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(Chest. 2000;117:1449-1454.)
© 2000 American College of Chest Physicians

Nd-YAG Laser Ignition of Silicone Endobronchial Stents*

Thomas A. Scherer, MD, FCCP{dagger}

* From the Pulmonary Division, Department of Internal Medicine, Triemli Hospital, Birmensdorferstrasse 497, 8063 Zurich, Switzerland. {dagger} Currently at LungenZentrum Hirslanden, Witellikerstrasse 36, 8008 Zuerich.

Correspondence to: Thomas A. Scherer, MD, FCCP, LungenZentrum Hirslanden, Witellikerstrasse 36, 8008 Zurich, Switzerland, e-mail: thsche{at}swissonline.ch


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Study objectives: To test the incendiary characteristics of various silicone endobronchial stents under the impact of the Nd-YAG laser.

Design: In vitro study in the laser laboratory of a university-affiliated city hospital.

Setting: The experiments were performed in a reaction chamber under controlled oxygen concentrations. The radiolucent and radiopaque Dumon silicone stent (Novatech; Aubagne, France) and the tracheal part of the Dynamic stent (Ruesch AG; Kernen, Germany) were tested. The Dumon stents were either clean, covered with a thin layer of blood, or mounted on fresh pig tracheal wall. The laser was aimed on them perpendicularly from distances of 1.0 cm and 0.5 cm.

Interventions: Minimal oxygen concentration to allow ignition and impact time for power outputs (POs) between 10 W and 80 W were determined.

Measurements and results: The lowest oxygen concentration that allowed ignition of some stents was 40%. The clean radiolucent stent could not be ignited with up to 100% ambient oxygen concentration. Radiopacity, presence of blood, tracheal wall, and metal, as well as higher PO and shorter distance of the laser probe decreased impact time to ignition. The radiopaque blood-covered stent was most easily ignited. For this stent, at a PO of 40 W, impact time to ignition was 1.5 ± 0.2 s, and at 30 W was 2.6 ± 0.3 s.

Conclusions: At ambient oxygen concentrations >= 40%, silicone stents can catch fire. Depending on the condition of the stent, the distance of the laser probe, and PO, ignition can occur after short impact times. To prevent stent ignition, oxygen concentration should be kept < 40%. When unusual circumstances require working with higher oxygen concentrations, pulse duration needs to be limited or stent removal might be considered before firing the laser.

Key Words: adverse effects • burns • endoscopy • laser surgery • oxygen • stents


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
The Nd-YAG laser plays an important role in endoscopic treatment of endobronchial lesions. Endobronchial fires caused by combustion of endotracheal tubes are well-known dangers. The incendiary characteristics of various endotracheal tubes have been thoroughly tested.1 2 3 4 5

Silicone endobronchial stents are used for relief of endobronchial obstruction. The Nd-YAG laser is often concurrently applied to coagulate and vaporize tissue. To avoid complications during laser surgery, it is recommended to keep the fraction of inspired oxygen (FIO2) < 0.40 to 0.50, and to limit pulse duration to 1 to 2 s and power output (PO) to 40 to 50 W.6 7 8 These safety precautions were established to avoid massive bleeding, hypoxemia, perforation, and endobronchial fires. The circumstances under which laser treatment can be safely performed in the vicinity of silicone stents and their incendiary characteristics are not yet defined.

The aim of our study was to test the incendiary characteristics of silicone endobronchial stents in vitro under various conditions that were chosen to imitate the conditions occurring during bronchologic interventions.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
The stents used were the radiolucent and radiopaque Endoxane silicone stent (Dumon; Novatech; Aubagne, France), and the tracheal part of the Dynamic stent with reinforcing metal hoops (Ruesch AG; Kernen, Germany).

For all experiments, the Dornier MediLas Fibertom 4100 (Dornier Medizintechnik; Munich, Germany) was used with standard light probes irrigated with room air (flow of 1 L/min). The fiber core diameter was 600 µm. While performing the experiments, calibration was done every 30 min using the built-in power meter. Maintenance of the laser by the manufacturer was performed after every 100 h of operation or at least every 6 months.

