Acquired pulmonary artery stenosis in four dogs

Brian A. Scansen Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.

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Karsten E. Schober Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.

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John D. Bonagura Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.

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Daniel D. Smeak Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210.

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Abstract

Case Description—4 dogs with acquired pulmonary artery stenosis (PAS) were examined for various clinical signs. One was a mixed-breed dog with congenital valvular PAS that subsequently developed peripheral PAS, one was a Golden Retriever with pulmonary valve fibrosarcoma, one was a Pembroke Welsh Corgi in which the left pulmonary artery had inadvertently been ligated during surgery for correction of patent ductus arteriosus, and one was a Boston Terrier with a heart-base mass compressing the pulmonary arteries.

Clinical Findings—All 4 dogs were evaluated with 2-dimensional and Doppler echocardiography to characterize the nature and severity of the stenoses; other diagnostic tests were also performed.

Treatment and Outcome—The mixed-breed dog with valvular and peripheral PAS was euthanized, surgical resection of the pulmonic valve mass was performed in the Golden Retriever, corrective surgery was performed on the Pembroke Welsh Corgi with left pulmonary artery ligation, and the Boston Terrier with the heart-base mass was managed medically.

Clinical Relevance—Acquired PAS in dogs may manifest as a clinically silent heart murmur, syncope, or right-sided heart failure. The diagnosis is made on the basis of imaging findings, particularly results of 2-dimensional and Doppler echocardiography. Treatment may include surgical, interventional, or medical modalities and is targeted at resolving the inciting cause.

Abstract

Case Description—4 dogs with acquired pulmonary artery stenosis (PAS) were examined for various clinical signs. One was a mixed-breed dog with congenital valvular PAS that subsequently developed peripheral PAS, one was a Golden Retriever with pulmonary valve fibrosarcoma, one was a Pembroke Welsh Corgi in which the left pulmonary artery had inadvertently been ligated during surgery for correction of patent ductus arteriosus, and one was a Boston Terrier with a heart-base mass compressing the pulmonary arteries.

Clinical Findings—All 4 dogs were evaluated with 2-dimensional and Doppler echocardiography to characterize the nature and severity of the stenoses; other diagnostic tests were also performed.

Treatment and Outcome—The mixed-breed dog with valvular and peripheral PAS was euthanized, surgical resection of the pulmonic valve mass was performed in the Golden Retriever, corrective surgery was performed on the Pembroke Welsh Corgi with left pulmonary artery ligation, and the Boston Terrier with the heart-base mass was managed medically.

Clinical Relevance—Acquired PAS in dogs may manifest as a clinically silent heart murmur, syncope, or right-sided heart failure. The diagnosis is made on the basis of imaging findings, particularly results of 2-dimensional and Doppler echocardiography. Treatment may include surgical, interventional, or medical modalities and is targeted at resolving the inciting cause.

A 26.9-kg (59.2-lb) 6-year-old spayed female German Shepherd Dog mix (dog 1) was examined after a single episode of collapse after exercise. The event had taken place 3 days prior to evaluation at the teaching hospital. Initiation of the event had not been witnessed, but the owner saw the dog lying in lateral recumbency with no apparent movement. Upon evaluation by the owner, the dog urinated, growled when assisted to rise, and was conscious but seemed disoriented. The duration of the event was estimated at < 2 minutes.

The dog's medical history included referral at 3.5 months of age for cardiovascular examination. Echo-cardiographic assessment at that time confirmed a diagnosis of valvular PAS and revealed a right ventricular outflow tract velocity of 4 m/s, equivalent to a peak systolic pressure gradient of 64 mm Hg. The dog also had a clinically significant left-to-right shunting patent ductus arteriosus and radiographie evidence of left-sided congestive heart failure. Medical treatment was initiated (furosemide, 2.4 mg/kg [1.1 mg/lb], PO, q 12 h; enalapril, 0.3 mg/kg [0.14 mg/lb], PO, q 12 h), and the ductus arteriosus was ligated without complications 10 days later via routine left thoracotomy1,2 Evaluation of radiographie views of the thorax obtained 7 days after ductal ligation indicated that congestive heart failure had resolved. Follow-up echocardiography again revealed valvular PAS, and on that occasion, outflow velocity was 4.78 m/s, corresponding to a peak right ventricular-to-PA systolic pressure gradient of 91.4 mm Hg. A dynamic component of right ventricular outflow tract obstruction associated with infundibular right ventricular hypertrophy was evident on 2-dimensional echo-cardiographic and spectral Doppler ultrasonographic imaging. Atenolol was prescribed (0.7 mg/kg [0.3 mg/ lb], PO, q 12 h) in an attempt to reduce the dynamic obstruction, and pulmonic balloon valvuloplasty was performed later, when the dog was 5 months old. Right ventricular cineangiography during the valvuloplasty procedure revealed an absence of ductal flow, moderate right ventricular hypertrophy, a thickened and stenotic pulmonary valve, poststenotic dilatation of the main PA, and patency of both PA branches (Figure 1). During the subsequent 5 years, no clinical signs referable to the cardiovascular system were seen except for 1 possible episode of exercise intolerance during a 20-minute walk. The dog was consistently medicated with atenolol at dosages that were increased according to weight. Serial echocardiographic examinations performed 1 month and 1,2, and 4 years after the valvuloplasty revealed a reduced and stable peak right ventricular-to-PA systolic pressure gradient (estimated at 65, 49, 48, and 43 mm Hg, respectively).

