Ann Thorac Surg 1998;66:95-100
© 1998 The Society of Thoracic Surgeons
Original articles: cardiovascular
Atrial ablation with an IRK-151 infrared coagulator
Hiroshi Kubota, MDa,
Akira Furuse, MDa,
Mika Takeshita, MDa,
Yutaka Kotsuka, MDa,
Shinichi Takamoto, MDa
a Department of Cardiothoracic Surgery, University of Tokyo, Tokyo, Japan
Accepted for publication February 17, 1998.
Address reprint requests to Dr Kubota, Department of Cardiothoracic Surgery, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
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Abstract
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Background. The purpose of this study was to develop a method of atrial ablation. In the IRK-151 infrared coagulator, light from a tungsten-halogen lamp is focused into a quartz rod. The distal exit plane is connected to a tip made of sapphire to allow linear ablation.
Methods. Thirty-six lesions were created in 9 mongrel dogs. The beating ventricular myocardium was ablated from the epicardium. In each dog, 4 lesions were created by using the following durations of application: 3, 9, 15, and 21 seconds. After the ablation, the myocardium was fixed and stained. A linear lesion on the beating right atrial free wall was created. Before and after the ablation, epicardial plaque–electrode mapping was performed. Three months after ablation, remapping was performed.
Results. The ablated myocardium had well-demarcated necrosis without carbonization or vaporization. The maximum depth was 10.3 ± 0.8 mm. The conducting pathway was blocked. The block, once made, continued for 3 months.
Conclusions. The IRK-151 produces well-demarcated lesions that were deep enough for atrial ablation to block the conducting pathway.
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Introduction
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The maze procedure for treating atrial fibrillation has become widespread. Since this procedure requires long and complicated cutting and suturing of both atria, a longer aortic cross-clamping time is necessary. However, no ideal method for atrial ablation has yet been reported. In the IRK-151 infrared coagulator, the distal exit plane of the light-conducting rod is connected to a tip. We made three kinds of artificial sapphire tips for this device to obtain linear photocoagulation. When each tip is touched to the epicardium, light energy is absorbed by the myocardium, which becomes photocoagulated. In this study, the characteristics of the three kinds of tip were first compared in a dog heart. The depth and width of the myocardial lesions were measured, and their histopathologic features were observed. Changes in the atrial conducting pathway immediately after and 3 months after ablation then were recorded by epicardial mapping. The purpose of this study was to develop a method of atrial linear ablation.
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Material and methods
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Device
The IRK-151 infrared coagulator (Infrarot-Kontaktkoagulator; Messerschmidt-Bolkow-Blohn, Frankfurt, Germany) was originally developed to secure hemostasis in bleeding parenchyma as an alternative to high-frequency electrocoagulation or laser coagulation (Fig 1A). The single-application time is limited to within 3 seconds by a timer. In this device, light from a tungsten-halogen lamp is focused by a reflector into a light-conducting quartz rod with a diameter of 10 mm. It emerges as 35 W/cm2 of near-infrared light energy (wavelength, 400 to 1600 nm; peak wavelength, 850 nm). The distal exit plane of the light-conducting rod is connected to the tip. We made three kinds of original tips using artificial sapphire (tapered, coated, and angled) by polishing the surface of a cylindrical sapphire slantwise (Fig 1B). All the tips have a rectangular (1.5 x 10.0 mm) edge to allow linear atrial myocardial ablation.
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Fig 1. (A) The IRK-151 infrared coagulator. (B) Three kinds of artificial sapphire tips: (left) tapered, (middle) coated, and (right) angled. The coated tip has the same shape as the tapered tip, but the slanting surfaces are coated with nickel and tin dichloride. The angled tip has a 4-mm straight nose to prevent unnecessary ablation caused by direct contact of the slanting surface. When these sapphire tips are pressed into tissue, light energy is absorbed by the myocardium, which is then photocoagulated. (a = tip; b = light-conducting quartz rod; c = reflector; d = grip; e = timer.)
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The tapered tip has two bilateral slanting surfaces. The coated tip has the same shape as the tapered tip, and the slanting surfaces are coated with nickel and tin dichloride to prevent leakage of unnecessary energy in a lateral direction. The angled tip has a 4-mm straight nose to prevent unnecessary ablation resulting from direct contact with the slanting surface. Artificial sapphire is transparent to light and does not adhere to coagulated tissue. When these sapphire tips are pressed onto the epicardium, light energy is absorbed by the myocardium, resulting in photocoagulation.
