Case Studies In Small Animal

Cardiovascular Medicine

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Case 20

Mitral Stenosis Section from "Small Animal Cardiovascular Medicine" On-Line

Text from "Small Animal Cardiovascular Medicine"

MITRAL VALVE STENOSIS

Mitral valve stenosis is defined as a narrowed mitral valve orifice due to an abnormal mitral valve. The increased resistance to blood flow results in a pressure gradient across the mitral valve and an increase in left atrial pressure.

Prevalence

Mitral stenosis (MS) is a rare cardiac abnormality in dogs and cats. From other’s and our experience, it appears that this abnormality is diagnosed in approximately 1 dog per year in a busy referral university hospital. We diagnosed MS in 12 dogs between 8/1/86 and 8/1/96. When mitral stenosis is identified in young dogs, it is often diagnosed in conjunction with another cardiac abnormality, most commonly subaortic stenosis in one series of cases. Three (two Newfoundlands) of our 12 cases had subaortic stenosis in conjunction with MS. We have also seen it in conjunction with pulmonic stenosis and with mitral dysplasia leading to mitral regurgitation. However, MS can be diagnosed in younger and older dogs with no coexisting abnormalities. In older dogs, it is often impossible to determine if the abnormality is congenital or acquired. Acquired mitral stenosis in humans is almost always due to rheumatic fever. Rheumatic fever does not exist in the dog or cats and so postulating an etiology for acquired mitral stenosis in dogs and cats is difficult. Of the 17 cases of dogs with mitral stenosis in the literature, five have been diagnosed in Bull terriers. In four of these dogs, mitral stenosis was the only lesion that was present. The other dog also had subaortic stenosis. Postmortem examination of one of these dogs revealed mitral valve dysplasia with short, thickened, and fused chordae tendineae that inserted into the mitral valves behind the margins. The papillary muscles were abnormal, being shifted upward and hypertrophied. This dog was eight years old when it presented in atrial fibrillation. The one bull terrier that we have seen with MS also had subaortic stenosis. Two of the cats reported with MS have also had evidence of mitral valve dysplasia when examined echocardiographically. These cats were 14 and 16 years old at presentation. Because of these data, the authors believe that most mitral stenosis in dogs and cats is probably congenital.

Pathophysiology

Mitral stenosis produces obstruction to blood flow from the left atrium to the left ventricle in diastole. The increased resistance to blood flow results in an increase in left atrial pressure with resultant increases in pulmonary vein and pulmonary capillary pressures. Consequently, severe mitral stenosis causes pulmonary edema. Exercise increases left atrial pressure even further which may result in exercise-induced dyspnea. The obstruction to blood flow may also result in exercise-induced syncope in dogs. Syncope may occur when flow cannot increase appropriately during exercise resulting in inadequate flow to maintain systemic pressure during exercise-induced vasodilation or may be due to a tachyarrhythmia. Alternatively, syncope may occur when the inadequate left ventricular return during exercise results in a decrease in left ventricular chamber size, enhanced sympathetic response to the decrease in stroke volume, ventricular mechanoreceptor stimulation, and reflex vagally-mediated peripheral vasodilation and bradycardia.

Reflex pulmonary vasoconstriction and pulmonary pathology leading to pulmonary hypertension, with or without right heart failure, is common in humans with mitral stenosis. Although mild pulmonary hypertension has been reported in two dogs with isolated mitral stenosis, this amount of elevation is most likely purely due to the fact that pulmonary artery diastolic pressure must equal pulmonary capillary pressure such that if pulmonary capillary pressure is increased, pulmonary artery pressure is also increased, albeit only mildly increased. No dog or cat with isolated mitral stenosis has been reported with severe pulmonary hypertension or right heart failure.

 

Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.