Subaortic Stenosis in Dogs

Subaortic stenosis (SAS) is one of the most common congenital cardiac defects in dogs. This condition involves a narrowing (stenosis) of the left ventricular outflow tract just below the aortic valve, resulting in obstruction to blood flow from the left ventricle into the aorta.

Pathophysiology #

Subaortic stenosis typically manifests as a fibromuscular ridge or ring below the aortic valve. The stenotic lesion can vary from a thin fibrous ridge to a thick fibromuscular collar or tunnel-like obstruction. The pathogenesis involves abnormal embryological development of the outflow tract, with excessive growth of fibrous tissue that forms during cardiac development.

The obstruction creates pressure overload on the left ventricle, resulting in compensatory concentric hypertrophy. This hypertrophy helps maintain cardiac output initially but eventually leads to increased myocardial oxygen demand while simultaneously reducing coronary perfusion. The pressure gradient across the stenosis directly correlates with disease severity.

The stenotic lesion accelerates blood flow through the narrowed region, creating turbulence that damages the endocardium and aortic valve cusps. This turbulence may lead to secondary aortic valve insufficiency in approximately 15-20% of cases. The high-velocity jet can cause endocardial fibrosis and predispose to bacterial colonization, potentially leading to infective endocarditis.

Chronic pressure overload eventually causes left ventricular diastolic dysfunction, increased left atrial pressure, and potentially left-sided congestive heart failure. Myocardial ischemia occurs due to increased oxygen demand from hypertrophy coupled with decreased coronary perfusion, especially during exercise. This ischemia can trigger ventricular arrhythmias, which are a common cause of sudden death in dogs with severe SAS.

Clinical Presentation and Signalment #

Breed Predisposition #

SAS shows a strong breed predisposition, most commonly affecting:

• Newfoundlands
• Golden Retrievers
• Boxers
• German Shepherds
• Rottweilers
• Bull Terriers

The inheritance pattern appears to be polygenic in most breeds, with an autosomal dominant pattern suggested in Newfoundlands.

Clinical Signs #

Clinical presentation varies with the severity of obstruction:

• Mild cases (pressure gradient <40 mmHg) are typically asymptomatic
• Moderate cases (40-80 mmHg) may show exercise intolerance
• Severe cases (>80 mmHg) often present with syncope, exercise intolerance, and potentially sudden death

Common clinical signs include:

• Systolic ejection murmur loudest at the left heart base (characteristic finding)
• Weak femoral pulses
• Exercise intolerance
• Syncope or collapse, especially after exercise
• Arrhythmias
• Sudden death (often the first and only sign in severe cases)

Most dogs are diagnosed as puppies during routine examinations or pre-breeding cardiac screenings when the characteristic murmur is detected. However, some dogs remain undiagnosed until adulthood when clinical signs emerge or sudden death occurs.

Medical Management #

The management of SAS focuses on reducing the risk of complications and improving quality of life, as there is no definitive cure short of surgical intervention.

Medical Therapy #

Beta-blockers (most commonly atenolol) are the mainstay of medical therapy for moderate to severe SAS. Beta-blockers:

• Decrease heart rate, improving diastolic filling and coronary perfusion
• Reduce myocardial oxygen consumption
• Decrease the likelihood of arrhythmias
• Typical dosing: atenolol at 0.5-1.0 mg/kg PO q12h

Additional medical management may include:

• Activity restriction, especially avoiding intense exercise
• Antibiotic prophylaxis prior to dental procedures or surgeries due to increased risk of endocarditis
• Treatment for congestive heart failure if present (diuretics, ACE inhibitors)
• Antiarrhythmic therapy if significant arrhythmias are documented

Surgical Options #

Balloon valvuloplasty has been attempted with variable success rates. This procedure involves catheter-based balloon dilation of the stenotic region. However, studies show limited long-term benefits, with frequent restenosis occurring within months.

Open surgical techniques, including open resection of the fibrous ring, have shown better outcomes but carry significant perioperative risks. These procedures are performed at specialized referral centers and remain controversial regarding long-term benefit versus risk.

Anesthetic Considerations #

Dogs with SAS present significant anesthetic challenges due to potential myocardial ischemia, arrhythmias, and hemodynamic instability. The anesthetic approach should be tailored to the severity of stenosis and requires careful planning.

Preoperative Considerations #

• Comprehensive cardiac evaluation including echocardiography to determine disease severity
• Continue beta-blocker therapy until the day of surgery
• Premedication with anxiolytics to reduce catecholamine release
• Consider prophylactic antibiotics
• Thorough physical examination with attention to hydration status

Intraoperative Management #

• Maintain normal to slightly elevated heart rate to maintain cardiac output. Most beta blockers (atenolol or sotalol) tend to keep heart rate low during anesthesia. That often can create low cardiac output when blood pressure is heart rate dependant
• Avoid drugs that cause significant bradycardia or myocardial depression
• Maintain adequate preload while avoiding volume overload
• Implement arterial blood pressure monitoring. Since the obstruction is internal to aortic valve and aortic outflow tract, systemic arteriolar dialation can severely decrease coronary perfusion resulting in myocardial ischemia, especially if myocardial oxygen demand is iatrogenically increased with drugs like atropine.
• Maintain normotension; avoid hypotension which may reduce coronary perfusion
• Maintain normal body temperature to avoid increased metabolic demands during recovery

Postoperative Care #

• Close monitoring for arrhythmias
• Pain management to minimize sympathetic stimulation
• Careful fluid management
• Resume beta-blocker therapy as soon as possible
• Monitor for signs of heart failure or pulmonary edema

Prognosis #

The prognosis for dogs with SAS depends primarily on the severity of obstruction:

• Mild SAS: Generally good prognosis with near-normal life expectancy
• Moderate SAS: Guarded prognosis; median survival 3-5 years with medical management
• Severe SAS: Poor prognosis; high risk of sudden death within the first 3 years of life

Early detection and appropriate medical management can improve quality of life and potentially extend survival, though the condition cannot be cured medically.

References #

1. Kienle RD, Thomas WP, Pion PD. The natural clinical history of canine congenital subaortic stenosis. J Vet Intern Med. 1994;8(6):423-431.
2. Meurs KM, Lehmkuhl LB, Bonagura JD. Survival times in dogs with severe subvalvular aortic stenosis treated with balloon valvuloplasty or atenolol. J Am Vet Med Assoc. 2005;227(3):420-424.
3. Eason BD, Fine DM, Leeder D, et al. Influence of beta blockers on survival in dogs with severe subaortic stenosis. J Vet Intern Med. 2014;28(3):857-862.
4. Linde A, Koch J. Screening for aortic stenosis in the Boxer: Auscultatory, ECG, radiographic and echocardiographic evaluation. Aust Vet J. 2006;84(9):331-337.
5. Visser LC, Scansen BA, Schober KE. Hemodynamic quantification of cardiac performance in dogs with subaortic stenosis using pressure-volume loops. J Vet Intern Med. 2018;32(1):57-63.
6. Martin MWS, Johnson MJS, Celona B. Canine dilated cardiomyopathy: a retrospective study of signalment, presentation and clinical findings in 369 cases. J Small Anim Pract. 2009;50(1):23-29.
7. Carter J, Conlon P. Anesthetic management of patients with cardiac disease undergoing non-cardiac surgery. Vet Anaesth Analg. 2021;48(1):29-46.
8. Orton EC, Herndon GD, Boon JA. Influence of open surgical correction on intermediate-term outcome in dogs with subvalvular aortic stenosis: 44 cases (1991-1998). J Am Vet Med Assoc. 2000;216(3):364-368.