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(Chest. 2001;120:934-940.)
© 2001 American College of Chest Physicians

Nd-YAG Laser Damage to Metal and Silicone Endobronchial Stents*

Delineation of Margins of Safety Using an In Vitro Experimental Model

Julius J. Dalupang, MD; Thomas G. Shanks, MPH and Henri G. Colt, MD, FCCP

* From Interventional Pulmonary Services, Pulmonary and Critical Care Medicine Division, University of California, San Diego Medical Center, San Diego, CA.

Correspondence to: Henri G. Colt, MD, FCCP, Director, Interventional Pulmonary Services, UCSD-La Jolla, 9310 Campus Point Dr, San Diego, CA 92037; e-mail: hcolt{at}ucsd.edu


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Study objective: To identify margins of safety within which bronchoscopic Nd-YAG laser resection can be performed without damaging indwelling tracheobronchial stents.

Design: Experimental in vitro study simulating a patient-care environment.

Methods: Uncovered and covered metal Wallstent (Schneider; Zurich, Switzerland) and Dumon (Bryan Corporation; Woburn, MA) silicone stents were deployed in the tracheobronchial tree of a ventilated and oxygenated (fraction of inspired oxygen, 40%) heart-lung block of a dead canine. Rigid bronchoscopic Nd-YAG (1,064 nm) laser procedures were performed in order to deliver laser energy using fiber-to-target distances of 10 mm and 20 mm, and noncontact, continuous-mode, 1-s pulses at power settings of 10 W, 30 W, and 40 W. The major outcome measure was laser-induced stent damage, defined as discoloration, ignition, or breakage. This was assessed using six power densities: 75 W/cm2, 172 W/cm2, 225 W/cm2, 300 W/cm2, 518 W/cm2, and 690 W/cm2.

Results: The uncovered Wallstent and the silicone stent remained intact at power densities of 75 W/cm2 (10 W, 20 mm) and 172 W/cm2 (10 W, 10 mm), but were damaged at power densities > 225 W/cm2 (30 W, 20 mm). The covered Wallstent was damaged at all power densities tested.

Conclusion: Uncovered Wallstent and silicone stents are not damaged when Nd-YAG laser energy is delivered using power densities <= 172 W/cm2 (10 W, 10 mm). Covered Wallstents, however, had a high likelihood of ignition at all power densities studied.

Key Words: bronchoscopy • complications • laser physics • laser/tissue interactions • metal stents • Nd-YAG laser resection • power density • silicone stents


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Tracheobronchial stents are increasingly used to palliate benign or malignant airway disease. Although recommendations exist to ensure safe bronchoscopic Nd-YAG (1,064 nm) laser resection for central airways obstruction, guidelines for laser usage in patients with indwelling endobronchial stents have not been established.

The paucity of data pertaining to laser/stent interactions is increasingly important because recurrent obstruction from either granulation tissue or tumor occurs in 9 to 67% of patients with metal stents1 2 3 4 5 6 7 8 and in 6 to 15% of patients with silicone stents.9 10 11 In these patients, laser-induced stent damage may cause substantial morbidity from airway burn injury in case of stent ignition, or airway wall and vascular perforation in case of metal stent rupture.

In patients without indwelling stents, it is generally recommended that laser resection be performed using laser pulse durations of 1 to 2 s, a power output of <= 40 W, and a fraction of inspired oxygen (FIO2) <= 0.40.12 Using these recommendations as a starting point, the purpose of this study was to identify margins of safety defined by the classic measures of power density and pulse frequency within which Nd-YAG laser energy could be delivered without damaging indwelling silicone and metal stents.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Theoretical Basis for Study Design
When a laser beam reaches a target material, radiant energy is absorbed, reflected, and transmitted. This distribution of energy, defined by Beer’s law,13 is dependent on the optical properties of the material and the wavelength characteristic of the laser light (1,064 nm in the case of Nd-YAG laser). As absorbed energy is converted into heat, a rise in the temperature of the target material occurs. Based on Fourier’s law of heat transfer14 and on laws of thermodynamics, the temperature of the material will rise above threshold levels only if the absorbed power density exceeds the capacity of the material to conduct heat away from the impact site. Power density, therefore, is directly proportional to the power setting of the laser (in watts) but inversely proportional to spot size (in centimeters squared) of the laser beam. The relationships between these optical and thermal phenomena can be mathematically represented as15 :

where {kappa} is the thermal conductivity, tP the duration of irradiation, {rho} the density of the material, CV the specific heat per unit volume, R the reflectivity, and I0 the laser intensity. The expression {kappa}tP1/2- is the thermal diffusivity or penetration depth of the laser beam, while (1-R) is the absorption coefficient.

