New Technologies: Selection of Stents
World Small Animal Veterinary Association World Congress Proceedings, 2007
David Holt, BVSc, DACVS
University of Pennsylvania School of Veterinary Medicine

Interventional radiology and stenting began in 1953 with the description of percutaneous arterial catheterization by Dr Seldinger. In this technique a sharp, hollow needle punctures a vessel, organ, or cavity. A guidewire is advanced through the lumen of the needle and a sheath or dilator is passed over the guidewire. This technique allowed the development of angioplasty and is routinely used in veterinary medicine for the placement of multi-lumen catheters, PDA coils, and nephrostomy/cystostomy tubes. Building on the work of Seldinger, Charles Dotter, Andreas Gruentzig, and others developed the angioplasty procedure. The dual lumen balloon developed by Gruentzig is the model for the balloon dilation catheters we now use to treat esophageal strictures. Interventional radiology has evolved in human medicine has evolved into a discipline with several subspecialties. In veterinary medicine, interventional radiology has a growing list of applications that include stenting of collapsed tracheas and urethral malignancies, coil embolization of intrahepatic shunts, embolization of non-surgical tumors, placement of cystostomy and nephrostomy tubes and retrieval of dislodged catheters.

Using interventional radiology techniques in veterinary patients is usually less invasive and, with experience, less time consuming than surgery. They generally result in less perioperative morbidity and shorter hospital stays. Some procedures, including chemoembolization or palliative stenting of malignancies obstructing the urethra offer treatments for conditions not amenable to surgery. The disadvantage of these procedures is the equipment necessary--fluoroscopy , ideally with digital subtraction, and the initial investment in guidewires, catheters, balloons, stents, and coils. A traditional fluoroscopy unit is adequate for many procedures, but a C-arm unit allows multiple tangential views of the area of interest. Occasionally ultrasonography is useful to guide the insertion of the needle or catheter into vessels, the bladder, or the kidney.

Tracheal Collapse

Tracheal collapse is a progressive disease in which the cartilage rings of the trachea loose their structural integrity. This results in collapse of the trachea during forceful respiration. Because of the changing transmural pressures on the trachea during respiration, the extrathoracic trachea collapses during inspiration and the intrathoracic trachea and mainstem bronchi collapse during expiration. Many affected animals live relatively comfortably with few episodes of severe collapse. Others can be successfully managed with a combination of anti-inflammatory drugs and cough suppressants. Weight loss, restricted exercise, and removal of allergens or second-hand smoke from the animal's environment can further minimize clinical signs.

Surgery or stenting is indicated in animals with worsening clinical signs that have failed medical therapy. Surgical treatment is generally performed on only the extrathoracic trachea. The surgical technique currently recommended involves placing polypropylene rings around the trachea through a ventral midline cervical incision. The largest study reported a 75-85% success rate for reduction of clinical signs in a population of 90 dogs. Five percent of dogs treated died peri-operatively, and 11% developed laryngeal paralysis. This is likely due to damage to the recurrent laryngeal nerves which run adjacent to or on the trachea in the caudal neck. Nineteen percent of dogs in the study required permanent tracheostomies; the median survival all dogs was 25 months.

A decision on surgery or stenting should be based on the animal's age, concurrent diseases, and the severity and location of the collapse. This is best evaluated by fluoroscopy or tracheoscopy. Animals with only cervical collapse are reasonable candidates for surgical ring placement. Animals with concurrent intra- and extrathoracic collapse are probably candidates for stenting, although some surgeons report improvement with cervical rings alone. When severe bronchial collapse is present, the decision on treatment becomes unclear. If dyspnea is the major clinical sign and intra-thoracic tracheal and bronchial collapse is present, a tracheal stent can help relieve the dynamic obstruction. If the patient's primary problem is coughing, then it becomes difficult to determine if stenting will improve the problem. Persistent coughing will cause cyclic loading of the stent and may result in stent fracture or the formation of granulation tissue. The question of intratracheal stent placement in young animals is unresolved, as long term (> 5 year) follow up of tracheal stents is not yet available.

Not all stents are created equal for tracheal collapse. The following table briefly outlines the advantages and disadvantages of commonly used stents.

Type

Material and Type

Advantages

Disadvantages

Palmaz

Stainless steel
Balloon expandable

Balloon expandable

Compresses easily then stays compressed
Sharp edges

Wallstent

Stainless steel
Self expanding

Re-constrainable
Low incidence of stent fracture

Will foreshorten
Sharp ends--granulation tissue?

