Ann Thorac Surg 1996;61:164-169
© 1996 The Society of Thoracic Surgeons
Original Articles: General Thoracic
In Vitro Study of Ablated Lung Tissue in Nd:YAG Laser Irradiation
Noriyoshi Sawabata, MD,
Kunimoto Nezu, MD,
Takashi Tojo, MD,
Soichiro Kitamura, MD
Department of Surgery III, Nara Medical College, Nara, Japan
Accepted for publication August 15, 1995.
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Abstract
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Background. Neodymium:yttrium-aluminum garnet lasers are used to reduce lung volume. An assessment of the relationship between the histologic and gross findings in the lung irradiated by a laser would be helpful in laser-assisted pneumoplastic procedures.
Methods. In vitro lung lobes surgically resected for pulmonary carcinomas were irradiated with a neodymium:yttrium-aluminum garnet laser at three energy levels in three modes: contact rubbing, contact pointing, and noncontact. Pleural degeneration in 216 samples from 24 lobes was classified as coagulative, amorphous, or destructive. At all energy levels, the laser was applied for 1.5 seconds.
Results. Noncontact mode at 7.5 W or 15 W and contact rubbing at 5 W caused coagulative or amorphous degeneration but no destructive degeneration. The energy level correlated with the color of the degenerated pleura. The incidence of destructive pleural degeneration, which led to air leaks as revealed by an air inflation test, was 0% in pink and white samples, 59% in brown samples, and 100% in black samples (p < 0.0001, white versus brown samples).
Conclusions. In neodymium:yttrium-aluminum garnet laser ablation of lung tissue, the color of the degenerated pleura correlates with the intensity of the applied laser energy and the degree of pleural degeneration.
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Introduction
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Volume-reduction surgical intervention is helpful in some patients [1–3]. Ablation of the pleura and pulmonary bullae is one of the surgical techniques to reduce lung volume (volume-reduction pneumoplasty). This method is applied using video-assisted thoracoscopic techniques in patients with diffuse emphysematous bullous disease [4, 5]. The potential damage to the lung tissue caused by optosurgical and electrosurgical devices differs with the equipment and the technique. The electrosurgical unit causes the most extensive damage, followed by the neodymium:yttrium-aluminum garnet (Nd:YAG) laser and the carbon dioxide laser [6]. The most commonly used laser in thoracic operations is the Nd:YAG laser in both contact and noncontact modes. In the present study, we conducted a morphologic examination of in vitro lung tissue damage caused by the Nd:YAG laser to assess the relationship of the histologic to the gross findings.
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Material and Methods
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Pulmonary lobes that had been surgically resected during treatment of lung carcinoma were washed in normal saline solution for several minutes to remove blood from the surface. We examined laser-irradiated lung samples from 24 patients, 10 men and 14 women, ranging in age from 58 to 72 years (mean age ± the standard deviation, 63.4 ± 6.8 years).
Nd:YAG Laser Irradiation
An Nd:YAG laser (model CL50; SLT Japan, Tokyo, Japan) was set to define the pulse-wave mode and the air-cooling mode (2.0 L/min). Pulmonary lobes were irradiated in one of three patterns: (1) noncontact mode, in which the lung tissue was irradiated with maintenance of a spot size 1 cm in diameter at the power of 7.5, 15, or 30 W for 1.5 seconds; (2) contact rubbing, in which the lung tissue was rubbed with a round sapphire tip 3 mm in diameter at 5, 10, or 15 W for 1.5 seconds; and (3) contact pointing, in which the lung tissue was touched with the contact tip at 5, 10, or 15 W for 1.5 seconds.
Air Inflation Test
After irradiation, the lung lobe was submerged in normal saline solution and inflated through the airway with a plastic tube and a ventilator with the airway pressure held constant at 25 cm H2O to detect any air leak in degenerated lung tissue.
Microscopic Examination
After the air inflation test, the lung irradiated by the Nd:YAG laser was harvested for fixation in 2.5% phosphate-buffered glutaraldehyde for 2 hours. For light microscopic examination, the samples were divided in two, dehydrated in a graded alcohol bath, fixed in paraffin, sliced, and stained with hematoxylin and eosin and elastica Van Gieson's. For scanning electron microscopic examination, samples were stained with osmic acid, dehydrated in a graded alcohol series, substituted by isopentyl acetic acid, dried with a critical-point drier, and spatter-coated with gold ion colloid.