Figure 1—
Figure 1—

Diagnostic images obtained in a mixed-breed dog with acquired PAS (dog 1). Right-ventricular angiogram obtained at 5 months of age depicting thickened pulmonic valve leaflets (arrow), poststenotic dilatation of the main PA, patency of both left and right PAs, and a filling defect in the right ventricular outflow tract, indicating hypertrophy of the supraventricular crest (A). Right parasterna short-axis echocardiogram obtained when the dog was 6 years of age, revealing moderate right ventricular hypertrophy and dilatation, systolic doming of the pulmonic valve leaflets (arrow), poststenotic dilatation of the main PA, hyperechoic tissue in the area of the left PA, and focal stenosis at the origin of the right PA with poststenotic dilatation of the distal portion of the right PA (B). Color Doppler mage of the PA branches revealing turbulence originating at the origin of the right PA with no flow detected in the left PA (C). Latera radiographie view obtained during right ventricular angiography at 6 years of age revealing severe dilatation of the main PA with a 1- to 1.5-cm stenosis (arrow) at the origin of the right PA, poststenotic dilatation of the right PA, and no opacification of the left PA. Mild tricuspid valve régurgitation is also evident (D). MPA = Main pulmonary artery. RA = Right atrium. RV = Right ventricle. RPA = Right pulmonary artery. LPA = Left pulmonary artery. SVC = Supraventricular crest. 5, 10, and 15 on the sides of echocardiograms indicate the number of centimeters of field depth.

Citation: Journal of the American Veterinary Medical Association 232, 8; 10.2460/javma.232.8.1172

At the time of evaluation for exertional collapse, the dog's medication regimen consisted solely of atenolol administration (dosage, 0.46 mg/kg [0.21 mg/lb], PO, q 12 h). Physical examination at that time revealed a grade III/ VI right-sided systolic murmur and a grade II/V1 left-sided systolic murmur, heard loudest at the heart base. Both lung fields auscultated clearly Vital signs were within reference limits, and systolic arterial blood pressure obtained via Doppler oscillometry on the left forelimb was 121 mm Hg. Results of a neurologic examination were considered normal. Values from a CBC and serum biochemical analyses were within reference limits, except for mild monocy-tosis (1.9 × 109 monocytes/L; reference range, 0 to 1.2 × 109 monocytes/L) and mild hyperglobulinemia (3.5 g/dL; reference range, 2.2 to 2.9 g/dL). Serum total thyroxine concentration was normal, and results of a heartworm antigen test were negative. Thoracic radiography revealed mild right-sided cardiomegaly with prominence of the right atrium and main pulmonary trunk; the appearance of the pulmonary parenchyma was considered unremarkable. Electrocardiography revealed sinus rhythm with occasional premature ventricular complexes and a right ventricular hypertrophy pattern with a mean electrical axis of 150° in the frontal plane.