Methods
Developing a suitable tip for obtaining linear atrial ablation
Nine adult mongrel dogs weighing 13.5 ± 1.7 kg were anesthetized using ketemine hydrochloride (20 mg/kg) intramuscularly and sodium pentobarbital (16 mg/kg) intravenously and were ventilated. All animals received humane care in compliance with the "Guide for the Care and Use of Laboratory Animals" published by the National Institutes of Health (NIH publication 85-23, revised 1985). Through a right thoracotomy, the beating right ventricular myocardium was ablated using the IRK-151 from the epicardium. The tip was compressed 2 to 3 mm into the epicardium unless this induced paroxysmal ventricular contraction. Four lesions were created using different durations of application (3, 9, 15 and 21 seconds). The three different sapphire tips were applied to 3 dogs each. Therefore, a total of 36 lesions were created. After ablation, the hearts were rapidly excised and the myocardium was fixed in 10% formalin. After adequate fixation, thin sections were cut and stained (hematoxylin-eosin and azan). The width, depth, and shape of ablation, and the resulting histopathologic changes were observed by light microscopy. To obtain linear coagulation, the ideal tip should create deep and narrow coagulation. Therefore we devised the linear index, to describe this: 
. The linear indices using the three kinds of tips at 21 seconds of application were calculated and compared for each tip. Ventricular myocardium was used instead of atrial myocardium to assess the depth of coagulation because the atrial myocardium is too thin for this evaluation.
To evaluate the influence of tip pressure on the epicardium, the depth of the lesion when the tapered tip was compressed 2 to 3 mm into the epicardium and that when the same tip merely touched the epicardium were compared for various durations of application (3, 9, 15 and 21 seconds) using 3 adult dogs.
Atrial ablation with an infrared coagulator
In 3 dogs a linear lesion on the beating right atrial free wall was created from the annulus of the tricuspid valve to the intraatrial sulcus by using the tapered tip. This lesion was created by several overlapping applications (15 seconds for each application) from both the epicardial and endocardial sides (Fig 2). First, the endocardial ablation was performed by insertion of the light-conducting rod into the right atrium through a purse-string suture on the right atrial appendage. After that, epicardial ablation was performed on the same line as endocardial ablation. It was easy to define the lesion of endocardial ablation from outside because the ablated atrial wall was discolored. To ensure complete coverage, 2.0 mm of overlapped lesions were made. Before and after ablation, epicardial plaque–electrode mapping (24-channel bipolar electrodes, 33 x 45 mm) was performed to determine the conduction pathway in the right atrial free wall. The recording conditions were spontaneous beating and overdrive pacing from the low right atrium (R = 140/min). In two other dogs, the same line as that to be ablated was clamped with a Satynsky clamp. The atrial wall was then cut and sutured. After unclamping, epicardial mapping was performed in the same way. The result of postablation epicardial mapping was contrasted with the mapping of the cut and sutured atrial wall. The collected data were recorded and analyzed by the HPM-7100 mapping system (Fukuda Denshi, Tokyo, Japan). After the experiment, the chest wall was closed. Three months later, right atrial epicardial plaque–electrode mapping was performed again through another thoracotomy. After the completion of mapping, the right atrial free wall was excised, fixed, and stained in the same way as the ventricle, and the histopathologic changes were observed by light microscopy.
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Fig 2. A linear lesion was created by several overlapping applications from both the epicardial and endocardial sides.
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Data are presented as mean ± standard deviation. Means were compared between the groups by analysis of variance or Student’s t test.
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Results
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Developing a suitable tip for obtaining linear atrial ablation
Depth of lesion
The depth of the lesion was correlated with increased coagulation time (Fig 3). The maximum depth was 10.3 ± 0.8 mm at 21 seconds using the tapered tip, which was significantly deeper than with the other kinds of tips. The depth of the lesion using the angled tip was 5.2 ± 0.3 mm, and that with the coated tip was 7.7 ± 0.3 mm at 21 seconds.
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Fig 3. Average depth of the lesion. The maximum average depth of the lesion was 10.3 ± 0.8 mm at 21 seconds with use of the tapered tip. (n.s. = not significant.)
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Width of lesion
The narrowest lesion width, 13.7 ± 1.5 mm at 21 seconds of ablation (Table 1), was created by the tapered tip. The linear index obtained using the tapered tip (0.76 ± 0.13 mm) was higher than that for the other tips (angled tip, 0.25 ± 0.02 mm; coated tip, 0.43 ± 0.03 mm).