Classification and Measurement of Degenerated Pleura
The treated visceral pleura was classified by color as pink, white, brown, or black by macroscopic inspection. Microscopic findings in the pleural tissue were categorized into one of three patterns on the basis of a previously reported classification [7]: (1) coagulative degeneration, in which much of the cellular outline and tissue architecture was still discernible; (2) amorphous degeneration, in which cells formed a continuous amorphous mass with no resemblance to the original architecture except for the presence of elastic fiber; and (3) destructive degeneration, in which cells formed debris without recognizable cellular structures (Fig 1
). Coagulative degeneration was distinguished from normal or amorphous degeneration by the surface structure of the degenerated pleura as assessed by scanning electron microscopy. When the net formation was still discernible by scanning electron microscopy, the area was considered to be coagulative, whereas it was defined as being amorphous when no discernible structure was present (Fig 2). The surface area (mm2) of each type of degenerated pleura was evaluated using scanning electron microscopy.
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Fig 1. . (A) Normal pleura. (B) Coagulative degeneration. Elastic fibers and collagen are contracted but still discernible. (C) Amorphous degeneration. Collagen fibers have become amorphous, but severely contracted elastic fibers are still discernible. (D) Destructive degeneration. The pleura is destroyed, and neither elastic fibers nor collagen is discernible. (Elastic van Gieson's; x400 before 28% reduction.)
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Fig 2. . (A) Scanning electron micrographs showing (A) coagulative degeneration and (B) amorphous degeneration. In the former, collagen is still discernible, but in the latter, the collagen has become amorphous and flat. (x5,000 before 28% reduction.)
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Statistical Analysis
Data were evaluated using a computer statistical package (StatView II; Abacus Concepts, Inc, Berkeley, CA). Analysis of variance, 
2 tests, and Fisher's exact tests were used as appropriate. Significance was accepted at a p value of less than 0.05.
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Results
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Twenty-four pulmonary lobes were treated at each energy level by each laser method. The mean area of degenerated pleura in the 24 lobes (total, 216 samples) after treatment is shown in Table 1. In the majority of the combinations of energy power and method, amorphous degeneration was the most common pattern, although noncontact mode at 7.5 W and contact rubbing at 5 W frequently resulted in coagulative degeneration. By all methods, the total area of pleural degeneration increased with the irradiation energy level. Of the two contact methods, pointing resulted in a larger area of destructive degeneration than rubbing at every power level.
The frequency distribution of air leaks is shown in Table 2. The air inflation test revealed that leakage occurred in samples with pleural destruction without parenchymal coagulation, whereas in samples without pleural destruction or parenchymal coagulation, air leak did not occur. Moreover, samples with pleural destruction as well as underlying parenchymal coagulation did not show leakage (Fig 3). Some of the samples irradiated using noncontact mode at 15 or 30 W or by contact pointing at 15 W developed an air leak. At 15 W, noncontact spotting produced air leaks more frequently than did contact rubbing.
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Fig 3. . Degenerative patterns in lung tissue treated with neodymium:yttrium-aluminum garnet (Nd:YAG) lasers. (A) The pleura demonstrates destructive degeneration, and the parenchyma shows no coagulation (air leak pattern). (B) The pleura shows amorphous degeneration, and some portions of the parenchyma have no coagulation. (C) The pleura demonstrates destructive degeneration, and the parenchyma shows coagulation. In cases of amorphous or coagulative degeneration of the pleura (such as B) or coagulated parenchyma (such as C), slight air leak is seen on the air inflation test. (Top: scanning electron microscopy; x500 before 33% reduction; bottom: hematoxylin and eosin; x100 before 33% reduction.)
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The frequency distribution of pleural tissue degeneration caused by the Nd:YAG laser classified according to the most severely degenerated portion is shown in Table 3. At 15 W, no destructive degeneration was observed with the noncontact mode, but it was seen in 21 (88%) of the samples that underwent contact rubbing (p < 0.0001) and all 24 of the samples that underwent contact pointing (p < 0.0001). Noncontact mode at either 7.5 or 15 W and contact rubbing at 5 W caused coagulative or amorphous degeneration but no destructive degeneration. In contrast to contact rubbing at 5 W, contact pointing at 5 W resulted in destructive degeneration in 12 samples (50%) (p < 0.0001).
