Thermal energy to disrupt or ablate diseased tissue has been used for over 50 years. Its application within the respiratory system was limited until more recently as technologic advancements have facilitated access. A number of delivery vehicles now are available for airway applications. For benign airway stenoses, there are two main goals of ablative therapies. First, endoscopic resection and vaporization of obstructing granulation tissue or scar can be accomplished. Second, dense focal scars can be incised to create pathways of least resistance so that subsequent rigid or pneumatic dilatations can be more predictably and safely performed.
Several energy sources can create thermal injury and result in tissue ablation. The choice of source depends on the nature of the stenosis, and includes location, cause, thickness, and length of airway involvement. Having the gamut of technologies is essential. The spectrum of ablative interventions, as well as flexible and rigid video bronchoscopes (telescopes) should be available within the designated interventional suite.
The development of lasers with short wavelengths that can be transmitted through thin quartz filaments has made endoscopic application possible. The carbon dioxide laser operates at a longer wavelength (10.6 μm), which limits its use to superficial structures.7 The neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, however, operates at wavelengths one-tenth as long, making it the more common source. Fine fiberoptic fibers can be passed through the working channel of a standard flexible bronchoscope and can be manipulated with a high degree of accuracy. Stenoses can be endoscopically resected primarily by vaporization of tissues. Since Nd:YAG laser light is poorly absorbed by both water and hemoglobin; it penetrates more deeply into surrounding tissues than other ablative therapies. Accordingly, the risk of airway perforation and injury to adjacent structures is higher. However, the wider energy dispersion field results in effective coagulation.8
A rare but important complication of endoscopic ablative therapies is airway ignition.8 This is more commonly associated with laser procedures performed through the flexible bronchoscope than through a rigid scope. Reduction of inspired FIO2 to <40%, avoidance of flammable endotracheal tubes (although there currently is no “laser-safe” endotracheal tube on the market), and use of reinforced endotracheal tubes or LMA are preferred during any thermal procedure. Awareness of this rare complication is critical.
Electrocautery and Argon Plasma Coagulation
Endoscopic electrocautery (diathermy) offers the ability to rapidly snare and remove large granulomas or polyps bypassing the tedious process of vaporization. Alternatively, cautery can be used similarly to the laser. Risk of airway ignition is slightly less with electrocautery because less heat is generated by electrocautery device. The depth of penetration of the thermal injury depends on contact time with tissues and, consequently, is perhaps, slightly better controlled with electrocautery than with other thermal devices.
The flow of electrons through tissues generates heat for coagulation because of the high resistance within the target tissue. Electrocautery probes require direct contact with the target tissue to initiate this effect. Argon plasma coagulation (APC) uses ionized argon gas to conduct electrons into the target, providing a noncontact mode of treatment (lightning effect).8 Argon gas flows flexibly, and therefore, can travel in a nonlinear fashion to the desired target (i.e., bend around corners). Moreover, as tissues are coagulated, their intrinsic resistance rises, redirecting the argon beam to adjacent, nontreated, lower-resistance tissues.8 This feature distinguishes APC from the laser. Also, APC can treat more superficial tissues than either laser or diathermy. Argon gas is pressurized and there is some small risk of gas emboli reaching the systemic circulation if the tip of the APC probe is kept too close to an active bleeding source.
Cryotherapy has been commonly used in dermatology, otolaryngology, and gastroenterology applications. More recently it was introduced for endobronchial treatment of both malignant and benign lesions. The mechanism of action is cellular necrosis caused by intracellular crystallization from freeze–thaw cycles.9,10 Initially, there can be some edema following therapy, but subsequent sloughing of treated tissue (with or without balloon dilatation) results in recanalization of the airway lumen. There is very little bleeding encountered at the time of cryoablation, and the resulting microvascular thrombosis permits only minimal delayed bleeding even after tissue is sloughed and regardless of concomitant or subsequent balloon dilatation.11
Histological analysis posttreatment has shown sloughing of the epithelium and deeper tissues to include the submucosal glands with preservation of the connective tissue. This appears to reepithelialize the radial margin of the injury.12 The use of this therapy has been limited by the cryoprobe design, which was somewhat cumbersome. Recent advances have included development of noncontact cryotherapy with liquid nitrogen. This permits more uniform application of the cryogen.12 Given the advances in this technology, conventional probe-delivered cryotherapy is useful for debulking endoluminal masses (benign and malignant), treating asymmetric scar, and foreign-body (and retained clot) extraction. Importantly, cryotherapy has no potential for airway fire. Spray cryotherapy is currently under active investigation, although its introduction into practice will be complicated by the need for highly pressurized cryogen (liquid nitrogen) which may lead to gas emboli and pneumothorax.
Microdebrider therapy is another newer modality useful in the endoscopic management of subglottic and tracheal cicatricial stenosis and malignant or granulation tissue-related airway obstruction. A microdebrider is a rotary cutting tool that attaches to a suction source that simultaneously debrides and evacuates tissue to recanalize the obstructed airway. Recently, this device has become popular for removing obstructing subglottic glands, airway papillomas, tracheal scars, as well as obstructing airway cancers.13 The suction component of the device continually clears debris and maintains visualization of the lumen. Nevertheless, the limitations of the technology remain accidental injury to normal tissue and bleeding from the raw debrided surface which may ultimately require thermal (or cryo) cauterization.9 The microdebrider probe must be passed through a rigid scope and visualized with a telescope or flexible video bronchoscope. This current setup does reduce the degrees of freedom of microdebrider movement.
In addition to the above-described endoscopic modalities, topical agents have been used as an adjunct to dilatational and ablative strategies. Topical endoscopic application of mitomycin C (~0.4 mg/mL) appears to be a safe and moderately efficacious adjuvant therapy, although limited experimental14 and clinical data15-17 exist. Mitomycin C can be applied topically to the treated areas, and surprisingly, this does not appear to affect epithelial regrowth; rather, it mainly interferes with refibrosis.10 Treated areas should be surveyed regularly for squamous metaplasia. Similarly, some data support the use of intralesional steroid therapy to modulate the local inflammatory process, and this may have some benefit in the management of tracheal stenosis.18 It is common to use mitomycin C and intralesional steroids alternately during chronic endoscopic palliation of a recurrent tracheal stenosis. Although the data is scant on these topical treatments, no appreciable complications have been reported in the literature.