Shape of lesion
Longitudinal sections showed that the longitudinal length of the lesions created by the three kinds of tips was preserved from the surface to the bottom. The cross-section of the lesion created by the tapered tip showed an elliptical shape. The coated tip created a cone-shaped lesion and the angled tip created a convex lesion at the center.
Influence of tip pressure on the epicardium
When the probe merely touched the epicardium, the maximum depth of the lesion was 5.7 ± 0.3 mm at 21 seconds. By compressing the tip into the epicardium, it was possible to obtain a maximum of 1.8 times this depth at 21 seconds of ablation (Fig 4).
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Fig 4. Influence of tip pressure on the epicardium. When the tip was pressed into the epicardium, the deeper lesion was created.
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Histopathologic findings
The epicardium was preserved, and the ablated myocardium had well-demarcated photocoagulation necrosis without carbonization or vaporization. The nuclei of the myocardial fibers were concentrated. The myocardial fibers were atrophic and degenerated. The fundamental histopathologic structure was the same irrespective of whether ablation time was short or long (Fig 5). All of the vessels were patent.
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Fig 5. Histopathologic findings of ventricular myocardium. The ablated myocardium had well-demarcated photocoagulation. The vessels were patent. The lesion reached the endocardium with use of the tapered tip at 21 seconds.
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Atrial ablation using the infrared coagulator
Epicardial mapping
Before linear ablation on the right atrial wall, electrical potential was conducted regularly from the sinus node. After ablation, it was conducted from the node to the annulus of the tricuspid valve through the lateral side (intraatrial sulcus side) of the ablation area. Under overdrive pacing from the low right atrium, it was conducted from the low right atrium to the right atrial appendage through the lateral side of the ablation area (Fig 6). Mapping after cutting and suturing the atrial wall showed the same pattern of conduction. Mapping 3 months after ablation also showed the same pattern of conduction.
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Fig 6. Right atrial epicardial mapping (A). Location of the electrodes. (B) The preablation map (left) and the postablation map (right) (sinus rhythm). The electrical conduction of the right atrium was blocked by the linear zone of photocoagulation. (C) Right atrial epicardial mapping (overdrive pacing from the low right atrium) (left) and electrocardiogram (right). Electrical potential conducted from the low right atrium to the right atrial appendage. The electrical conduction was blocked by the same area as in B. Electrodes 17 and 18 were on the lower side, and 19 and 20 were on the higher side near the lesions.
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Histopathologic findings
The atrial epicardium was preserved. The ablated myocardium had well-demarcated transmural photocoagulation necrosis without carbonization or vaporization, as in the ventricular myocardium. The deposition of hemosiderin, invasion of macrophages, increased capillary vessels, and increased juvenile elastic fibers were observed in chronic phase. The myocardium did not revive. The endocardium was thickened, and elastic fibers also appeared (Fig 7).
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Fig 7. Right atrial free wall 3 month after ablation. The endocardium was thickened. Density of juvenile elastic fibers was higher. The vessels were patent.
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Comment
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Recently, since Cox and associates [1–4] published the maze procedure for treating atrial fibrillation, surgical treatment for atrial fibrillation has attracted considerable attention and become widespread. The cause of atrial fibrillation is considered to be intraatrial multiple macroreentrant circuits [5]. The intention of the maze procedure is to prevent the occurrence of macroreentrant circuits by dividing the atrial wall into a size smaller than the critical mass of atrial myocardium that can cause macroreentry. Because the maze procedure requires a long and complicated suture line on both atria, it requires a longer aortic cross-clamping time. Especially in high-risk patients, because the longer aortic cross-clamping time may cause complications postoperatively, not all patients with atrial fibrillation are eligible for this procedure.
Cryoablation takes a long time to ablate the myocardium, and the resulting cryolesion is larger than necessary. In laser ablation, it is difficult to control the depth of ablation, and the equipment needed is large and expensive. The purpose of this study was to develop a method of atrial ablation with a shorter aortic cross-clamping time during the maze procedure. An IRK-151 infrared coagulator was used for this study. This device was developed by Nath and associates [6] in Germany. In relation to arrhythmia, Nakajima and associates [7] reported the electrocardiographic changes after photocoagulation using the IRK-151 on sinus node, right bundle branch, and His bundle in canine heart.