Echocardiographic evaluation revealed a normal-appearing left atrium and left ventricle. The right side of the heart had evidence of severe ventricular remodeling with a mixed-hypertrophy pattern (eg, dilatation with wall thickening). Severe right atrial enlargement was also seen, in conjunction with severe tricuspid valve régurgitation. The pulmonic valve appeared thickened and partially fused, with prominent systolic doming and poststenotic dilatation of the main PA (Figure 1). The PA branches were difficult to image, with hyperechoic tissue seen at the origin of the left PA. A stricture was suspected at the origin of the right PA. Color Doppler interrogation revealed turbulent flow in the right PA and no flow in the left PA. Turbulent flow was noticed at the level of the pulmonic valve, with a flow velocity predicting a peak right ventricular-to-PA systolic pressure gradient of 51 mm Hg. Spectral Doppler interrogation of the tricuspid valve régurgitant jet predicted a right ventricular-to-right atrial pressure gradient of 181 mm Hg. These data were interpreted as consistent with moderate valvular PAS with severe PA hypertension, likely caused by stenoses of the proximal PA branches (ie, stenosis in series). The atenolol dosage was increased (to 0.93 mg/kg [0.42 mg/ lb], PO, in the morning, and to 0.46 mg/kg, PO, in the evening), and the dog was scheduled for cardiac cath-eterization to further assess the right ventricular outflow tract and PA branches and possible means of therapeutic intervention.

Four weeks later, the dog underwent cardiac cath-eterization. At that visit, the murmurs had increased in intensity to grade IV/VI systolic on the right and grade III/VI systolic at the heart base on the left. The dog was anesthetized, and contrast transthoracic echocardiogra-phy was performed via IV injection of agitated saline (0.9% NaCl) solution to evaluate patency of the atrial septum; no flow across the atrial septum was detected. Cineangiography of the right ventricle revealed moderate to severe hypertrophy of the supraventricular crest, valvular pulmonic stenosis, dilation of the main PA, no opacification of the left PA, and a discrete 1- to 1.5-cm narrowing of the proximal portion of the right PA with poststenotic dilatation of the distal portion of the right PA (Figure 1). Pressure recordings revealed a pulmonary capillary wedge pressure of 6 mm Hg, distal right PA pressure of 21/14 mm Hg, main PA systolic pressure of 135 mm Hg, right ventricular pressure of 150/0 mm Hg, and a mean right atrial pressure of 4 mm Hg. A diagnosis of valvular PAS and peripheral PAS was made, and balloon dilation of the valve fusion was performed with a 5-cm × 25-mm balloon dilatation catheter.a The balloon catheter provided a balloon-to-annulus ratio of 0.9:1 (measured pulmonary annulus diameter, 28 mm) and was used despite being slightly undersized because no larger balloon catheters were available. A double-balloon technique may have overcome this difficulty, but the result was satisfactory with use of the single-balloon technique. Postdilatation right ventricular systolic pressure was 98 mm Hg. The dog recovered from anesthesia without complications. The following day, echocardiographic assessment revealed laminar flow across the pulmonic valve at a rate of 1.4 m/s (right ventricular-to-PA systolic pressure gradient, 7.7 mm Hg) and a tricuspid valvular régurgitant jet of 6.5 m/s (right ventricular-to-right atrial pressure gradient, 170 mm Hg), consistent with elimination of the valvular stenosis and persistence of the peripheral PAS. Recommendations were made for follow-up examination with computed tomography-assisted angiography and possible surgical palliation of the peripheral PAS. The client did not return the dog for follow-up evaluation, and contact with the referring veterinarian revealed that the dog had been euthanatized 4 months after the cardiac catheterization because of progressive syncopal events. Postmortem examination was not performed.

A 37-kg (84.1-lb) 4-year-old spayed female Golden Retriever (dog 2) was referred for evaluation of refractory seizures of 1 month's duration. Nine seizures had been seen by the owner during the 30 days prior to admission, each lasting for 30 to 90 seconds and characterized by a period of excitement prior to the event, collapse, limb rigidity, jaw clenching, and a 1-hour period of weakness or sedation afterward. Phénobarbital had been prescribed by the referring veterinarian, and the dog was being treated at a dosage of 3.5 mg/kg (1.6 mg/lb), PO, every 12 hours at the time of evaluation at the teaching hospital.

Physical examination revealed vital signs to be within reference range and a grade III/VI musical systolic ejection murmur at the heart base on the left side, with radiation to the right hemithorax. Neurologic evaluation was unremarkable. Results of a CBC, biochemical analyses, and urinalysis revealed no clinically significant abnormalities. Serum phénobarbital concentration was 24.5 μg/mL (therapeutic range, 15 to 40 μg/mL). Thoracic radiography and abdominal ultrasonography revealed no abnormalities. Echocardiography revealed normal left-sided structures and function, but a 3 × 2-cm mass was attached to and moving with the arterial surface of the pulmonic valve, resulting in moderate-to-severe acquired pulmonic valve stenosis (estimated peak right ventricular-to-PA systolic gradient, 75 mm Hg; Figure 2). The right atrium was moderately dilated, and the right ventricle had mixed hypertrophy with mild dilatation and wall thickening. These findings were consistent with valvular neoplasia, thrombus, or a sterile endocarditic vegetation. The clinical events were presumed to be syncopal and secondary to right ventricular outflow obstruction; however, a primary neurologic cause could not be excluded. Administration of aspirin (1 mg/kg [0.45 mg/lb], PO, q 12 h) was added to the treatment regimen.

