Mitral Insufficiency in a Dog

History. A 12-year-old cocker spaniel is brought to a veterinary hospital because of a recent deterioration in its condition. The dog has been a faithful pet and has always enjoyed exercising with its owner, but over the past few months the owner has noticed an increasing reluctance to exercise.

Clinical Examination. You have examined this dog on many occasions, and it has always been friendly, but when you walk into the examination room, the dog greets you with only a modest tail wag. It stands with its head down and its tongue hanging out; it is panting. It walks reluctantly toward you when you call it. The dog was formerly fat but is now in about normal flesh, so over the past few months it has lost some weight.

You lift the dog onto the examination table and begin by looking at the mucous membranes, which appear normal in color. The dog’s temperature is normal. The dog is panting, which makes auscultation of the chest difficult, but on the occasions when the dog breathes without panting, you notice some increased sounds in the trachea and in all the lung fields, which sound like fluid bubbling within the air spaces of the lungs. The heart rate is dramatically increased, and a loud murmur is audible over the mitral area during systole. You tell the owner that you suspect that the dog has a heart problem, which is leading to the accumulation of fluid in the lungs.

You take chest radiographs and an arterial blood sample for measurement of blood gas tensions. The chest radiograph shows an enlarged heart, particularly the left ventricle. The lungs are diffusely more dense than normal, and the densities have a fluffy appearance, which suggests that they are in the alveoli.

An echocardiogram is performed to assess further the mitral valve regurgitation. The echocardiogram demonstrates thickening of the septal leaflet as well as the lateral leaflet. There is a flail of the mitral valve into the atria during ven­tricular systole. The left ventricle appears dilated, and left ventricular contractility is decreased.

Comment. Left-sided heart failure is accompanied by mitral valve insufficiency, and thus blood leaking back into the left atrium during systole creates a murmur. The elevation in left atrial pressure as a result of mitral regurgitation is leading to increased pressure in the pulmonary veins and capillaries. The increased pulmonary capillary hydrostatic pressure causes fluid filtration into the interstitium and now into the alveolar air spaces. This condition has likely been progressing for some time, and only when it became severe enough for fluid to accumulate in the air spaces of the lung did the owner notice the deterioration in the dog’s condition.

The hypoxemia is a result of ventilation/perfusion mis­matching because of accumulation of fluids within the alveolar spaces. These fluid-filled spaces are still perfused, but the blood passing through this region does not pick up a sufficient amount of oxygen. This results in hypoxemia. The hypoxemia stimulates ventilation, and the increase in total ventilation to the lung is sufficient to eliminate more carbon dioxide than normal, so Paco₂ is 30 mm Hg rather than the normal level of 40 mm Hg.

Treatment. The dog is treated with a diuretic and a digitalis glycoside. The diuretic causes fluid elimination by the kid­neys, which reduces vascular volume and intravascular pres­sures and therefore reduces the amount of fluid being filtered into the lung. Over time this leads to resolution of the edema.

The digitalis glycoside increases the contractility of the heart and thus the dog’s cardiac output, which improves its ability to exercise.