For example, at low power densities, the poor absorption of the Nd-YAG laser and its pronounced Mie and Rayleigh scattering result in slow homogeneous heating of a large volume of tissue without serious mechanical damage to the tissue surface.16 At high power densities, however, the temperature 2 to 3 mm below the tissue surface rises rapidly, prompting vaporization of water content and a pocket of steam with a pressure high enough to rupture overlying tissues.17 Based on these principles, we hypothesized that certain quantities of laser energy, defined by power density, would cause local heating without the extreme temperature elevation required to disrupt stent integrity or prompt stent ignition.

In Vitro Experimental Model
In order to enhance applicability of our results to the clinical setting, the following experimental environment simulating a rigid bronchoscopic intervention in patients was designed. An intact heart-lung-tracheobronchial tree was harvested from a dead adult canine and cleaned thoroughly. The entire structure was suspended at a slope of 60° (Fig 1 ). Throughout the experiment, this lung-tracheobronchial model was intubated with a rigid bronchoscope (EFER-Dumon Rigid Bronchoscope; Bryan Corporation; Woburn, MA) and connected to a Harvard respirator pump for animals (Harvard Apparatus; Holliston, MA) delivering a tidal volume of 400 mL per breath. Respiratory frequency was set at 12 cycles per minute, and oxygen delivery through the system was set at an FIO2 of 40%. The rigid telescope, laser fiber, and suction catheters were introduced through sealed rubber obturator caps, maintaining a closed system so that two-lung ventilation was maintained throughout the intervention. This protocol was reviewed and approved by the University of California-San Diego (UCSD) Institutional Animal Care and Use Committee (IACUC) [IACUC tissue request No. T 99154]. No animals were killed for the sole purpose of this study; tissue was transferred from a previously killed animal in a separate IACUC-approved protocol.



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  		Figure 1.. The heart-lung block, including the tracheobronchial tree, harvested from an dead adult canine.

 
Experimental Protocol
Under direct bronchoscopic guidance, stents were sequentially inserted into the central bronchi. In three series of experiments, stents studied were as follows: (1) uncovered 10-mm by 30-mm Wallstents (Schneider; Zurich, Switzerland) made of cobalt-based superalloy Elgiloy filaments braided into a cylindrical mesh; (2) 10-mm by 30-mm Wallstents (Schneider) covered with 1.0-mm thick polyurethane (European model stent); and (3) 10-mm by 30-mm stents made of molded silicone with external surface studs (Bryan Corporation). A new stent was used for each individual experiment.

For all experiments, a 100-W–capacity Nd-YAG laser (model 813; Laserscope; San Jose, CA) and saline solution-cooled, 0.6-mm–diameter coaxial fiber (Endostat; Laserscope) were used. The fiber was placed through the rigid bronchoscope and directed tangentially toward the inner side of the proximal extremity of each indwelling stent. The helium-neon focusing laser beam was used to ensure that the laser was fired parallel to the airway wall in the direction of the target surface.

Using noncontact, 1-s duration, and continuous mode, laser energy was delivered at power settings of 10 W, 30 W, and 40 W. At least 1 s separated each pulse of laser energy delivered. The distance between the distal aspect of the laser fiber and the target was precisely measured and maintained by fixing the fiber to a ruler attached to the obturator cap of the rigid bronchoscope. For each of the three power settings, laser fiber-to-target surface distances of 10 mm and 20 mm were used.

The average power density delivered for each setting was computed based on the formula18 : PAW = 0.8635 x power (watts)/{pi} r2 where {pi} r2 is the spot size, and PAW is average power density. Because the laser beam diverges 11° around the fiber core, the spot size at a distance was calculated using the following equation: spot size = {pi} [(fiber radius in centimeters) + (distance in centimeters) x tan 5.5°]2. Using fiber-to-target distances of 10 mm and 20 mm, therefore, spot sizes were 0.05 cm2 and 0.155 cm2, respectively. Computed power densities for each experimental condition are shown in Table 1 .