Figure 2—
Figure 2—

Diagnostic images from a Golden Retriever with acquired PAS (dog 2). Echocardiographic right parasternal short-axis image revealing mild eccentric right ventricular hypertrophy. Notice the hyperechoic mass (arrow) attached to the arterial surface of the closed pulmonic valve (A). Intraoperative photograph of pulmonary arteriotomy and mass resection. Notice that the right ventricular outflow tract patch is open and the PA has been incised. The tan soft tissue mass (arrow) is being resected from the caudal wall of the pulmonic valve annulus, which is seen best in the close-up inset (B). AV = Aortic valve. PV = Pulmonic valve. See Figure 1 for remainder of key.

Citation: Journal of the American Veterinary Medical Association 232, 8; 10.2460/javma.232.8.1172

Alterations in the antiepileptic regimen were made during the next 2 weeks, including addition of potassium bromide (27 mg/kg [12.3 mg/lb], PO, q 12 h) and leveteracetam (20.3 mg/kg [9.2 mg/lb], PO, q 12 h), but resulted in little change in the frequency or nature of the seizure events. Two weeks after the initial evaluation at the teaching hospital, the pulmonic valve mass was reassessed echocardiographically and was found to be unchanged in size with increased severity of the stenosis (estimated pressure gradient, 100 mm Hg). Magnetic resonance imaging of the brain was performed, with results interpreted as normal.

Because the dog did not respond to appropriate antiepileptic treatment, surgical intervention was undertaken to remove the pulmonic valve mass. In brief, mild hypothermia was induced (rectal temperature, 33.9° to 34.4°C [93° to 94°F]) and a median sternotomy was performed. The pericardium was reflected, and a fluo-ropolymer membrane patchb was sutured to the right ventricular outflow tract. The main PA and pulmonic valve annulus were incised once all venous return to the heart had been occluded, and the mass was dissected off the pulmonic valve (Figure 2). The mass was firmly adhered to the caudal wall of the PA, and excision was estimated as being 90% complete. The patch graft was closed, and venous blood flow was restored. After surgery, the dog was mechanically ventilated and remained stable until increased bleeding from the chest tube was noticed 6 hours after surgery. Blood products were administered, and the decision was made to reexplore the thorax, but the dog developed cardiac arrest prior to surgical exploration. An attempt at open-thorax resuscitation was unsuccessful. On postmortem examination, the patch graft appeared well-adhered to the epicardial surface, and a cause for the postoperative hemorrhage was not readily apparent. Results of histologie evaluation of the mass and the remaining tissue in the main PA were consistent with an intracardiac fibrosarcoma arising from the distal surface of the pulmonic valve.

A 5-kg (11-lb) 14-week-old female Pembroke Welsh Corgi (dog 3) was evaluated on a referral basis because of a patent ductus arteriosus. The dog had undergone surgery for the patent ductus approximately 2 weeks previously at a local veterinary clinic. The dog's pre- and postoperative clinical status had been unremarkable. On physical examination, a grade V/ VI continuous murmur was detected at the heart base on the left side. Thoracic radiography revealed moderate left atrial and ventricular enlargement, severely decreased vascularity in the left hemithorax with partial atelectasis of the left lung, overcirculation and increased vascularity in the right hemithorax, and no evidence of pulmonary edema (Figure 3). Echocar-diographic imaging revealed moderate left atrial and ventricular dilatation with preserved systolic function, continuous turbulent flow entering the main PA consistent with a left-to-right shunting patent ductus arteriosus, and focal constriction at the origin of the left PA with dilatation of the left PA distal to this constriction. It was not possible to interrogate flow and obtain the pressure gradient across the stenotic left PA because of contamination by the ductal flow signal. However, the velocity of the ductal flow (systolic velocity of 5 m/s corresponding to an aorta-to-PA pressure gradient of 100 mm Hg) predicted normal main PA systolic pressure. These findings were suggestive of left PA ligation during the prior surgery and continued patency of the ductus arteriosus. The thorax was explored via left lateral thoracotomy at the fourth intercostal space. A consolidation of granulation tissue was seen overlying the heart base and great vessels and was bluntly dissected with minor associated bleeding. The patent ductus arteriosus was isolated and double ligated with size-0 polypropylene suture materials Remnants of suture material from the prior surgery were seen caudal to the ductus arteriosus, although the anatomic location of these sutures was not readily apparent. Further dissection of the suture remnants was not possible given the concern about hemorrhage. Surgical closure was routine, and the dog recovered from anesthesia without complications.

