Interventional radiology (IR) involves the use of contemporary imaging modalities such as fluoroscopy to gain access to specific structures in order to deliver different materials for therapeutic purposes. These techniques are routinely used in human medicine, however, despite potential applications these procedures have not yet been widely adopted in veterinary medicine. The relatively high morbidity and mortality rates associated with surgery of the cervical and intra-thoracic trachea as well as the difficulty in accessing certain areas of the nasopharynx and lungs make IR techniques particularly well suited for this location. This section will present the advantages offered by IR techniques, some of the equipment involved, and some of the techniques currently being used in veterinary patients with conditions affecting the respiratory tract.
Advantages and Disadvantages
The use of IR techniques in veterinary patients offers a number of advantages compared to more traditional therapies. These procedures are minimally invasive and can therefore lead to reduced peri-operative morbidity and mortality, shorter anesthesia times and shorter hospital stays. Some less equipment-intensive procedures can result in reduced costs as well. Perhaps most importantly, these techniques can offer alternative treatment options for certain patients in which traditional therapies have failed or are unavailable, declined, or not indicated.
The primary disadvantages of IR include the required technical expertise, the specialized equipment necessary (fluoroscopy), and the large initial capital investment necessary to provide a suitable inventory of catheters, guidewires, balloons, stents and coils. Fortunately, the majority of IR procedures performed within the respiratory tract are not equipment intensive, and most are non-emergent cases in which the necessary equipment can be purchased when needed. In addition, for those whom do not have fluoroscopy readily available, some of these techniques have been performed with endoscopic guidance alone.
As tracheal IR procedures are performed through the endotracheal (ET) tube, traditional sterile operating rooms are not required. The author performs the majority of these types of IR procedures in clean radiography / angiography suites while wearing sterile latex gloves. In general, the largest diameter ET tube possible should be selected (at least 4mm inner diameter) to facilitate unrestricted passage of devices through the tube while permitting simultaneous oxygen delivery and ventilation during the procedure. An ET tube with a radio-opaque line or markers should always be used when possible to help avoid inadvertent deployment of stents within the tube. The use of sterile ET tubes is debatable and not routinely required by the author as the trachea is not generally considered a sterile environment.
The radiation exposure during conventional or C-arm fluoroscopy can be substantial. The operator should review radiation safety guidelines, minimize exposure time and beam size, and maximize shielding and distance from the beam. The surgeon, anesthetist, and assistants should always wear full lead gowns and lead thyroid shields during these procedures. A traditional fluoroscopy unit is sufficient for the large majority of tracheal IR procedures currently performed. A C-arm fluoroscopy unit has the advantage of mobility of the image intensifier, permitting multiple tangential views without moving the patient.
A general review of IR equipment is beyond the scope of this chapter, but a brief discussion of certain devices is necessary to understand how one performs these procedures:
Guidewires: These are flexible, often angled-tipped and hydrophilic (slippery) coated wires used to gain access to different structures under direct fluoroscopic visualization.
Catheters: These catheters are long (typically 65cm to 100cm) and designed to be advanced over a guidewire. For these procedures, "marker catheters" with radio-opaque centimeter markings are typically used in order to calculate radiographic magnification and determine the length and diameter of both the normal and narrowed segments.
Stents: These devices are designed to hold the tracheal lumen open in the presence of tracheal collapse, stenosis, or malignant obstruction. Self-expanding metallic stents (SEMS; Figure 1) are most commonly used in the trachea, however balloon-expandable metallic stents (BEMS) are typically used in the nasopharynx. These devices are typically made of nitinol (nickel-titanium alloy) or stainless steel and are compressed onto very narrow delivery systems which can be introduced through the ET tube and placed across a narrowed area.