Impact time to ignition was defined as the shortest impact time required to cause a continuous burning of the stent at 50% ambient oxygen concentration. Laser impulses were fired in the single pulse mode with the light guide protection system (LPS) activated. This system protects the tip of the laser probe from overheating by shutting down laser power. Impact time to ignition was determined by firing at the preset pulse duration of 0.1, 0.2, 0.3, 0.5, 0.7, 1.0, 2.0, 3.0, 5.0 and 15.0 s. Between 5 s and 15 s, a stopwatch was used. Maximal exposure time was 15 s. For every setting, 10 different measurements were performed. Laser PO was first set at 10 W, and then increased in 10 W steps up to 80 W, if impact time was >= 15 s.

To ensure a controlled oxygen environment, the experiments were performed in a reaction chamber with continuous flow of oxygen and room air. The oxygen concentration in the reaction chamber was continuously analyzed in a circonia cell (MedGraphics Corporation; St. Paul, MN) by aspiration through a small suction catheter and displayed online on a monitor. The circonia cell was calibrated every 30 min. To determine the lowest oxygen concentration needed for ignition, the oxygen concentration in the reaction chamber was increased from 21 to 30%, and then in 10% steps up to 100%. PO was first set to 10 W and further increased in 10 W steps if ignition did not occur after 15 s of laser impact.

Before firing the laser, the stents were meticulously cleaned with soap and thereafter dried. For certain experiments, they were covered with a small layer of blood or mounted on a piece of fresh pig tracheal wall. The laser was actuated perpendicularly on the stents from distances of 0.5 cm and 1.0 cm. The blood layer was put on the side facing the laser probe and the tracheal wall on the opposite side.

For each impact, the laser was aimed at different spots that were clean and free of any soot or discoloration. In the experiments using the Dynamic stent, the laser beam was aimed at the metal hoops.

According to van der Spek and coworkers,9 laser power density was calculated as PO (in watts) divided by impact surface (square centimeters).

Stents and Settings
ST1: radiolucent, clean, 1.0-cm distance; ST2: radiopaque, clean, 1.0-cm distance; ST3: radiolucent with metal struts, clean, 1.0-cm distance; ST4: radiolucent with blood layering, 1.0-cm distance; ST5: radiopaque with blood layering, 1.0-cm distance; ST6: radiolucent, soot covered, 1.0-cm distance; ST7: radiolucent, mounted on pig tracheal wall, 1.0-cm distance; ST8: radiopaque, mounted on pig tracheal wall, 1.0-cm distance; ST9: radiolucent, mounted on pig tracheal wall, 0.5-cm distance; ST10: radiopaque, mounted on pig tracheal wall, 0.5-cm distance.

Statistics
Student’s t test for dependent samples was used to compare impact times at 0.5-cm and 1.0-cm distances, Student’s t test for independent samples was applied to compare impact times between different stents, and analysis of variance with Duncan’s Multiple Range Test for post hoc comparison was used for impact times at increasing PO for the same stents. Statistica for Windows software (Statsoft; Tulsa, OK) was used for the calculations. A p < 0.05 was considered significant. Numbers are given in mean ± SEM unless otherwise indicated.


   Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Power Density
At 0.5-cm distance, the diameter of the laser spot was 1.5 mm; at 1.0-cm distance, the diameter was 2.3 mm. Corresponding power densities are shown in Table 1 .


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  		Table 1.. Power Densities at 0.5-cm and 1.0-cm Distance of the Laser Probe

 
Oxygen Concentration and PO Required for Ignition
At 40% ambient oxygen concentration, a continuous burning of ST2 and ST5 could occasionally be caused. At 50% oxygen concentration, continuous burning could be achieved in all stents except for ST1. Higher oxygen concentrations caused a more intense and rapid burning. The clean radiolucent stent (ST1) could not be ignited at any PO. Because the soot-covered radiolucent stent (ST6) exhibited an intense flash like fire even at low PO, the LPS shut off power before ignition occurred.

