Dummy Foal: Hypoxemic Ischemic Encephalopathy

History. A 14-year-old I horoughbred mare is presented for dystocia (difficult birth). The foal (a filly) was pulled with some difficulty. The filly was slow to stand and did not nurse voluntarily for several hours.

Clinical Examination. The foal has a slightly low tem­perature and increased pulse and respiratory rates. The mucous membranes are tacky to the touch (dehydrated) and dark pink in color (indicating poor perfusion and/or poor oxygenation). Capillary refill time is prolonged. The foal has a marked murmur similar to that heard with a patent ductus arteriosus. Peripheral pulses are decreased, and distal extremities are cool. Gastrointestinal motility is decreased. The foal appears mature physically, but she is acting dysmature when she attempts to stand, nurse, or lie down. Blood analysis reveals that the foal is not septic, but she is hypoxemic, has evidence of poor kidney function, and is acidotic.

Comment. Hypoxemic ischemic encephalopathy (HIE) occurs when a foal receives decreased oxygen for some time. This can occur before, during, or after foaling. With a dystocia, after the water breaks and while the foal is being pulled, the oxygen supply to the foal is decreased. The foal must rely on anaerobic metabolism during the period of low oxygen, which results in acidosis. Decreased oxygen elevates pulmonary vascular resist­ance (hypoxic vasoconstriction). The resulting accumulation (backup) of blood in the pulmonary artery» right ventricle, and right atrium raises the pressures there. If pressures in the right side of the heart exceed those in the left side, blood flow persists (from right to left) through the foramen ovale. (When a normal foal begins to breathe, pressures in the right side fall below those in the left side of the heart, so the foramen ovale closes.) The blood that flows through the foramen ovale in this foal reaches the aorta without passing through the lungs, and therefore without being oxygenated at all (right-to-left shunt).

Treatment. The foal needs oxygen to reverse the hypoxic pulmonary vasoconstriction and therefore the hypoxemia. This can be provided by nasal insufflation (tube placed in nasal cavity for delivery of oxygen). Additionally, the foal will be given drugs, such as dopamine, to increase cardiac contractility, cardiac out­put, and blood pressure. This treatment, in addition to intraven­ous fluids, will likely improve blood flow to the vital organs, including the brain and kidneys. Improved renal and respiratory function will reverse the acidosis. Foals with HIE often develop other complications, which need to be addressed as they arise.