South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Exercise-associated hyponatremia: For human athletes, the recommendation of liberal drinking
during endurance exercise was strongly promulgated during the last couple of decades of the 20th
century. In combination with a dramatic increase in marathon running towards the end of that
century, this recommendation actually led to development of a new problem among human
endurance athletes: exercise-associated hyponatremia. Hyponatremia, induced by excessive fluid
replacement with water or a variety of hypotonic sports drinks, actually resulted in serious
neurological impairment and death in a number of “weekend warrior” marathon competitors.
Fortunately, despite aggressive management of endurance horses to encourage fluid intake,
primarily by frequent administration of electrolytes before and during competition, riders have
never been able to get their horses to drink excessive amounts of rehydration fluids (Figure 3).
Thus, hyponatremia has not yet been recognized as a serious problem of endurance horses.
Figure 3. In contrast to their human counterparts, horses can absorb substantial amounts of water and electrolytes
from the large colon during prolonged exercise, leading to a typical “tucked-up” appearance to the abdomen toward
the later stages of competition (left). Further, oral electrolyte pastes are commonly used to stimulate drinking in
endurance horses (right).
Decrease in effective circulating volume and autonomic dysregulation: As liberal use of
intravenous fluids became the norm for treatment of collapsed marathon runners and exhausted
endurance horses, a common clinical observation was that athletes of both species often recovered
rapidly in response to administration of a volume of fluids that was substantially less than that
required for correction of estimated fluid losses. This observation led to the suggestion that
decreased effective circulating volume, rather than the overall magnitude of body fluid loss, may
be a more important pathophysiologic mechanism for either collapse or exhaustion in human and
equine endurance athletes. In fact, Noakes and coworkers in South Africa called into question the
importance of the overall loss of body fluids during endurance exercise and have encouraged
researchers to focus greater attention on potential dysregulation of autonomic vascular tone as a
potentially more important factor in development of collapse, especially in the immediate postexercise period. Rather than administration of intravenous fluids as emergency treatment of
collapsed athletes (that have a normal core body temperature), these researchers have advocated
resting in the supine position with the lower extremities elevated for several minutes. This
recommendation has led to good success in initial triage of collapsed endurance athletes,
furthering support for altered autonomic vascular tone as an important mechanism leading to
collapse. Dysregulation of autonomic tone has been hypothesized to lead to pooling of blood in
the lower extremities on cessation of prolonged endurance exercise. As a consequence, postural
hypotension at the finish, accompanying the decrease in heart rate and cardiac output following
cessation of exercise, may be a more important cause of collapse than the magnitude of body fluid
loss.
In an attempt to investigate whether or not autonomic dysregulation occurs in endurance horses,
the author and colleagues recently measured indirect blood pressure in a group of horses that
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