Chapter 65: Pulmonic Stenosis

Pulmonic Stenosis

Amara H. Estrada, Gainesville, Florida

Herbert W. Maisenbacher, III, Gainesville, Florida

Pulmonic stenosis (PS) is one of the most common congenital heart defects encountered in dogs. Obstruction can occur anywhere along the right ventricular (RV) outflow tract into the main pulmonary artery, and the lesion can be classified as subvalvular, valvular, or supravalvular.

Typical valvular PS in children is caused by commissural fusion of thin to somewhat thickened valve leaflets that assume a dome-shaped appearance during systole as the valve pushes into the pulmonary artery. This form of PS is observed in some dogs. In contrast, valvular stenosis caused by pulmonary valve dysplasia is characterized by markedly thickened valve leaflets, with or without annular hypoplasia or commissural fusion. This dysplastic form is the most common type observed in dogs and usually is associated with leaflet mobility and fusion along the commissural edges. Pulmonary valvular stenosis also can occur due to a subvalvular fibrous obstruction or a supravalvular ring. In some dogs multiple levels of obstruction are evident.

In English bulldogs and boxers PS often is associated with an anomalous origin of the left coronary artery. In these dogs, a single large coronary artery originates from the right aortic sinus (of Valsalva) and then divides into the left and right main branches. The left coronary artery then encircles the RV outflow tract just below the pulmonary valve. This vascular malformation, termed an R2A coronary anomaly, is associated with a subvalvular obstruction (Buchanan, 1990).

Diagnosis

The diagnosis of PS can be made with reasonable certainty using knowledge of breed predilection, physical examination findings, and some simple diagnostic tools found in most veterinary hospitals. A presumptive diagnosis of PS often is suggested by results of physical examination, electrocardiography, and radiography. Echocardiography is used to make a definitive diagnosis, determine disease severity, identify any coexisting cardiac abnormalities, and guide treatment recommendations. In situations in which the dog is clinically healthy, the heart rhythm is normal, and client resources are limited, ancillary tests usually can be deferred (or supplanted) by a comprehensive Doppler echocardiographic study.

Breeds Affected

PS is encountered most commonly in terrier breeds, English bulldogs, Samoyeds, Chihuahuas, miniature schnauzers, Labrador retrievers, Mastiffs, chow-chows, Newfoundlands, basset hounds, and cocker spaniels. It has been shown to be inherited in beagles.

Clinical Signs

Most dogs with PS are asymptomatic as puppies. Auscultation of a cardiac murmur during a routine examination is the first clue that PS (or another congenital heart defect) might be present. Clinical signs are more likely to occur in dogs older than 1 year of age. Clinical signs can develop earlier in dogs with extremely severe stenosis or with other congenital cardiac abnormalities such as a patent foramen ovale, atrial septal defect, ventricular septal defect, or tricuspid valve dysplasia.

Initial clinical signs are related to low cardiac output, including exercise intolerance and apparent shortness of breath. Dogs with extremely severe stenosis can have exertional dyspnea and syncope caused by the inability of the RV to increase its output in response to exercise or excitement. Arrhythmias and reflex-mediated syncope are other causes of collapse; sudden death has been reported and is most common in severely affected dogs. Signs of right-sided congestive heart failure (ascites, pleural effusion, peripheral edema) usually occur later in the course of the disease but can occur earlier in patients with concurrent tricuspid valve dysplasia or with development of atrial fibrillation. Right-to-left shunting of blood can occur in dogs with an atrial or ventricular communication such as a patent foramen ovale, atrial septal defect, or ventricular septal defect. Cyanosis and polycythemia usually are minimal unless a large defect is present. Growth usually is normal in patients with PS, regardless of the severity of the obstruction.

Physical Examination Findings

Auscultation of a dog with PS typically reveals a left-sided systolic murmur heard best over the cranial heart base and radiating cranially and dorsally. In general, the intensity of the murmur increases with the severity of obstruction. The classic murmur is described as ejection sounding, with a crescendo or crescendo-decrescendo timing and harsh quality. Rarely, a soft diastolic murmur of pulmonic insufficiency also is heard. Occasionally, a pulmonary ejection click is evident. The click corresponds to the time when the doming pulmonary valve reaches its open position and snaps open. When moderate to severe tricuspid regurgitation is present a blowing or holosystolic murmur is heard over the right apical region.

Jugular veins usually are normal in dogs with PS, but a prominent A wave sometimes is observed. Moderate to severe tricuspid regurgitation is another cause of a prominent jugular pulse, and if atrial fibrillation or congestive heart failure develops, jugular venous distention may be recognized. Femoral artery pulse quality usually is normal in dogs with PS.