Figure 3—
Figure 3—

Diagnostic images from a Pembroke Welsh Corgi with acquired PAS (dog 3). Dorsoventral radiographie view depicting hypo-vascularity in the left lung field with hypervascularity in the right lung field, partial atelectasis of the left cranial lung lobe, and mild left ventricular enlargement (A). Right-parasternal short-axis echocardiographic image of the great vessels from the same dog revealing focal stenosis (arrow) at the origin of the left PA (B). See Figures 1 and 2 for key.

Citation: Journal of the American Veterinary Medical Association 232, 8; 10.2460/javma.232.8.1172

The dog remained clinically normal after surgery, and ovariohysterectomy was performed without complications 4 months later. Thoracic radiography and echocardiography 3.5 years after the initial surgery revealed persistent stenosis of the left PA and reduced vascularity in the left hemithorax.

An H-kg (24.3-lb) 13.5-year-old castrated male Boston Terrier (dog 4) was evaluated for abdominal distension and suspected cardiac disease. Distention of the abdomen had been noticed by the owners for 1 week. The medical history was unremarkable except for chronic mild rectal prolapse and year-round administration of milbemycin oximed for heartworm prevention.

On physical examination, sinus tachycardia and a heart rate as high as 160 beats/min were detected in conjunction with a grade III/V1 left apical systolic murmur and a grade II/VI systolic murmur at the apex on the right side. Occasional premature beats and an intermittent gallop rhythm were auscultated. Palpation of the abdomen revealed a prominent abdominal fluid wave and moderate hepatomegaly. Cytologie analysis of an abdominal fluid sample obtained via abdominocentesis indicated that the fluid was a modified transudate, with a nucleated cell count of 1,450 WBCs/μL. and protein concentration of 4.5 g/dL. Serum biochemistry values were within reference range. An ECG revealed frequent premature atrial complexes, occasionally in bigeminal distribution. Thoracic radiography revealed a rounded soft tissue mass separate from and silhouetting with the left cranial margin of the cardiac silhouette that was causing dorsal and rightward deviation of the trachea; mild pleural effusion; and enlargement of the right ventricle, left ventricle, and left atrium (Figure 4). Echocardiography revealed a 4.8 × 5.8-cm mass of mixed echogenicity at the heart base that was partially compressing the dorsal wall of the left atrium. The main PA was observed in close proximity to the mass, with both PA branches surrounded by suspected neoplastic tissue. Acceleration of flow out both the left and right PAs was confirmed on both color and spectral Doppler ultrasonographic imaging. Moderate right ventricular and right atrial enlargement with a prominent central jet of tricuspid valve régurgitation were seen; the 4.18 m/s velocity of the tricuspid valve régurgitant flow was predictive of high right ventricular systolic pressures (estimated peak right ventricular-to-right atrial systolic pressure gradient, 70 mm Hg). Both atrioventricu-lar valves were moderately thickened and had changes compatible with degenerative valve disease (ie, endocardiosis). Mild mitral valve régurgitation and left atrial enlargement were noticed. The pulmonary veins were difficult to image and were in close proximity to the mass but appeared patent with laminar flow of blood into the left atrium.

Figure 4—
Figure 4—

Diagnostic images from a Boston Terrier with acquired PAS (dog 4). Ventrodorsal (A) and right lateral (B) thoracic radiographie views revealing a soft tissue mass obscuring the left cranial margin of the cardiac silhouette, resulting in rightward and dorsal deviation of the trachea. Cardiomegaly can also be seen, consistent with right ventricular, left ventricular, and left atrial enlargement.Thin pleura fissure lines are evident between the right middle and right caudal lung lobes and between the left caudal lobe and the caudal subseg-ment of the left cranial lung lobe, consistent with mild pleural effusion (arrows). Echocardiographic images reveal a large mass of mixed echogenicity at the heart base apparently compressing the dorsal wall of the left atrium (C). Right parasternal short-axis echocardiographic image depicting the same heart-base mass with both the LPA and RPA coursing through and partially compressed by the mass (D). HBM = Heart-base mass. See Figures 1 and 2 for remainder of key.