VetStent

Nitinol
Interwoven

Re-constrainable
No sharp ends

Foreshortening
Stent fracture
Fracture leads to unraveling

SMARTR stent

Nitnol
Laser cut

Does not foreshorten

Limited use
Rapid fracture?

The stent is placed with the animal under general anesthesia and the endotracheal tube placed in the proximal trachea. A marker catheter is placed in the esophagus and positive pressure ventilation is applied. The maximal diameter of the trachea is measured and compared to the known length of the marker catheter to correct for radiological magnification, which varies between machines. The diameter is usually chosen to be ~10-20% greater than the maximal tracheal diameter to minimize chances of subsequent stent migration. Stents should be available in 2mm diameter increments (i.e., 8mm, 10mm, 12mm, and 14mm diameter) and in the most commonly used lengths. The cervical trachea is routinely larger in diameter than the intra-thoracic trachea. Stent sizing can be complicated when the difference in these two measurements varies dramatically. Several types of stents will be longer than their stated length when they cannot open to their full diameter. This needs to be taken into consideration to avoid the stent opening into the larynx.

The stent is prepared, saline flushed, and introduced into the trachea through a right-angle bronchoscope adapter. All manipulations should be performed under fluoroscopic guidance. The animal is positioned so the cervical and intra-thoracic trachea lies in a straight line to facilitate unrestricted passage of the relatively inflexible delivery system. To initiate stent deployment, with one hand on the hub (or the cannula), and the other hand on the Y-piece (sheath), gently withdraw the Y-piece (sheath) while simultaneously advancing the hub (cannula) in equal proportions. If done appropriately, as stent deployment proceeds, the distal end of the stent will remain in the same location throughout deployment. The stent may be reconstrained if necessary by simultaneous withdrawal of the cannula and advancement of the sheath in order to avoid dragging the stent across the tracheal mucosa.

Clinical improvement has been reported in 75-90% of dogs treated with self expanding intraluminal stents. Reported perioperative mortality is 10%. Late complications include stent shortening, fracture, granulation tissue formation, and progressive tracheal and bronchial collapse.

Intrahepatic Portosystemic Shunts

Congenital portosystemic shunts (PSS) are generally single anomalous communications between the portal and systemic venous circulations. Intrahepatic PSS are surgically demanding procedures. Reported perioperative mortalities range from 10-65%. Interventional treatment of intrahepatic PSS is performed through a sheath in the jugular vein. The shunt is identified and an appropriately sized stent is placed in the caudal vena cava covering the opening of the shunt. Coils are then placed into the shunt through the interstices of the stent. The stent prevents the coils from "launching" into the vena cava.

Palliative Stenting for Malignant Obstructions

Conventional palliative therapy for non-resectable malignant obstructions caused by transitional cell carcinoma include stents placed by cystostomy or urethrostomy and sutured in place or tube cystostomy placement. Alternatively balloon-expandable (BEMS) and self-expanding metallic stents (SEMS) can be placed across malignant urethral obstructions in dogs using fluoroscopy to re-establish urethral patency. In male dogs, a vascular sheath is placed into the penile urethra once a hydrophilic guide wire has been advanced past the obstruction and into the bladder. In female dogs the bladder is punctured with a needle under ultrasonographic guidance and the guidewire placed into the bladder and out the urethra. The sheath is then advanced over the guidewire. A positive contrast urethrogram is performed to determine the maximal diameter of the normal urethra to assist in selecting an appropriately sized stent. Palliative stenting has also been performed in the gastrointestinal, cardiovascular, and respiratory systems.

Percutaneous Arterial Embolization and Chemoembolization

Although arterial embolization does not use stent placement, it is an interventional radiology technique that has been very useful in controlling hemorrhage , occluding vascular malformations, including hepatic arteriovenous malformations, and slowing tumor growth. Chemoembolization has been used to deliver intra-arterial chemotherapy to non-resectable tumors. This results in 10-50 fold increases in the intra-tumoral drug concentration when compared to intravenous chemotherapy. Obstruction of the tumor vasculature with subsequent particle embolization results in tumor necrosis and decreases tumor drug wash out, limiting systemic chemotherapy toxicity.

Hepatic arteriovenous malformations are uncommon, but often difficult to treat surgically. Selective catheterization and embolization of the hepatic arterial branches feeding the arteriovenous malformations has been successful in ameliorating the clinical signs in a limited number of dogs.

Speaker Information
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David Holt, BVSc, DACVS
University of Pennsylvania School of Veterinary Medicine
Pennsylvania, USA


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