Snares / Stone Baskets: These devices are similar to their endoscopic counterparts (and often used interchangeably) for use in retrieving tracheal foreign bodies under fluoroscopic guidance.
|Figure 1. Self-expanding metallic stent.|
In contrast to the dynamic obstruction often identified with tracheal collapse, animals can develop fixed obstructions from congenital or acquired causes (e.g., nasopharyngeal stenosis). Historically, stenting has been avoided in benign human diseases because of the risk of potential long-term complications such as stricture and granulation tissue formation or stent fracture. The introduction of removable (and more recently bio-absorbable) stents helps to avoid these potential long-term complications. These technological advancements have led to re-evaluation of these devices for use in benign disease. Initial balloon dilation under endoscopic or fluoroscopic guidance remains the standard of care for veterinary patients with benign strictures in most locations, particularly those not amenable to easy surgical excision or repair. However, certain cases either fail repeated balloon dilation therapy or the repeated procedures may be declined by pet owners. IR techniques have been used to perform palliative stenting for benign strictures in the airway(nasopharynx and trachea) when conventional therapies have failed or were declined (Figure 2).
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Figure 2. Balloon-expandable metallic stent in nasopharynx of cat with nasopharyngeal stenosis.
Tracheal collapse is a progressive, degenerative disease of the tracheal cartilage rings predominantly affecting small and toy-breed dogs. Hypocellularity and decreased glycosaminoglycan and calcium contents result in subsequent dynamic tracheal collapse during respiration. Clinical signs range from a mild, intermittent "honking" cough to severe respiratory distress and are usually controlled with medications including anti-inflammatories, cough suppressants, sedatives/tranquilizers, and bronchodilators. In addition, weight loss, restricted exercise, and removal of second-hand smoke or inhaled allergens can further palliate symptoms. Those patients that have failed aggressive medical and environmental management, and have had other causes of respiratory disease either treated or ruled out, become candidates for surgical or interventional treatment.
Various surgical techniques have been described, however the currently recommended surgical therapy for patients with extra-thoracic tracheal collapse is extra-luminal polypropylene ring prostheses. This technique involves placing extraluminal support rings around the trachea during an open cervical approach. Although one study reports a 75%-85% overall success rate in 90 dogs for reducing clinical signs, this technique is not without complications. 1 The same study reports that 5% of animals died peri-operatively, 11% developed laryngeal paralysis from the surgery, 19% required permanent tracheostomies (half within 24 hours), and ~23% die of respiratory problems with a median survival of 25 months. In addition, while all dogs had extra-thoracic tracheal collapse, only 11% of the dogs in this study had intra-thoracic tracheal collapse. Upon review of their cases, the authors subsequently concluded that the use of this technique in animals with intra-thoracic tracheal collapse cannot be recommended as the associated morbidity was unacceptably high.
The absence of low morbidity surgical options for animals with tracheal collapse has led to the investigation of human-designed stents for veterinary tracheal use. The advantages of intra-luminal tracheal stenting include minimal invasiveness, avoiding dissection around the peri-tracheal neurovascular structures, shorter anesthesia times, and access to the entire intra-thoracic trachea. A number of stents have been evaluated in the canine trachea, primarily stents made of stainless steel or nitinol (laser-cut, knitted or mesh).2-4 Clinical improvement rates in 75%-90% of animals treated with self-expanding, intra-luminal stainless steel stents have been reported (Figure 3).3,4 Immediate complications were mostly minor although there was a peri-operative mortality rate of approximately 10%. Reported late complications have included stent shortening, excessive granulation tissue, progressive tracheal collapse, and stent fracture.
The decision to perform surgery or stenting is a complicated and unresolved one. Regardless, all animals should receive aggressive medical and environmental management before considering either of these treatment options. Failure to administer medications is not a valid reason to pursue these techniques as the majority of these animals will likely require additional medical management in the future. Patients must be treated on an individual case basis, however some basic guidelines can be used. In the author's opinion, if only cervical tracheal collapse is present, then surgical rings should be considered. An exception may be in a geriatric patient or one with excessive co-morbidities (extensive cardiac or pulmonary disease, endocrinopathies, etc.) in which prolonged anesthesia or healing could preclude more invasive surgery. In addition, the author would prefer to avoid intra-luminal stent placement in younger animals as long-term follow-up (> 5 years) in tracheal stented animals has not yet been performed. If only intra-thoracic tracheal collapse is present then surgery is unlikely to resolve the problem and will likely be associated with excessive morbidity; therefore an intra-luminal stent should be considered. Patients with both cervical and intra-thoracic tracheal collapse can be managed with either a single long stent spanning both segments, or with surgical rings in the cervical trachea and a stent for the intra-thoracic trachea.
