2021 Volume 85 Issue 8 Pages 1394-1399
Background: It is unclear whether balloon size can influence lesion formation. The aim of this study was to evaluate the impact of balloon size on lesion formation during laser balloon procedures in an in vitro model.
Methods and Results: Laser energy was applied to chicken muscle using a first generation laser balloon. Laser ablation was performed with 2 different balloon sizes (18 mm and 32 mm) using 2 different power settings (12 W/20 s and 8.5 W/20 s) on the chicken muscle. The lesion characteristics, including maximum lesion depth, maximum lesion diameter, surface diameter and depth at maximum diameter, were compared between the 18-mm and 32-mm balloon groups at 12 W/20 s and 8.5 W/20 s, respectively. We created 40 lesions using laser energy at 12 W/20 s and 80 lesions at 8.5 W/20 s. At both power settings, the maximum lesion depth and the depth at the maximum diameter were larger in the 18-mm than in the 32-mm balloon group. At both power settings, the maximum lesion diameter and the surface diameter were smaller in the 18-mm than in the 32-mm balloon group.
Conclusions: The balloon size could affect the lesion formation during laser balloon ablation. The lesion with the larger balloon size was wider and shallower than the lesion with the smaller balloon size.
Catheter ablation is a widely accepted therapy for atrial fibrillation (AF), and pulmonary vein (PV) isolation is still the cornerstone of AF ablation.1 Not only radiofrequency focal tip catheter ablation, but also other balloon systems are used to achieve a PVI, such as with a cryoballoon2 or visually guided laser balloon (VGLB).3,4 The lesion formation with laser energy occurs with a different mechanism than that with radiofrequency current or cryothermal energy.5,6 The VGLB (HeartLight Endoscopic Ablation System; CardioFocus, Marlborough, MA, USA) includes a compliant balloon and a small endoscope to deliver laser energy with a 30° arc/spot. Various power settings can be selected depending on the myocardial thickness.
With radiofrequency current energy, it has been reported that the lesion size is related to the contact force and power settings.7 Nagase et al reported in an in vitro model that the lesion size created with laser energy depends on the power as well as the total laser energy.6 The VGLB is a unique ablation system, which does not require contact between the energy source and lesion; however, this system delivers ablation energy with an angle, and the distance between the energy source and lesion can be influenced by the balloon size. The aim of this study was to determine whether the balloon size influenced the lesion formation in an in vitro model.
The experimental models were designed according to a previous report using the VGLB.6 Commercially available chicken skeletal breast muscle was used. An 18-mm or 32-mm diameter hollow aluminum cylinder was stabbed into the muscle to create a laser balloon insertion site. The muscles were wrapped in plastic bags and indirectly warmed to 37–38℃ in a circulating bath maintained at ∼38℃. The muscles were transferred from the bathtub to a laser shielded box. Laser applications were immediately delivered using the first generation VGLB (HeartLight Endoscopic Ablation System; CardioFocus, Marlborough, MA, USA) to 5 segments per each specimen. No circulating bathtub was used during the laser application as previously reported,6 because clinically, the laser energy is titrated into the heart tissue under an optimal pulmonary vein occlusion to remove the blood. Two types of laser energy settings (8.5 W/20 s: 170 J, 12 W/20 s: 240 J) were applied with 2 different balloon sizes (18 mm, 32 mm). Laser energy was applied to the materials perpendicularly. Forty laser ablation lesions were created at 12 W/20 s with 18-mm and 32-mm balloon size and 80 lesions at 8.5 W/20 s with 18-mm and 32-mm balloon size. The experimental set-up is shown in Figure 1. Our local ethical committee approved this study.
Experimental set-up. (A) Laser balloon was inserted into the hole in the chicken skeletal breast muscle. The hole was created using an 18-mm or 32-mm diameter cylinder. (B) Laser applications were delivered to 5 segments per each specimen.
The lesion size was measured immediately after the final ablation of each muscle using electronic calipers. The muscles were sectioned to measure the lesion size (maximum depth, maximum diameter, depth at the maximum diameter, and surface diameter) (Figure 2). The laser energy is absorbed by the water in the tissue, and the maximum heat reaches a few millimeters under the endocardial level and propagates further.5 Therefore, we defined that a lesion was inadequate for evaluation when the surface lesion formation was assessed to be an unclear lesion by 2 observers.
Illustration of the lesion measurements. The lesion size was measured immediately after the final ablation of each chicken muscle using electronic calipers. The muscles were sectioned to measure the lesion size (maximum depth, maximum diameter, depth at the maximum diameter, and surface diameter).
Continuous variables are presented as the mean±SD and categorical variables as exact numbers and percentages. The parametric data were compared using an independent Student’s t-test or Fisher’s exact test. Values of a P<0.05 were considered statistically significant. The statistical analyses were performed using a commercially available statistical package (JMP Pro14; SAS Institute, Inc., Cary, NC, USA).