The radiopaque blood-covered stent (ST5) was ignited at the lowest PO, followed by the radiolucent stent with metal hoops (ST3), the radiolucent blood-covered stent (ST4), and the radiolucent stent mounted on tracheal wall (ST7; Table 2 ).


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  		Table 2.. Lowest Laser Power Output Causing Ignition and Corresponding Impact Time (mean ± SEM) for Various Stents*

 
Effect of Material and Surface on Impact Time to Ignition
Radiopacity, metal, tracheal wall, and cover with a small layer of blood all reduced impact time to ignition (Fig 1 ). Among the ignitable stents, ST2 required the longest impact time and ST5 was most easily ignited. At 50 W, the respective impact times to ignition for ST2 were 9.7 ± 0.7 s; for ST7, 4.0 ± 0.3 s; for ST4, 2.1 ± 0.2 s; for ST3, 2.0 ± 0.2 s; and for ST5, 0.7 ± 0.1 s. With 60 W, the respective impact times for ST2 were 5.4 ± 0.6 s; ST7, 2.4 ± 0.2 s; ST4, 1.3 ± 0.2 s; ST3, 1.3 ± 0.2 s; and ST5, 0.4 ± 0.03 s. Except for ST3 and ST4, all differences were highly significant.



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  		Figure 1.. Comparison of impact time to ignition for various stents at POs of 50 W and 60 W. Distance between tip of the laser probe and stent, 1 cm; ambient oxygen concentration, 50%. The differences between ST2 and ST7, ST7 and ST4, and ST3 and ST5 are highly significant.

 
Effect of PO on Impact Time to Ignition
The effect of PO on impact time to ignition of ST5 is shown in Figure 2 . The respective impact times for 10 W were 14.1 ± 0.3 s; for 20 W, 5.2 ± 0.3 s; for 30 W, 2.6 ± 0.3 s; for 40 W, 1.5 ± 0.2 s; for 50 W, 0.7 ± 0.1 s; and for 60 W, 0.4 ± 0.03 s. Between 10 W and 50 W, the differences were highly significant.



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  		Figure 2.. Comparison of various POs on impact time to ignition for ST5. Ambient oxygen concentration, 50%; distance between laser probe and stent, 1.0 cm. Differences are highly significant except for 50 W and 60 W.

 
Effect of Distance of the Laser Probe
To test the effect of distance of the laser probe on impact time to ignition, ST8 and ST10 were used. As shown in Figure 3 , distance had a significant effect. The respective impact times (1.0-cm and 0.5-cm distance) for 50 W were 13.0 ± 0.3 s and 7.1 ± 0.3 s, respectively; for 60 W, 6.5 ± 0.5 s and 3.8 ± 0.3 s, respectively; for 70 W, 3.8 ± 0.3 s and 2.2 ± 0.1 s, respectively; and for 80 W, 2.8 ± 0.3 s and 0.9 ± 0.05 s, respectively. The differences were all highly significant.



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  		Figure 3.. Effect of distance (1.0 cm and 0.5 cm) between tip of the laser probe and radiopaque stent on tracheal wall (ST8 and ST10) on impact time to ignition. PO, 50, 60, 70, and 80 W; ambient oxygen concentration, 50%. The impact time to ignition was significantly reduced by decreasing the distance from 1.0 to 0.5 cm.

 
Impact Time to Ignition at PO of 30 W and 40 W
Impact time at PO of 30 W and 40 W were compared for various stents. At 40 W, impact time to ignition for ST5 was 1.5 ± 0.2 s; for ST3, 2.4 ± 0.2 s; and for ST4, 3.3 ± 0.3 s. At 30 W, ST5 was ignited after 2.6 ± 0.3 s, ST3 was ignited after 3.4 ± 0.4 s, and ST4 was ignited after 3.8 ± 0.3 s.


   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The presented results show that ignition of silicone endobronchial stents is easily achieved and that ambient oxygen concentration, amount of absorption of the laser light, PO, and distance of the laser probe are important variables.

The effect of ambient gas concentration on flammability and ignition of silicone devices has been shown by several authors10 11 12 13 14 15 ; they all could demonstrate that oxygen concentration plays an important role.