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Table 3. . Classification of Neodymium:Yttrium-Aluminum Garnet Laser–Ablated Samples by Most Severe Degeneration Presenta
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The frequency distribution of the color of the degenerated pleural sample is shown in Table 4. The intensity of and the difference in the color (pink, white, brown, black) of the tissue after laser ablation correlated directly with the intensity of the applied laser energy. Black pleural changes were not observed in noncontact spotting, and pink pleural changes were not seen in contact pointing.
The frequency distribution of the type of degeneration classified by tissue surface color is shown in Table 5. Of 16 pink pleural samples, 10 (63%) showed coagulative degeneration. The white pleural samples frequently showed amorphous degeneration, and the brown samples had both amorphous and destructive degeneration. All black samples showed destructive degeneration.
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Comment
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The surgical techniques for the treatment of bullous disease using video-assisted thoracoscopic surgical procedures include resection of bullae [8, 9] and ablation of bullae and pleura without destruction [4, 5]. Although a useful device in the resection of bullae [9], the stapler sometimes cannot be applied in cases of diffuse bullous emphysema. Neodymium:yttrium-aluminum garnet lasers [9], carbon dioxide lasers [4, 5], contact electrocauterization [10], and argon beam coagulators (noncontact electrocauterization) [8] are used to ablate the bullae and the pleura. Experimentally, laser devices produce less tissue damage than electrocautery, and their application accelerates the healing of incisions [11]. In volume-reduction pneumoplasty, therefore, laser devices appear to be more suitable than electrosurgical devices because they have less potential to cause lung tissue damage [6, 12].
Two irradiation modes, continuous wave and pulse wave, can be applied for Nd:YAG laser irradiation [13, 14]. Continuous-wave irradiation sometimes causes tissue effects distant from the surface at the point where the laser beam makes contact with the tissue. In contrast, pulse-wave irradiation causes degeneration near the contact area [15]. Therefore, we chose the pulse-wave mode to avoid remote thermal injury that could cause pulmonary damage.
The most common perioperative complication in volume-reduction pneumoplasty is an air leak occurring secondary to pleural degeneration [4, 5]. Pleural degeneration can be classified into three types: coagulative, amorphous, and destructive degeneration [7]. Laser-associated amorphous tissue degeneration is caused by contraction of elastic fibers and formation of amorphous structures from collagen fibers, both of which are induced by a photothermal reaction. The incidence of destructive degeneration in the pleura corresponds to the incidence of air leak, as shown by this study.
High-energy contact pointing and high-energy noncontact mode, which cause destructive degeneration of the pleura, are not suitable for ablation therapy. Contact rubbing under the energy conditions of 5 W and noncontact mode at either 7.5 or 15 W are the techniques with the lowest risk of development of air leak because they produce coagulative and amorphous degeneration with only slight destructive degeneration in the pleura, even though the total amount of pleural degeneration increases with the irradiation energy. Even when destructive degeneration occurred in the pleura, parenchymal coagulation protected against air leak. Cole and Wolfe [16] studied the histologic changes of laser-treated lung tissue over time and noted that parenchymal coagulation produced intensive fibrosis and that the underlying lung showed alveolar hyperemia 1 week after laser treatment.
During operation, one guide to assess the degree of pleural degeneration is the gross appearance by color of the tissue. In our experiments, the intensity of and difference in tissue color correlated with the intensity of the applied laser energy and the lung tissue degeneration. Amorphous degeneration occurred in many white specimens, whereas no white specimens showed destructive degeneration. In contrast, the brown samples showed both amorphous and destructive degeneration. The brown degenerated pleura exhibited a 59% incidence of destructive degeneration after Nd:YAG laser irradiation. Contact rubbing at 5 W or noncontact mode at 7.5 W did not produce brown discoloration of the pleura, whereas noncontact mode at 15 W produced brownish discoloration in 25% of the pleural samples.
The color of the treated pleura correlated with the intensity of the applied laser power and the degree of the pleural degeneration. This information may be useful in video-assisted thoracoscopic surgical intervention for volume-reduction pneumoplasty using Nd:YAG lasers.
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Footnotes
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Address reprint requests to Dr Sawabata, Department of Surgery III, Nara Medical College, 840 Shijho-cho Kashihara City, Nara, Japan 634.
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References
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Abstract
Introduction