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  		Table 1.. Power Densities at Each Power Setting and Fiber-to-Target Distance*

 
Outcome Measures and Statistics
Laser energy was delivered until stent damage occurred or a total of 60 impulses were delivered. Stent damage was defined as visualization of blackish discoloration, blister formation, perforation, actual ignition, or rupture of the integrity of the stent. Rupture of integrity of the metal stent was defined as a spontaneous break in the continuity of the metal mesh during laser application.

The total number of pulses and the total energy required to damage a stent during each experimental condition were recorded and expressed as mean ± SD. Comparisons of mean number of pulses and mean total energy delivered were made using paired Student’s t test at a level of significance of 0.05.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
The uncovered Wallstent was not damaged despite 60 pulses at power densities of 75 W/cm2 (10 W, 20 mm) and 172 W/cm2 (10 W, 10 mm). The wire mesh broke, however, at power densities >= 225 W/cm2 (30 W, 20 mm; Table 2 ).


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  		Table 2.. The Total Number of Pulses and Total Energy Required To Damage the Uncovered Wallstent, Covered Wallstent, and Silicone Stents*

 
The covered Wallstent was damaged using a power density of only 75 W/cm2 (10 W, 20 mm), prompting a black discoloration at the impact site. This occurred after delivering a mean of 36 pulses (Table 2) . Black discoloration of the stent was also observed at 172 W/cm2 (10 W, 10 mm). At power densities of >= 225 W/cm2, black discoloration was immediately followed by perforation of the covering of the Wallstent (Fig 2 ). This occurred after a mean of only 2.6 pulses were delivered. At the highest power density tested, 690 W/cm2 (less than two pulses at 40 W, 10 mm), the stent abruptly burst into flames.



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  		Figure 2.. The perforated and discolored covered Wallstent after application of laser energy at 518 W/cm2 (30 W, 10 mm).

 
The silicone stent was not damaged despite 60 pulses at power densities of 75 W/cm2 (10 W, 20 mm) and 172 W/cm2 (10 W, 10 mm). At power densities of >= 225 W/cm2, a slight discoloration was noted at the impact site after a mean of <= 3.6 pulses (Table 2) . A blister formed, accompanied by focal black discoloration at the impact site at a power density of 690 W/cm2 after less than two pulses were delivered, although the stent did not ignite (Fig 3 ).



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  		Figure 3.. The discolored studded silicone stent after application of laser energy at 690 W/cm2 (40 W, 10 mm).

 
When distance was maintained constant at 10 mm or 20 mm (p < 0.05; Table 2 ), the mean number of pulses and mean total energies (in joules) at which stents were damaged significantly decreased as power settings were increased. The mean number of pulses and mean total energies at which stents were damaged were not significantly different, however, when fiber-to-target distance increased from 10 to 20 mm while maintaining power settings constant. Consequently, no definite relationship was found between power density and total energy at which stent damage occurred (Fig 4 ).



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  		Figure 4.. The relationship of power density (watts/centimeters squared) and the total energy (joules) required to destroy the stents. Prolonged irradiation using the safe power densities (shaded area) can be performed in uncovered Wallstent and silicone stents without stent destruction. The uncovered Wallstent was destroyed at all power densities applied.

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
This experimental study realistically simulates a patient-care setting in which rigid bronchoscopic laser resection is performed in the presence of an indwelling stent while using an FIO2 <= 40%. Results clearly identify a margin of safety for laser resection when power density is <= 172 W/cm2, or when a power setting of 10 W is applied with a laser fiber-to-target distance of at least 10 mm.

Our observation that 60 pulses at 10 W applied 10 mm from the target did not damage either the uncovered Wallstent or the silicone stent reinforces that of Witt et al,19 who found that uncovered Wallstent, Strecker, and Palmaz metal stents were destroyed when noncontact Nd-YAG laser was used at power settings > 10 W. In fact, these investigators19 found that metal stents were destroyed after just one 25-W pulse delivered 4 mm from the target, prompting them to conclude that Nd-YAG laser should never be used in patients with indwelling metal stents.