At a 12 W/20 s setting, all lesions ablated by the VGLB with 18-mm and 32-mm balloons were adequate for evaluating. Forty lesions in each group were analyzed. The maximum depth and depth at the maximum diameter were deeper in the 18-mm balloon group than in the 32-mm balloon group (4.7±0.7 vs. 3.8±0.4, P<0.001, 2.0±0.3 vs. 1.9±0.4, P=0.020, respectively). The maximum diameter and surface diameter were smaller in the 18-mm balloon group than in the 32-mm balloon group (7.2±0.6 vs. 7.7±0.7, P<0.001, 5.8±0.6 vs. 7.0±0.5, P=0.020, respectively) (Table 1, Figures 3A,4A).
| 12 W/20 s 18 mm (n=40) |
12 W/20 s 32 mm (n=40) |
P value | |
|---|---|---|---|
| Maximum depth (mm) | 4.7±0.7 | 3.8±0.4 | <0.001 |
| Maximum diameter (mm) | 7.2±0.6 | 7.7±0.7 | <0.001 |
| Surface diameter (mm) | 5.8±0.6 | 7.0±0.5 | <0.001 |
| Depth at maximum diameter (mm) | 2.0±0.3 | 1.9±0.4 | 0.020 |
Differences in the laser ablation lesion size for the 18-mm and 32-mm balloons with 12 W/20 s and 8.5 W/20 s. (A) Examples of laser ablation lesions with 12 W/20 s. With the 18-mm balloon, the maximum diameter and maximum depth were 5.7 mm and 4.7 mm, respectively. With the 32-mm balloon, the maximum diameter and maximum depth were 8.1 mm and 3.4 mm, respectively. (B) Examples of the laser ablation lesions with 8.5 W/20 s. With the 18-mm balloon, the maximum diameter and maximum depth were 4.9 mm and 3.7 mm, respectively. With the 32-mm balloon, the maximum diameter and maximum depth were 5.2 mm and 3.2 mm, respectively.
Lesion measurements (mean±SD) with 12 W/20 s and 8.5 W/20 s for the 18-mm and 32-mm balloons. (A) With 12 W/20 s, the maximum depth (4.7±0.7 vs. 3.8±0.4, P<0.001) and depth at the maximum diameter (2.0±0.3 vs. 1.9±0.4, P=0.020) were deeper in the 18-mm balloon group than in the 32-mm balloon group. The maximum diameter (7.2±0.6 vs. 7.7±0.7, P<0.001) and surface diameter (5.8±0.6 vs. 7.0±0.5, P=0.020) were smaller in the 18-mm balloon group than in the 32-mm balloon group. (B) With 8.5 W/20 s, the maximum depth (4.2±0.4 vs. 3.4±0.5, P<0.001) and depth at the maximum diameter (2.0±0.3 vs. 1.7±0.4, P<0.001) were deeper in the 18-mm balloon group than in the 32-mm balloon group. The maximum diameter (5.5±0.6 vs. 6.6±0.7, P<0.001) and surface diameter (4.7±0.5 vs. 6.1±0.5, P<0.001) were smaller in the 18-mm balloon group in the than 32-mm balloon group.
At a 8.5 W/20 s setting, all lesions ablated by the VGLB with 18-mm and 32-mm balloons were adequate for evaluating. Eighty lesions in each group were analyzed. The maximum depth and depth at the maximum diameter were deeper in the 18-mm balloon group than in the 32-mm balloon group (4.2±0.4 vs. 3.4±0.5, P<0.001, 2.0±0.3 vs. 1.7±0.4, P<0.001, respectively). The maximum diameter and surface diameter were smaller in the 18-mm balloon group than in the 32-mm balloon group (5.5±0.6 vs. 6.6±0.7, P<0.001, 4.7±0.5 vs. 6.1±0.5, P<0.001, respectively) (Table 2, Figures 3B,4B).
| 8.5 W/20 s 18 mm (n=80) |
8.5 W/20 s 32 mm (n=80) |
P value | |
|---|---|---|---|
| Maximum depth (mm) | 4.2±0.4 | 3.4±0.5 | <0.001 |
| Maximum diameter (mm) | 5.5±0.6 | 6.6±0.7 | <0.001 |
| Surface diameter (mm) | 4.7±0.5 | 6.1±0.5 | <0.001 |
| Depth at maximum diameter (mm) | 2.0±0.3 | 1.7±0.3 | <0.001 |
The present study demonstrated the influence of balloon size in lesion formation during VGLB ablation. We found that wider and shallower lesions were created in the 32-mm balloon group than in the 18-mm balloon group at both the 8.5 W/20 s and 12 W/20 s settings.
