility of linear lesion generation in dogs and to describe the basic characteristics of a catheter system that will have the potential to ablate Afib by efficiently creating linear contiguous atrial lesions. Our present hypothesis is that successful ablation of Afib will require a system that is easy to manipulate, can conform to various atrial sizes and shapes and can create effective contact by maintaining firm and uniform catheter–tissue pressure. Firm contact with the atrial tissue during RF energy application increases the likelihood of transmural, contiguous lesion formation.
Although the results from animal studies may not be fully extrapolated to the human heart, this catheter technology shows great promise in creating the type of lesions that will likely be necessary to ablate Afib in humans. The data obtained with regard to linear lesion contiguity and transmurality, local electrogram changes and pacing thresholds reflect the results obtained from this specific catheter design and cannot be extrapolated to other designs.
The loop catheter system is capable of creating long linear lesions (12 to 16 cm) at predefined anatomic locations. The deployment of the catheter requires minimal radiation exposure. It caused no perforations or valvular injuries. However, the catheter could tear the septum if it is placed transseptally, the sheath is retracted into the RA and the catheter is deflected while still traversing the septal hole. In addition, if it is deployed across the mitral ring it may interfere with valvular closure. Once the catheter is positioned, no further movements are needed until the completion of the linear lesion. The methods described in this report provide the operator with multiple means of assessing the quality of lesion formation. Ideal lesions are likely to result when thermistors are used to maintain the temperature at 70°C, the local electrical activity amplitude is reduced by 50% of its baseline value, splitting of the atrial depolarization is noted and the atrial pacing threshold increases. Atrial fibrillation was never a consequence of creating a linear lesion in any of the 12 dogs that were tested for inducible arrhythmia, but sustained atrial flutter was inducible after the completion of all the lesions in three dogs (25%).
Limitations. In these acute studies, the dogs were not anticoagulated. Long-term complications such as pericarditis or pericardial effusion could not be assessed, but there were no atrial perforations in any of the procedures. The potential correlation between the microscopic analysis of the tissues for lesion contiguity and transmurality and the inducibility of atrial flutter postablation was not assessed. However, the high density epicardial maps and the appearance of split potentials postablation both indicate the formation of conduction block across lesions. In those dogs with inducible atrial flutter, such flutter is likely a result of a reentry around an incomplete lesion. These ablation procedures were carried out in dogs with normal hearts. Further investigation of this technology and technique needs to be performed in a chronic model of Afib to assess its curative efficacy.
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Footnotes |
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This article has been cited by other articles:
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  B. Avitall, A. Urbonas, S. Millard, D. Urboniene, and R. Helms Ablation of atrial fibrillation in the rapid pacing canine model using a multi-electrode loop catheter J. Am. Coll. Cardiol., May 1, 2001; 37(6): 1733 - 1740. [Abstract] [Full Text] [PDF]
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A. Natale, K. H. Newby, E. Pisano, F. Leonelli, R. Fanelli, D. Potenza, S. Beheiry, and G. Tomassoni Prospective randomized comparison of antiarrhythmic therapy versus first-line radiofrequency ablation in patients with atrial flutter J. Am. Coll. Cardiol., June 1, 2000; 35(7): 1898 - 1904. [Abstract] [Full Text] [PDF]
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