Citation: Journal of the American Veterinary Medical Association 232, 8; 10.2460/javma.232.8.1172

A tentative diagnosis of heart base neoplasia with external compression of the PA branches and resultant precapillary PA hypertension and right-sided heart failure was made. The dog was discharged to the owner's care on medical treatment that included fu-rosemide (2.3 mg/kg [1 mg/lb], PO, q 12 h), enalapril (0.23 mg/kg [0.1 mg/lb], PO, q 12 h), and spironolactone (1.1 mg/kg [0.5 mg/lb], PO, q 12 h). This treatment regimen did not fully control accumulation of ascites fluid, and therapeutic abdominocentesis was performed by the referring veterinarian every 2 to 4 weeks.

The dog was reevaluated 4 months after initial diagnosis. Substantial weight loss of 1.6 kg (3.5 lb) and dorsal muscle wasting had developed during the preceding 4 months. The dog had mild dyspnea and exercise intolerance secondary to peritoneal fluid accumulation, which resolved after abdominocentesis. At that visit, respiratory rate, blood pressure, and heart rate were within normal limits; normal sinus rhythm was auscultated; and renal variables indicated only mildly high values for SUN concentration (29 mg/dL; reference range, 5 to 20 mg/dL). Echocardiography revealed minimal change in the size of the mass (4.6 × 6.3 cm) and persistence of PA stenosis. An estimate of right ventricular systolic pressure made on the basis of tricuspid valve régurgitant flow velocity was comparable to the initial pressure (right ventricular-to-right atrial systolic pressure gradient, 66 mm Hg). The treatment regimen was altered by increasing the furosemide dosage (2.7 mg/kg [1.2 mg/lb], PO, q 8 h), adding an injectable furosemide treatment (2.7 mg/kg, SC, q 48 h), and supplementing omega-3 fatty acids (38.3 mg/kg [17.4 mg/lb], PO, q 24 h of eicosapentaenoic acid; 25.5 mg/ kg [11.6 mg/lb], PO, q 24 h of docosahexaenoic acid). At the time of manuscript preparation (9 months after initial diagnosis), the dog remained stable on this regimen with therapeutic abdominocentesis required every 2 weeks, although progressive cachexia and weight loss had developed despite normal appetite.

Discussion

Congenital PAS in various forms is well-described in dogs3-6 and is the third most commonly diagnosed congenital heart defect in dogs in North America.7 More recently, a congenital form of peripheral PAS has been described.8 Although congenital PAS is well-described in dogs,3-6,8 the pathogenesis and natural history of acquired causes of PAS are not sufficiently documented in the veterinary literature, with only 2 case reports describing acquired PAS in dogs of which the authors are aware.9,10 In the present report, 4 dogs with acquired PAS that were evaluated at The Ohio State University Veterinary Teaching Hospital from 2003 to 2006 are described with the intent to highlight this clinical disorder and describe potential causes, treatments, and outcomes.

In humans, acquired forms of PAS are less common than congenital forms.11"17 Acquired PAS in humans has been reported secondary to extrinsic causes, including external compression by pulmonary or medi-astinal neoplasia13,18,19 and cysts20; compression from aortic aneurysms,21,22 infectious mediastinitis,17 idio-pathic pericarditis,15 and pericardial calcification23; in-traluminal conditions such as neoplasia,11,24 systemic vasculitides,12,25 pulmonary thromboembolic disease,26 and idiopathic fibrous intimai proliferation16,27; and valvular abnormalities associated with rheumatic heart disease,28 malignant carcinoid syndrome,29 and infective endocarditis.28 A report9 of acquired PAS secondary to a heart-base tumor in a Boston Terrier was published approximately 40 years ago. More recently, acquired PAS has been reported after decortication secondary to constrictive pericarditis in a Labrador Retriever as well as after surgical patch graft placement for congenital supravalvular PAS in a Cocker Spaniel.10

