Older Horse with Exercise Intolerance

History. A 22-year-old Thoroughbred mare is presented for exercise intolerance. The owner uses her for trail riding and some low-level eventing. The mare has had some mild arthritis during her career, but in the last 2 to 3 months she seems reluctant to work, takes longer to recover after rides, and seems lethargic.

Clinical Examination. The mare appears to be slightly underweight. She is responsive but quiet (more quiet than normal, according to the owner). Her temperature is normal; pulse and respiration are slightly increased. Her mucous mem­branes are darker pink than normal (suggesting reduced blood flow), but capillary refill time is not abnormally long. She has a grade IV systolic murmur on the left side, most consistent with mitral regurgitation. Her lungs are normal on auscultation. No other abnormal findings are noted on physical examination. The mare is lunged for several minutes and reauscultated. No additional abnormalities are detected, except the mare’s heart rate and respiratory rate seem to take longer than normal to return to their resting levels. A blood sample is taken for analysis.

Comment. Results of the complete blood count (CBC) and serum chemistry are within normal limits. Echocardiography reveals mitral regurgitation associated with fibrotic thickening of the mitral valve. The chordae tendinae are intact. There is Somedilation (eccentric hypertrophy) of the left ventricle, but not of the left atrium.

Mitral valve thickening and insufficiency often develop with age, and mitral regurgitation is likely limiting this mare’s left ventricular performance. The resulting tendency for ineffi­cient pumping of blood into the systemic circulation can account for the decreased perfusion of the mucous mem­branes at rest and for the exercise intolerance and Iistlessness noticed by the owner. With each systolic contraction, the left ventricle is pumping blood both forward, into the aorta, and backward, through the leaky mitral valve and into the left atrium. The mild left ventricular hypertrophy and dilation are likely adaptive responses to this increased volume work. Animals with more severe mitral regurgitation also have left atrial dilation, associated with a much poorer prognosis than if there is no dilation or only left ventricular dilation.

Treatment. No medical treatment is indicated at this time. However, the owner needs to decrease the work by the mare. The mare should have only light, nonstressful activity. A follow-up examination is recommended in 3 to 6 months to assess the rate of progression of the mitral valve disease. If marked progression is noted at that time, the mare should be retired.