Brisket Disease in a Heifer
History. A 2-year-old Hereford heifer was kept during the winter on a farm in the foothills of the Rocky Mountains outside Denver. In the Iate spring the heifer was transported to Climax, Colorado (altitude, 3400 m), for summer grazing.
After 6 weeks the owners noticed that the animal was having some difficulty breathing, was reluctant to move around the pasture, and had developed an enlarged pendulous brisket and some swelling between the jaws.Clinical Examination. Inspection of the heifer reveals a lethargic animal in poor condition. The respiratory and heart rates are elevated, and air seems to be moving well through the nostrils. The most noticeable observation is an enlarged and pendulous brisket. The swelling extends up the neck, and there is also a pendulous area between the jaws. The jugular veins are distended.
Palpation of the swollen brisket reveals that it is heavy; when it is squeezed, the imprints of the fingers remain for some time. The swelling between the mandibles behaves in a similar manner when palpated. The mucous membranes of the heifer are a normal color, and the lung sounds are unremarkable.
Comment. The swelling in the brisket and between the mandibles, which pits on palpation, is evidence of accumulation of interstitial edema in the dependent areas of the heifer, in which there is loose connective tissue. Accumulation of edema in these regions is an indication of the increase in systemic venous pressure, which is also causing jugular distention. Both are caused by right-sided heart failure.
The most likely cause of right-sided heart failure in a heifer grazing at high altitude is diffuse vasoconstriction of the pulmonary circulation as a result of chronic hypoxia (inspired oxygen tension at 3400-m elevation is 97 mm Hg, compared with 150 mm Hg at sea level). The smooth muscle in the pulmonary arteries contracts in response to hypoxia; if this response is maintained for several weeks, the amount of smooth muscle in the pulmonary arteries increases.
Furthermore, the animal produces extra erythrocytes in an attempt to transport more oxygen. These extra erythrocytes increase the hematocrit and make the blood more viscous and difficult to pump through the lung. Maintenance of cardiac output in the presence of the elevated pulmonary vascular resistance and increased blood viscosity requires an increase in pulmonary arterial and right ventricular pressure. Persistently working against the increased pressure causes right-sided heart failure.Treatment. If this animal is returned to lowland pasture, it will recover. The vasospasm in the pulmonary circulation and the hematocrit diminish once the hypoxic stimulus is removed. For immediate treatment, this animal should be moved to an area of lower altitude and could be given oxygen to relieve the hypoxic stimulus. This would cause a reduction in pulmonary arterial pressure, which would cause some relief. However, pulmonary arterial pressure would not decrease to normal levels, because of the increased amount of smooth muscle now present in the pulmonary arteries. In early disease, this process may be reversible. Once cardiac signs develop, however, the prognosis is guarded. Additional treatments include digoxin and diuretics, if desired.