The biophysics of lesion formation have been described previously, especially in the field of radiofrequency focal tip ablation.7,8 The lesion size created by radiofrequency energy is correlated with ablation duration, energy settings, and contact force. The formula to predict lesion formation is clinically used and has been described to improve clinical outcomes.9,10 In the field of VGLB ablation, Nagase et al reported that the lesion size created by the VGLB depends on the power as well as the total energy.6 However, to the best of our knowledge, no data are available on the influence of the balloon size on lesion formation. We had the following hypothesis that laser energy is delivered with a 30° arc/spot. Therefore, the lesion surface area would be wider when a larger balloon size is adopted. Because the total energy was divided by a larger surface area, the energy/area would decrease (Figure 5). It is also though that energy absorption by heavy water was increased and that the energy was attenuated. However, it has been reported that very little laser energy is absorbed by heavy water,11 and it is considered that the effect of energy attenuation due to a large balloon size would be very small. Our result, that wider but shallower lesions were created by both the 8.5 W/20 s and 12 W/20 s setting in the 32-mm balloon group compared to that in the 18-mm balloon group, supported this hypothesis.
An illustration of the difference in the lesion surface area and energy/area according to the distance from the laser source to the lesion.
During VGLB ablation, a higher energy setting of ≥8.5 W has been reported to be effective for acute and chronic outcomes without comprising safety.12,13 Those reports did not include any information on the balloon size. The balloon used with the VGLB system is a compliant balloon. The balloon size is set from 1 to 9 and is changed by adjusting the circulating deuterium oxide flow rate. The balloon diameter is not always equal, even with the same settings. Also, the laser shaft may not be positioned in the center of the balloon due to the deformation of the balloon, so the distance between the laser generator and tissue surface would be the determinant, not the balloon size itself. Therefore, it is important to imagine the distance from the laser shaft to the lesion surface when considering the anatomical findings from endoscopy and fluoroscopy during VGLB ablation. Further studies to determine the optimal laser energy settings, including the balloon size, are needed to improve the clinical outcomes of VGLB ablation. It is also important to keep it in mind that irradiation on a narrow distal portion may result in an unexpectedly deeper lesion because of the pear shape, even though the balloon size and flow rate settings are the same.
Several studies have reported that a high-power short duration ablation using radiofrequency energy creates larger diameter lesions with a smaller depth and provide better clinical outcomes compared to conventional power settings.14–16 Our result that a larger balloon size would create wider and shallower lesions as compared to a smaller balloon size was not in-line with those previous studies. There remains doubt whether lesion depth created using a larger balloon is adequate to create transmural lesions in thick muscle such as the left pulmonary vein anterior ridge.17 The average maximum depth using the 32-mm balloon was 3.4 mm with the 8.5 W/20 s setting and 3.8 mm with the 12 W/20 s setting. The results of the lesion size were comparable to the previous report using VGLB ablation on a chicken skeletal muscle.6
Clinical ImplicationsThe myocardial wall thickness varies depending on the location. An unnecessary deeper lesion formation might introduce collateral organ damage, and an inadequate lesion formation might introduce an incomplete block or a late reconnection. When using VGLB ablation, an adjustment not only of the power setting, but also the balloon size according to the anatomy of the pulmonary veins and left atrium, would decrease the complications and improve clinical outcomes.
Study LimitationsThis study had some limitations. First, the experimental tissue was dead tissue. The results of the VGLB ablation lesion size on the chicken skeletal muscle cannot be adopted in humans because VGLB ablation lesions might be affected by the tissue color and absorption. In terms of the passive cooling effect, the room temperature was lower than the body temperature, and the experimental lesions in this study would have been smaller than that in real-world practice. However, in terms of the ablation stability, the experimental model was free from any heart beating and respirations, and the experimental lesions would have been larger in real-world practice. However, this experimental model was accepted as a VGLB experimental study type6,18 and we used the same material in different settings. In vitro experiments in medium-sized animals that are close to humans are desirable to prove the effects of the balloon size on lesion formation. Second, the anatomy of the experimental model was not same as that of the PV ostium. The laser energy was not always delivered perpendicularly at the human PV ostium. However, the concept of the distance depending on lesion formation is still important in clinical practice. Third, our experimental model did not include a blood circulator model during ablation. In a real VGLB PV isolation, blood floats around the occluded heart tissue. Blood reacts with the laser to produce heating effects, including carbonization, and cooling effects on heart tissue. According to the above described limitations, further in-vivo experimental studies are warranted. Finally, our findings were based on a gross pathological examination. It would have been better to have fixed the materials in formalin and to have quantified them with a histopathological analysis.
Lesion formation with visually guided laser balloon ablation was influenced by balloon size. A larger balloon size would create wider and shallower lesions than a smaller balloon size during VGLB ablation. Clinicians should consider adjusting not only the power setting, but also the balloon size according to the anatomy of the pulmonary veins and left atrium when performing VGLB ablation.
The authors wish to thank Mr. John Martin for his linguistic assistance with this manuscript. The authors thank Japan Lifeline for permitting us to use the HeartLight training center.
The authors declare no conflict of interest for this article.
None.
The authors declare no conflicts of interest for this article. Y.S. is a member of Circulation Journal’s Editorial Team.
Osaka Rosai Hospital (IRB study number: 2020-20) provided permission to conduct this study.