Development of complete left PA occlusion and partial right PA occlusion in dog 1 was suspected from echocardiographic findings and was confirmed by an-giography although the cause of the peripheral PAS remained uncertain. The possibility that the left PA was inadvertently occluded at the time of ductal ligation surgery was excluded because both the left and right PA were patent on the angiogram obtained during pul-monic balloon valvuloplasty 4 weeks after ductal ligation surgery. Furthermore, the large increase in right ventricular systolic pressure to 181 mm Hg was first observed when the dog was 6 years of age and not during the annual examinations throughout the 4 years following valvuloplasty. The estimated peak right ventricular systolic pressure measured 4 years after valvuloplasty by interrogation of the tricuspid valve régurgitant velocity correlated precisely with that predicted from spectral Doppler interrogation of the valvular PAS. The estimated right ventricular systolic pressure of 65 to 70 mm Hg was equivalent to the 43 mm Hg gradient measured across the pulmonic valve at the same time, assuming a systolic PA pressure of 25 mm Hg. Thoracic radiography and ultrasonography excluded the possibility of external compression from a mass lesion in the mediastinum or pulmonary parenchyma as a contributing factor. Aortic dissections and aneurysms have been reported as causes of external compression on the PA resulting in acquired PAS in humans,21,22 and aortic aneurysms have also been associated with patent ductus arteriosus in dogs.30 However, inspection of the levophase of the right ventricular angiogram in dog 1 revealed the aorta to be normal in structure and to have no aneurysm or ductal flow.

One possible etiology for the peripheral PAS, although speculative, is a delayed inflammatory reaction around the site of ductal ligation. Although silk is commonly used to ligate the ductus arteriosus of dogs and was used in dog 1, suture granuloma formation in humans is reported to be most severe with this material.31 The time course of 6 years between ductal ligation and development of peripheral PAS in this dog is inconsistent with suture granuloma formation as a cause of the acquired stenosis; however, delayed suture granulomas developing 18 and 25 years after surgery have been reported.32,33 The pathogenesis of acquired peripheral PAS in dog 1 is uncertain.

It was not surprising that dog 1 was syncopal, given the suprasystemic right ventricular pressures. Syncope is commonly described in the clinical syndromes of both congenital PAS4,6 and acquired PA hypertension.34 Syncope in both syndromes develops secondary to poor right ventricular cardiac output, reduced transpulmo-nary flow and subsequent hypoxemia, arrhythmias secondary to right ventricular hypertrophy, or inappropriate sympathetic withdrawal and high parasympathetic tone secondary to the Bezold-Jarisch reflex (ie, neu-rocardiogenic or vasodepressor syncope).34,35 The absence of postmortem findings is a limitation; however, the present case highlights the value of Doppler echo-cardiography in noninvasive estimation of intracardiac hemodynamics. The discrepancy between the main PA and tricuspid valvular régurgitant velocities led to the diagnosis of acquired peripheral PAS in conjunction with congenital valvular PAS.

Dog 2 had the rare condition of intracardiac neo-plasia resulting in acquired PAS. In a recent review36 of cardiac tumors in dogs, the incidence of intracardiac neoplasia was 0.19% among all dogs in the Veterinary Medical Database at Purdue University from 1982 to 1995, with 84% of such tumors of primary cardiac origin and 16% metastatic tumors. In that review, fibrosarcoma and unspecified sarcoma were rare, together accounting for only 2% of all primary heart tumors. The anatomic site within the heart of these sarcomas was not detailed in the review. However, in all case reports37-39 involving dogs with cardiac fibrosarcoma, the site of origin was the right atrium and no dogs had fibrosarcoma arising from a cardiac valve, as did the dog of the present report. In another report,40 2 dogs with neoplastic obstruction of the right ventricular outflow tract were described, one of which was an Irish Setter with a myxoma in the right ventricle, similar in some aspects to the dog in the present report. Although both dogs had cardiac murmurs and syncopal events, the myxoma in the Irish Setter was of a different cellular origin, was pedunculated and highly mobile, and was detected on the ventricular side of the valve as opposed to the arterial surface, which was involved in the dog in the present report. Fibrosarcoma or pleomorphic sarcoma arising from the pulmonic valve has been described in humans11 and is a recognized but rare cause of intraluminal acquired PAS. This case highlights the importance and difficulty of differentiating neurogenic collapsing events from those of cardiac origin.