A few researchers might contend that lower power densities are not frequently used in bronchoscopic practice in which some operators are accustomed to higher power settings and shorter laser fiber-to-target distances. We suggest that the effects of laser resection using these settings should be further investigated. For example, in an Nd-YAG dosimetry study performed in porcine bladder, Cos and Di Sant’agnese20 demonstrated that 10 W applied for 10 s produced far more effective penetration of laser energy than 100 W for 1 s. Although these investigators20 used a fiber-to-target distance of only 2 mm, it is possible that the effect would be similar if applied at a greater distance because of the dynamic change of absorption with repetitive irradiation.16 21 22

Although spectroscopic studies23 24 have shown similar alteration in optical properties of organic polymers with repetitive irradiation, the paucity of adverse effects from prolonged irradiation at 10 W may be related to the cooling times of the stent materials. Before metals and organic polymers are uniformly heated, absorbed laser energy is equitably partitioned among electrons. Expressed as "cooling time," this equipartition may be as short as 10-13 s in metals, and between 10-12 to 10-6 s in nonmetals.25 26

In our study, it is noteworthy that the covered Wallstent was damaged at an even lower power density (75 W/cm2; 10 W, 20 mm). The covering of the original model of the self-expanding Wallstent is a thermoplastic elastomer made with a urethane backbone. This material does not have the high-temperature resistance of conventional vulcanized rubber and cannot tolerate temperatures of 88°C to 100°C.27 This is probably why the covered Wallstent ignited immediately at a power setting of 40 W.

We also found that uncovered Wallstents, covered Wallstents, and silicone stents were all damaged at power densities > 225 W/cm2 and at power settings > 30 W. The thermal conductivity of most metals ranges from 0.66 to 4.180 W/cm2/°C/cm,28 while that of silicone rubber is 6 to 12 W/cm2/°C/cm.27 Comparatively, the thermal conductivity of water, a major constituent of biological tissues, is only 0.005 W/cm2/°C/cm.28 A radiant power density of 1460 W/cm2 is required to vaporize biological tissues.22 Despite their greater thermal conductivities, all stents were damaged at power densities commonly used to coagulate rather than to vaporize tissues. This might indicate that stent materials have greater absorptivity of Nd-YAG laser energy than biological tissues. Sainte-Catherine and coworkers29 observed, for example, that metal surfaces had greater absorptivity for Nd-YAG laser energy than CO2 laser energy. This finding is contrary to the known behavior of Nd-YAG and CO2 laser systems in human tissues.

We believe the potential for increased absorptivity of laser energy by stents is real because, as in the case of the covered Wallstent, temperatures rise rapidly, prompting stent ignition without warning. Obviously, the presence of dark pigment such as blood in or around the stent would further increase the absorption of Nd-YAG laser radiation, precipitating stent ignition and airway burn injury.

The silicone stent, although becoming discolored, never ignited. Perhaps this is because the polydimethylsiloxane chain, the basic skeleton of the silicone stent, can withstand temperatures ranging from - 70°C to + 250°C.27 We ceased laser firing, however, the moment we saw discoloration rather than continuing until stent ignition. The reason for this is that in actual clinical practice, the severely grave consequences of airway burn injury must be avoided, and it is well recognized that Nd-YAG laser irradiation of darkly pigmented or charred material increases surface absorption.30

Although our study suggests parameters that might allow granulation or tumor tissue surrounding a silicone stent to be resected without removing the stent, careful attention must be paid to these parameters and to the rules governing safe bronchoscopic laser resection. The importance of laser/tissue interactions was well demonstrated by Scherer,31 who, in a recently published in vitro study, was able to ignite blood or soot-covered silicone stents using multiple laser power settings, whereas clean silicone stents could not be ignited regardless of power density or oxygen concentration.

Our study might raise a question pertaining to the effect of local temperature on laser/tissue interaction. The threshold temperature at which materials are damaged by laser energy is more easily reached when the baseline temperature of a target material is higher and nearer the threshold temperature. The laser energy delivered in our study was applied to tissues residing within an experimental model at less than true body temperature, but greater than room temperature. Predictably, the stents placed in our model were also at higher baseline temperature, and thus more easily damaged by laser energy than if operated on at room temperature.