The use of intra-luminal tracheal stents in patients with bronchial collapse is currently debatable. Unfortunately, there is currently no data available to either support or refute the routine use of tracheal stenting in these patients. The author currently avoids placement of stents within collapsing mainstem bronchi as the benefit achieved will likely be minimal, and temporary, due to continued collapse of more distal bronchi. However, it seems that certain patients can benefit from tracheal stenting, even when concurrent bronchial collapse is present and remains untreated. These clients are warned that continued coughing should be anticipated as the bronchial collapse will remain following placement of the tracheal stent.
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Figure 3. Dog with tracheal collapse. A. Positive pressure ventilation for measurements. B. Delivery system placed through ET tube. C. Following stent placement.
Malignant Airway Obstructions and Benign Tracheal Stenoses
Veterinary patients can present with advanced stages of malignancy in which traditional therapies such as surgery, chemotherapy, or radiation therapy are associated with excessive morbidity, cost, or poor outcome. Presenting clinical signs may be associated with the tumor location and subsequent local effects rather than the systemic effects of the tumor burden; for example, a tracheal tumor resulting in subsequent airway obstruction (Figure 4). Benign conditions can also result in subsequent tracheal stricture formation leading to progressive airway obstruction. The same techniques described above have been used for treatment of tracheal malignancies or stenoses secondary to trauma when surgical procedures were declined, not indicated, or expected to result in excessive morbidity or mortality. The author has placed stents for intrinsic malignant tracheal obstructions5, however tracheal stents could also provide palliation for extrinsic compression of the airways.
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Figure 4. Pre- and post-stent placement for malignant tracheal obstruction in a cat.
Tracheal Foreign Bodies
IR techniques are useful for retrieval of obstructive tracheal or bronchial foreign bodies, particularly in very small patients in which surgery or endoscopy would either be dangerous or impossible. Occasionally, tracheal diameter restrictions require extubation before tracheoscopy / bronchoscopy can be performed. The endoscopes used are often occlusive in these small patients, impairing adequate ventilation. The techniques described above allow passage of very narrow snares or stone baskets through a bronchoscope adapter and ET tube while a complete anesthetic circuit is maintained (Figure 5).
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Figure 5. A. Bronchial foreign body (FB).
B. Guidewire beyond FB (arrowhead).
C. Stone basket (arrow) in bronchus.
D. Post-op films following FB removal.
Vascular Foreign Body Retrieval
Image-guided removal of vascular foreign bodies (catheter-fragments, etc.) can be performed relatively rapidly and minimally-invasively using IR techniques in order to avoid more aggressive surgical techniques or the risk associated with leaving these foreign materials in vivo. Similar to the human experience, the increased use of vascular access and implantable devices in veterinary patients will likely result in an increased frequency of dislodged or migrated vascular foreign bodies encountered and diagnosed by veterinarians.6
1. Buback JL, Boothe HW, Hobson HP. Surgical treatment of tracheal collapse in dogs: 90 cases (1983-1993) Journal of the American Veterinary Medical Association 1996; 208(3):380-384.
2. Radlinsky MG, Fossum TW, Waler MA, et al. Evaluation of the palmaz stent in the trachea and mainstem bronchi of normal dogs. Veterinary Surgery 1997; 26(2):99-107.
3. Norris JL, Boulay JP, Beck KA, et al. Intraluminal self-expanding stent placement for the treatment of tracheal collapse in dogs (abstr), in Proceedings, 10th Annual Meeting of the American College of Veterinary Surgeons 2000.
4. Moritz A, Schneider M, Bauer N. Management of advanced tracheal collapse in dogs using intraluminal self-expanding biliary wallstents. Journal of Veterinary Internal Medicine 2004; 18:31-42.
5. Culp WT, Cole S, Weisse C. Intra-luminal tracheal stent placement in 3 cats. Vet Surg 36:107-113, 2007.
6. Culp WT, Weisse C, Berent AC, et al. Percutaneous endovascular foreign body retrieval (poster). Proceedings, 17th Annual Veterinary Symposium, Chicago, IL, 2007.