In the experiments of Simpson et al14 and Simpson and Wolf, 15 silicone medical devices were ignited with a propane torch or a CO2 laser at oxygen concentrations of 19% and 20%, respectively. This is considerably lower than the 40% oxygen concentration needed in our experiments. Comparison with our results is not possible, because a propane torch and a CO2 laser were used and information about color and composition of the silicone material was not provided. Color significantly affects absorption of the Nd-YAG laser light,5 6 16 and the propensity to catch fire also depends on the type of laser. The CO2 laser is capable of igniting a polyvinylchloride tube,14 but Nd-YAG laser light is not capable of doing so, as long as it is fired at an unmarked spot.2

Sosis and Dillon2 could nicely demonstrate how blood and mucus affect the characteristics of polyvinylchloride tubes when exposed to the Nd-YAG laser. The clear unmarked tube was unaffected even at high PO (70 W) and impact times up to 60 s. However, when blood or mucus was present, impact time to ignition was reduced to < 6 s. These findings are in accordance with the presented results. In our experiments, the clean radiolucent stent could not be ignited, but in the presence of blood ignition was possible. The metal hoops of the Dynamic stent had a similar effect. Soot exhibited the highest absorption. Even with short impact times and low PO, an intense flash like fire was elicited, which activated the LPS before the stent caught fire. We also made the observation that barely visible discolorations or traces of soot or dark-colored dust absorbed the laser light much better than the clean material and shortened impact time. This had to be taken into account for these experiments. Stents were therefore meticulously cleaned before firing the laser, and for each measurement the beam was aimed at a different spot.

We also tested the effect of PO and distance on impact time. By reducing distance, the laser beam is focused on a smaller surface, therefore increasing power density.9 This resulted in a reduced impact time to ignition.

Principles of safety in application of Nd-YAG laser have been described previously.7 16 In order to avoid complications, it is recommended to limit PO to 45 to 50 W, pulse duration to 1 s, and to keep FIO2 <= 50%. In the case series of Dumon et al,7 where complications of 1,503 patients were analyzed, no perforations and no endobronchial fires occurred. Cavaliere and coworkers8 reported on interventions in 2,008 patients. They applied up to 50 W to vaporize residual tumor. They too report no perforations and no endobronchial fires. Both studies do not report the presence of endobronchial silicone devices. Ramser and Beamis6 recommend to keep FIO2 < 40%, and to limit pulse duration to 0.5 to 1 s and PO to 20 to 40 W. According to the presented results, these are safer margins in case silicone stents are in situ, and bronchoscopists that follow these guidelines should be able to avoid stent ignition. In view of the possible devastating complications of endobronchial fires, removal of silicone endobronchial stents is a consideration when the circumstances require an FIO2 > 0.40 and long pulse durations.

In summary, these experiments demonstrate that silicone endobronchial stents can easily catch fire when they are exposed to the Nd-YAG laser if ambient oxygen concentration is >= 40%. The PO of the laser light, the presence of blood or metal, and the distance of the laser probe significantly affect impact time to ignition. To avoid stent ignition, at PO of 30 or 40 W, pulse duration should be kept <= 2 s and <= 1 s, respectively, when working with an FIO2 >= 40%. For safety reasons, removal of silicone endobronchial stents before firing the laser is a consideration, when the situation requires working with a higher FIO2, high laser PO, or long pulse durations.


   Footnotes
 
Abbreviations: FIO2 = fraction of inspired oxygen; LPS = laser guide protection system; PO = power output; ST1 = radiolucent silicone stent, 1-cm distance; ST2 = radiopaque silicone stent, 1-cm distance; ST3 = radiolucent silicone stent with metal hoops, 1-cm distance; ST4 = radiolucent silicone stent, covered with blood, 1-cm distance; ST5 = radiopaque silicone stent, covered with blood, 1-cm distance; ST6 = radiolucent silicone stent, covered with soot, 1-cm distance; ST7 = radiolucent silicone stent, mounted on pig tracheal wall, 1-cm distance; ST8 = radiopaque silicone stent, mounted on pig tracheal wall, 1-cm distance; ST9 = radiolucent silicone stent, mounted on pig tracheal wall, 0.5-cm distance; ST10 = radiopaque silicone stent, mounted on pig tracheal wall, 0.5-cm distance

Presented in part at the Annual Meeting of The American Thoracic Society/American Lung Association in San Diego, CA, April 1999.