Electrocardiographic Findings

The electrocardiographic (ECG) findings may be normal in dogs with mild PS and therefore may be somewhat useful in assessing severity of obstruction. Dogs with more moderate to severe PS and RV hypertrophy usually have ECG evidence of RV enlargement (right-axis deviation; deep S waves in leads I, II, III, and aVF; prominent S waves in left-sided chest leads). Both supraventricular arrhythmias (atrial premature complexes, atrial flutter or fibrillation) and ventricular arrhythmias may be recorded in severe cases.

Radiographic Findings

Thoracic radiographs may show features supportive of the clinical diagnosis of PS; however, as with ECG, radiographic findings in mild cases of PS may be completely normal. Radiographs in dogs with moderate to severe PS usually demonstrate a prominent RV and poststenotic dilation of the main pulmonary artery. Poststenotic dilation of the main pulmonary artery may be visualized on a lateral view but is most impressive on a dorsoventral view at the 1 to 2 o’clock position. The pulmonary vasculature and parenchyma usually are normal, although sometimes they may appear undercirculated, especially in cases with concurrent right-to-left shunting.

Echocardiographic Findings

Definitive diagnosis and full characterization of the defect and related complications requires echocardiography with Doppler imaging. Obstruction in the RV outflow tract causes an increased resistance to flow and an increase in RV systolic pressure. The magnitude of the increase in RV pressure depends on the severity of obstruction. The more severe the stenosis, the more pressure the RV must generate to overcome the stenosed valve and maintain pulmonary perfusion. The response of the RV to this pressure overload is concentric hypertrophy.

Two-Dimensional Echocardiography

Two-dimensional echocardiography allows for subjective evaluation of the severity of stenosis. Typical features of PS can be demonstrated clearly from both the right-sided long-axis and short-axis views (Web Figure 65-1). In general, the thicker the RV, the more severe the stenosis. Concentric hypertrophy of the RV, narrowing in the region of the pulmonary valve, and poststenotic dilation of the pulmonary artery are the echocardiographic hallmarks of PS. In severe cases, septal flattening may occur as RV systolic pressure becomes higher than left ventricular systolic pressure (see Web Figure 65-1). Dogs with severe PS also may have hyperechoic regions visible within the RV or septum related to areas of ischemia and fibrosis.

Web Figure 65-1 Echocardiographic images from a dog with severe pulmonic stenosis. A, Right-sided four-chamber long-axis view. B, Right-sided short-axis view. Both echocardiographic images show dramatic right ventricular hypertrophy (arrows) and evidence of septal flattening, indicating higher right than left ventricular pressure. Notice also the increased echogenicity of the papillary muscles of the right ventricle, which indicates probable ischemia of this portion of the myocardium.

Three features of valvular stenosis that occur to varying degrees are fusion of leaflets, valve thickening, and hypoplasia of the annular region. Valve leaflets may appear prominent because of thickening. Systolic motion of the valve leaflets is restricted, with inward curving at the tips of the leaflets, known as doming, which is most apparent in stenosed but mobile valves. Dysplastic valves often appear very thickened and can be immobile, without the characteristic doming seen with the “typical” valvular stenosis in children. In cases with valve dysplasia or subvalvular stenosis, poststenotic dilation of the pulmonary artery may be absent, and the pulmonary annulus may be hypoplastic. The distinction between the two types of stenosis (typical leaflet fusion and valve dysplasia) often is not clear, and pathologic features of both forms usually are present in dogs. For prognostic reasons dysplastic valves should be assessed for mobility, commissural fusion, and hypoplasia. Annular hypoplasia and immobile valves are likely to respond less favorably to interventional therapy (Bussadori et al, 2001).

Evidence of dynamic infundibular narrowing caused by hypertrophy also may be seen on two-dimensional evaluation. During systole, as the walls of the hypertrophied infundibulum come together, obstruction in this region can occur in addition to the valvular obstruction (Web Figure 65-2). The severity of this dynamic stenosis may be difficult to estimate, but it is important to identify it before interventional procedures are undertaken because this type of obstruction can become immediately more severe following balloon valvuloplasty and may require β-adrenergic blockade (see the discussion of suicide RV in the section on treatment later in the chapter).

Web Figure 65-2 Echocardiographic images from a dog with severe pulmonic stenosis. Short-axis images obtained at the level of the pulmonary valve. The pulmonary valve is severely thickened and dysplastic. There is no evidence of poststenotic dilation of the pulmonary artery beyond the dysplastic valve. The annulus is normal in size. During diastole (A) the infundibular or subvalvular region is open without evidence of subvalvular obstruction. However, during systole (B) the walls of the hypertrophied infundibulum come together and may create a dynamic subvalvular obstruction (arrow).

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Tags: Kirks Current Veterinary Therapy XV

Jul 18, 2016 | Posted by admin in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off

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