Another potential concern about our study is that the laser beam was focused on each stent, rather than on surrounding tissues. During tissue removal of granulation tissue, the laser beam is usually focused on the tissue itself, which may or may not be directly adjacent to the indwelling stent. Because laser energy is easily scattered and reflected, however, collateral absorption is frequent. Laser energy delivered onto surrounding tissues, therefore, will also be frequently absorbed by adjacent foreign bodies. In our study, stents were frequently damaged after only one or two impulses at higher power densities, suggesting that substantial variation of target points during bronchoscopic laser resection would be necessary to prevent inadvertent stent damage.

Because uncovered Wallstent and silicone stents were damaged after just a few pulses at 30 W and 40 W, but were not affected by prolonged irradiation using 10 W, questions are raised pertaining to the concept of energy density per pulse (defined as power density [watts/centimeters squared] multiplied by pulse duration in seconds) as a defining factor of laser safety. We cannot address this issue, however, because we did not vary impact duration times in our study. In the study by Scherer,31 impact duration significantly contributed to the likelihood of silicone stent ignition, prompting that author’s recommendation to maintain impact duration to <= 1 s when using power settings of >= 40 W.

Finally, both in our study and in that of Witt et al,19 there is an absence of a significant relationship between power density and total energy (in joules) delivered before stent damage occurs. Probably, this is related to the lack of influence of distance on total energy delivered. It is tempting to presume that the total energy needed to damage a stent decreases as power density increases. As shown in Figure 4 , however, at various power densities damaging all three stents, the total energies delivered were similar. For instance, at power densities of 300 W/cm2 (40 W, 20 mm) and 690 W/cm2 (40 W, 10 mm), differences in total energy delivered to damage an uncovered Wallstent were not statistically significant (54.3 ± 21.8 J vs 56.3 ± 18.5 J, p < 0.85). Similar observations were made for silicone stents damaged at power densities > 225 W/cm2, and covered Wallstents, which were damaged at all power densities.

One reason why total energy may be an inconsequential measure of outcome for any given power density is that power density involves not only the time rate at which energy is expended but also the spatial extent of that energy.18 In biological tissues, for example, a wide range of power densities can achieve similar macroscopic effects (ie, tissue blanching). Although total energies delivered to achieve these effects might be similar, each power density will be different in terms of tissue margins and depth of coagulation. Energy delivered from a site near the target tissue, for instance, achieves narrower lateral margins and a deeper depth of coagulation than energy delivered from a site far from the target tissue (dependent on wavelength absorption characteristics).

In summary, our results suggest that a wide range of power densities can be damaging to an airway stent, and that total energy alone should not be used to delineate margins of safe laser resection. Applying our findings to the clinical setting, bronchoscopists should be most aware of power density, rather than concerned with power settings or total energy only, when contemplating laser/stent interactions.

At an FIO2 <= 40%, a margin of safety exists for Nd-YAG laser applications onto or around an uncovered Wallstent or silicone stent using power densities <= 172 W/cm2, or a power setting of 10 W with a fiber-to-target distance of 10 mm. The common practice of removing these stents, specifically the silicone stent, prior to laser resection may in this case be unnecessary. A margin of safety is not as readily apparent for covered Wallstents, which at higher power densities ignite easily and without warning.


   Acknowledgements
 
The authors thank Guilherme Carrilho Da Graca of UCSD Mechanical and Aerospace Engineering for technical advice. We also thank Ron Konopka and his staff at the UCSD Animal Laboratory; Ray Cox and Toni McCracken of Laserscope, San Jose, CA; and Drs. Ray Shee Lan and Toshiro Matsuo. We gratefully acknowledge the help of Frank Abrano, President of Bryan Corporation, Woburn, MA, for supplying us with silicone stents.


   Footnotes
 
Abbreviations: FIO2 = fraction of inspired oxygen; IACUC = Institutional Animal Care and Use Committee; UCSD = University of California-San Diego

Funded in part by University of California-San Diego Academic Senate Grant No. 07427A.

Received for publication September 11, 2000. Accepted for publication March 26, 2001.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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