Dog 3 was representative of an iatrogenic cause of acquired peripheral PAS. Surgical ligation of patent ductus arteriosus has been performed successfully for over 50 years in dogs with a low complication rate,1,2,41 and to the authors' knowledge, instances of inadvertent left PA ligation have not been reported. However, this complication is well-recognized in human cardiothoracic surgery, particularly in premature infants.42-45 Mistaken left PA ligation is more likely in infants with a large patent ductus arteriosus and concurrent PA hypertension or in those with an anatomically atypical ductus arising proximal to the left subclavian artery43 In the former circumstance (large patent ductus arteriosus with PA hypertension), the ductus may be as large as the aortic arch and may partially overlie the arch, hiding it from view. In the latter circumstance (anatomically atypical ductus arising proximal to the left subclavian artery), the recurrent laryngeal nerve does not pass under the ductus but instead passes under the aortic arch and complicates anatomic assessment. Whether such an anomaly affects dogs is unknown. Ductal ligation surgery must be performed by a surgeon with considerable experience to minimize complications, especially ductal tearing. In the dog of this report, persistence of a continuous murmur after surgery prompted further investigation, and the radiographie and echocar-diographic evidence of left PA ligation were confirmed on surgical exploration. Although the ligature around the left PA could not be dissected and removed at the time of the second surgery, the dog has continued to do well clinically with reduced arterial flow to the left lung lobes. The fact that dog 3 did better clinically than dog 1 was likely because the right PA remained fully patent and dilated in response to the increased flow. In rats subjected to pneumonectomy complete removal of the left lung resulted in compensatory hypertrophy of the remaining right lung and restoration of normal lung volume within 8 weeks.46 This compensatory capability of the lungs likely explains why dog 3 did not have clinical signs after ligation of the left PA. In contrast, dog 1 had complete stenosis of the left PA and partial stenosis of the right PA, leaving little pathway for blood to exit the right side of the heart. Furthermore, congenital stenosis of the pulmonic valve obstructed right ventricular outflow in dog 1, and the combination of valvular and near-complete peripheral PAS led to the clinical deterioration described.

Acquired PAS in the Boston Terrier, confirmed at necropsy, has been described as arising secondary to a tumor of the heart base.9 However, that report was published 4 decades ago, prior to the advent of echo-cardiographic imaging. Dog 4 of the present study had a similar condition, with compression of the PA branches by a large heart base mass. Given the predilection for dogs of this breed to develop neoplasia of the chemo-receptor cells at the heart base,36,47 the cellular origin of the neoplasm was presumed to be neuroendocrine. Given the lack of histologie confirmation, it is possible that the cellular origin of the tumor in dog 4 was not chemoreceptor tissue, and the possibility that the mass was an ectopic thyroid carcinoma or other atypical neoplasia may also be considered. However, the dog's sig-nalment and slow growth of the tumor during 1 year of monitoring support the likelihood of chemodectoma because thyroid carcinomas typically follow a more aggressive course.48

The Boston Terrier and other brachycephalic breeds of Bulldog ancestry are known to be at increased risk for developing chemodectomas, and it is speculated that chronic hypoxia in conjunction with genetics plays a role in the prevalence of this tumor in these breeds.36,47 These tumors are generally considered chemotherapy-and radiation-resistant, and treatment is directed at palliation of clinical signs; most commonly, such signs are referable to development of pericardial effusion. The clinical manifestation of right-sided heart failure in affected dogs is not unique, although dogs with chemo-dectoma typically develop right-sided heart failure secondary to cardiac tamponade and not as a consequence of acquired PAS, as in the dog described in the present report.

Treatment for acquired PAS depends on the inciting cause. External compression by pulmonary or me-diastinal masses may be cured with surgical excision of the compressing lesion. If the compression involves the heart base, however, excision may not be feasible. Intramural defects may be addressed with surgical resection, preferably with cardiopulmonary bypass if available, although inflow-occlusion techniques may also be considered. Additionally, interventional techniques are now available to palliate and, in some instances, cure the defect.10,49 Percutaneous balloon angioplasty with or without stent implantation, is the treatment of choice for uncomplicated congenital peripheral PAS in children49 and has similarly been used in idiopathic peripheral PAS in adults.16 Intraluminal stent placement has recently been reported as palliation for PAS in the dog10 and may be considered if sufficient catheteriza-tion expertise and financial resources are available.

In conclusion, 4 dogs had acquired PAS of various etiologies. Acquired forms of PAS, although rare, should be considered in dogs with clinical signs or physical findings of syncope and right-sided heart disease, and treatment should be directed at resolving the inciting cause.

ABBREVIATIONS

PA

Pulmonary artery

PAS

Pulmonary artery stenosis

a.

NuMED Inc, Hopkinton, NY.

b.

Gore-Tex, WL Gore & Associates Inc, Flagstaff, Ariz.

c.

Prolene, Ethicon Inc, Somerville, NJ.

d.

Interceptor, Novartis Animal Health US Inc, Greensboro, NC.

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