Supported by Astra Pharmaceutika, Dietikon, Merck Sharp and Dohme-Chibret, Glattbrugg, and Rhône-Poulenc Rorer, Thalwil, all in Switzerland.

Received for publication July 13, 1999. Accepted for publication November 17, 1999.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Sosis, MB (1989) What is the safest endotracheal tube for Nd-YAG laser surgery? A comparative study. Anesth Analg 69,802-804[Abstract/Free Full Text]
  2. Sosis, M, Dillon, F (1991) Hazards of a new, clear, unmarked polyvinylchloride tracheal tube designed for use with the Nd-YAG laser. J Clin Anesth 3,358-360[CrossRef][Medline]
  3. Kautzky, M, Fitzgerald, R, Dechtyar, I, et al (1993) Effects of the holmium: YAG and erbium:YAG lasers on endotracheal tubes. ORL J Otorhinolaryngol Relat Spec 55,105-109[Medline]
  4. Gonzalez, C, Smith, M, Reinisch, L (1990) Endotracheal tube safety with the erbium: yttrium aluminum garnet laser. Ann Otol Rhinol Laryngol 99,553-555[ISI][Medline]
  5. Geffin, B, Shapshay, SM, Bellack, GS, et al (1986) Flammability of endotracheal tubes during Nd-YAG laser application in the airway. Anesthesiology 65,511-515[CrossRef][ISI][Medline]
  6. Ramser, ER, Beamis, JF (1995) Laser bronchoscopy. Clin Chest Med 16,415-426[ISI][Medline]
  7. Dumon, JF, Shapshay, S, Bourcereau, J, et al (1984) Principles for safety in application of Neodynium-YAG laser in bronchology. Chest 86,163-168[Abstract/Free Full Text]
  8. Cavaliere, S, Venuta, F, Foccoli, P, et al (1996) Endoscopic treatment of malignant airway obstructions in 2,008 patients. Chest 110,1536-1542[Abstract/Free Full Text]
  9. Van der Spek, AFL, Spargo, PM, Norton, ML (1988) The physics of lasers and implications for their use during airway surgery. Br J Anaesth 60,709-729[Free Full Text]
  10. Hayes, DM, Gaba, DM, Goode, RL (1986) Incendiary characteristics of a new laser-resistant endotracheal tube. Otolaryngol Head Neck Surg 95,37-40[ISI][Medline]
  11. Pashayan, AG, Gravenstein, JS (1985) Helium retards endotracheal tube fires from carbon dioxide lasers. Anesthesiology 62,274-277[CrossRef][ISI][Medline]
  12. Simpson, JI, Wolf, GL (1988) Flammability of esophageal stethoscopes, nasogastric tubes, feeding tubes, and nasopharyngeal airways in oxygen- and nitrous oxide-enriched atmospheres. Anesth Analg 67,1093-1095[ISI][Medline]
  13. Simpson, JI, Schiff, GA, Wolf, GL (1990) The effect of helium on endotracheal tube flammability. Anesthesiology 73,538-540[ISI][Medline]
  14. Simpson, JI, Wolf, GL, Rosen, A, et al (1991) The oxygen and nitrous oxide indices of flammability of endotracheal tubes determined by laser ignition. Laryngoscope 101,981-984[ISI][Medline]
  15. Wolf, GL, Simpson, JI (1987) Flammability of endotracheal tubes in oxygen and nitrous oxide enriched atmosphere. Anesthesiology 67,236-239[CrossRef][ISI][Medline]
  16. Shapshay, SM, Beamis, JF (1986) Safety precautions for bronchoscopic Nd-YAG laser surgery. Otolaryngol Head Neck Surg 94,175-180[ISI][Medline]



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