In our experiment, the depth of coagulation was assessed in the ventricular myocardium. However, the atrial and ventricular myocardium have radically different structures. Therefore, our data for the width and depth of coagulation cannot be applied to atrial coagulation and should be regarded simply as reference data.
Because the atrial wall is thinner than the ventricular wall, atrial coagulation should not cause carbonization or vaporization, to prevent perforation. In our experiment, infrared photocoagulation caused well-demarcated and homogeneous necrosis within the lesions, without carbonization or vaporization, and the epicardium remained intact. To prevent postoperative thromboembolism, it is important to leave no foreign body such as thread. In this regard, preservation of endocardial continuity is important.
Comparison of precoagulation and postcoagulation atrial epicardial electrode mapping confirmed that transmural degeneration of the atrial wall caused electrical block concurrently. As epicardial mapping after cutting and suturing of the right atrium showed the same mapping pattern as the ablated atrium, this was reconfirmed. It is not fully clear from these mappings that there are no regions of slow conduction penetrating the lesions because the mapping system cannot clearly distinguish electrical block from slow conduction. Ideally to prove complete electrical block of activation, an atrial area should be isolated with the IRK-151, and mapping under pacing in the isolated region would be necessary. The electrical block, once made, continued for 3 months in all cases. These results support the use of this device for clinical application.
Linear ablation is more efficient than continuous overlapped ablation with a round tip. Usually, the volume of the lesion is calculated according to a formula using the epicardial radius (a) and myocardial depth (b): 
, where for oblate, a > b and for prolate, a < b [8, 9].
This formula is not suitable for evaluating the efficacy of linear ablation as it is neither oblate nor prolate. A compact index ( 
) was therefore devised. As the index increases, the ablation approaches linearity. Unexpectedly the tapered tip showed the highest index (0.76), and the coated tip the lowest index (0.43). This result may have occurred because the nickel and tin dichloride coating the surface could not reflect the infrared rays perfectly, leading to heating of the tip surface.
The degree of tip pressure on the myocardium is also important for determining the depth of the lesion. Our experiment showed that compressing the myocardium created a deeper lesion than merely touching it.
Mikat and associates [10] reported that an average depth of 5 mm was created when a cryoprobe was applied to perfused dog epicardium for 90 to 120 seconds. Hendry and associates [11] showed that an average depth of 9.9 mm was created when a cryoprobe was applied to extirpated canine hearts. They also showed that an average maximal depth of 11 mm was created by application of an argon beam coagulator to similar tissue. They mentioned that the lesion made by the argon beam coagulator could be applied more rapidly than cryothermia, resulting in a visible color change in the tissue and indicating that an area has been covered. In our experiment, the maximum average depth was 10.3 mm. Because the ablation reached the endocardium, the thickness of the myocardium itself was a limiting factor for ablation, and therefore deeper ablation might have been obtained. In such a case, the linear index was biased, showing a value lower than the actual one. Hunt and associates [12] described cryolesions that could be made with circular tips, requiring 2.5 mm of overlapped lesions to ensure complete coverage. Although the lesions made by an infrared coagulator also require overlapping to obtain electrical block, this is not so important, as it takes only a few seconds to obtain enough atrial lesion depth.
For atrial ablation, the ideal device should able to create linear transmural lesions in a short time without perforation or unnecessary ablation. The argon laser and neodymium:yttrium-aluminum garnet laser are currently used clinically for treatment of arrhythmia [13, 14]. These instruments are expensive and large, and it is difficult to obtain linear ablation using them. Furthermore, lasers may cause perforation of the thin atrial wall. The IRK-151 infrared coagulator produces well-demarcated lesions that are easily detected with the naked eye and whose depth is controllable by adjusting the duration of application. The depth was found to be adequate for atrial ablation aimed at blocking the conduction pathway.
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Acknowledgments
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We thank Dr Isabelle Brazzalotto, Anesthesia Service, for her assistance in preparing the drawings, and Prof Eliane Albuisson, Biostatistics and Medical Informatics Service, University of Clermont-Ferrand, France, for statistical assistance. We are also grateful for the assistance of Dr Bruno Miguel and Prof Charles de Riberolles, Cardiovascular Surgery, University of Clermont-Ferrand, in the preparation of the manuscript. This work was supported by Grant in Aid for Scientific Research of the Japanese Ministry of Education, 1994–1995.
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References
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Abstract
Introduction
