Case Studies In Small Animal
Text from "Small Animal Cardiovascular Medicine"
Valvular Pulmonic Stenosis
Pulmonary valve stenosis due to commissural fusion is characterized by a thin to moderately thickened, pliant, conical or dome-shaped valve, with a narrowed outlet ("windsock"). Typically, there are no discrete valve leaflets, although rudimentary raphe may extend from the orifice toward the wall of the pulmonary artery. Pulmonary valve stenosis due to pulmonary valve "dysplasia," on the other hand, usually consists of markedly thickened valve leaflets and annular hypoplasia without fusion of the commissures. Commonly, the pathologic findings of both commissural fusion and valvular dysplasia coexist and a more distinct subclassification cannot be made. Consequently, the term pulmonary valve dysplasia encompasses a variety of valvular abnormalities in dogs. The leaflets of dysplastic valves are usually relatively immobile. This, in concert with the annular hypoplasia, leads to a reduction in effective orifice size. The most common form of PS reported in dogs is valvular dysplasia (88%).
Balloon valvuloplasty has rapidly become the initial treatment of choice for relieving cardiovascular obstructions in humans. Balloon valvuloplasty has been particularly successful in humans with congenital valvular PS, resulting in at least partial reduction of the obstruction in most patients. The technique uses a catheter with a strong, cylindrical balloon at its end placed under fluoroscopic guidance via a vascular cut-down or percutaneous approach. The balloon is positioned across the obstruction and inflated with fluid under pressure. This fractures or stretches the obstructing tissue, increasing the effective size of the lumen. In general, balloon dilation techniques are simpler, less traumatic, less expensive, and less risky than open-chest surgical procedures.
The technique of transluminal angioplasty has been used for several decades in human patients for the treatment of obstructive vascular lesions. For more than 10 years balloon dilation techniques have been used for other congenital or acquired stenotic lesions in humans. Balloon valvuloplasty can be performed safely in dogs with pulmonic stenosis and the results parallel those seen in human medicine. We have been performing balloon valvuloplasty for the treatment of pulmonic stenosis at our institution for over 10 years.
Standard techniques for vascular access and routine cardiac catheterization are described in Chapter 7. Following hemodynamic and angiographic studies, a stiff, long guide wire (>200 cm in length, 0.03-0.038 inch diameter) is introduced into either the left or right branch of the PA through an end-hole catheter. An appropriate size balloon dilation catheter is introduced over the guide wire and is positioned with fluoroscopic guidance across the obstruction. The balloon is rapidly inflated by hand using a 1:1 mixture of saline and contrast medium using a 20-ml syringe. The inflated balloon completely obstructs the systolic outflow from the right ventricle. Consequently, the balloon is kept fully inflated for only a few seconds and then is rapidly deflated. Full inflation is maintained for 5-10 seconds, and the average time from inflation to 90% deflation is generally less than 20 seconds. Multiple inflation/deflations are performed until a satisfactory degree of dilation is produced. Initially when the balloon is inflated, an indentation of the balloon caused by the stenotic valve is visualized fluoroscopically. Upon successful balloon dilation, this indentation disappears (usually abruptly) during inflation of the balloon and is not evident on subsequent balloon inflations. Usually inward or outward traction on the guide wire by an assistant is necessary to maintain balloon position during inflation. Hemodynamic and angiographic studies are reevaluated after the dilation procedure.
Due to the variety of balloon catheters available it is necessary to choose an appropriate catheter for each individual patient. There are currently no standards available for balloon dilation catheter selection in veterinary medicine. Generally, the length of the balloon should extend from within the right ventricular outflow tract to several centimeters above the pulmonary valve. A balloon length of 4 - 10 cm should be appropriate for most dogs. The balloon diameter is chosen based upon pulmonary annulus size measured from the right ventricular angiogram or echocardiogram. Balloon diameters 1.2 to 1.5 times the pulmonary annulus diameter are generally safe and effective.
Bright, et. al. first reported on the use of balloon dilation valvuloplasty in a dog in 1987. Since then it has become the preferred technique for the initial treatment of PS in dogs. Successful reduction of the obstructive pressure gradient by 50% or more has been reported in about 75-80% of dogs treated. Complications are comparatively rare. Perforation of the heart with a catheter or guide wire may lead to cardiac tamponade, bleeding, and death. The most common complication is damage to the tricuspid valve. This may lead to right heart failure. We estimate that this complication occurs in about 5% of our patients. Minor complications are common and include hemorrhage at the access site, arrhythmias (ventricular premature contractions), and right bundle branch block.
Long-term follow-up studies in humans suggest excellent longevity and quality of life for patients following successful balloon dilation. Although fewer studies are available for dogs, the results appear to be similar. The initial reduction in the pressure gradient persists over succeeding months in most dogs, and the prognosis is probably improved, especially if the gradient can be reduced to <50 mmHg. In one report, untreated dogs had a 2.1 times greater risk of dying within the first two years compared to dogs following successful balloon dilation. Aggressive selection of the balloon diameter relative to the pulmonary annular diameter, and further experience with the technique, should further improve outcomes in the future. Additional work may also show that the technique can be applied to animals with PS that is part of more complicated defects such as tetralogy of Fallot.
©Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.