When horses are working hard, especially in hot weather, they are particularly prone to dehydration and electrolyte imbalances. The official term for one symptom of these problems is a mouthful: synchronous diaphragmatic flutter (SDF). Horse owners and veterinarians who have dealt with this condition through the years generally identify it with a simpler, more direct term: thumps.

Thumps is not a common condition. Many highly experienced veterinarians have never encountered a case, unless they have been involved in endurance races. The same is true of many horse owners.

“I have seen only three cases of thumps in my career,” says Wes Schroeder, DVM, a Maple Plain, Minn., practitioner who has officiated at a number of endurance races and competitive trail rides. “All of them were at endurance races, and fortunately all three of them recovered.”

The condition, when it does occur, normally appears in a horse that has not been properly conditioned for an endurance race or one which has refused to ingest sufficient quantities of water during competition on a hot day.

While no one is positive about everything that goes on within the horse’s body when thumps occurs, it is known that the phrenic nerve becomes hyperexcitable, possibly due to electrolyte imbalance. When hyperexcitable, this nerve (which runs across the heart) is easily stimulated by the electrical impulses that normally travel across the heart. The phrenic nerve controls contraction of the diaphragm (which controls lung movement and breathing), and thus hyperstimulation of this nerve results in hyperstimulation of the diaphragm–which then contracts simultaneo