Thumps can occur when excessive seating causes dehydration and massive electrolyte loss. As the bloodstream is depleted of salt ions, the function of the nervous system is compromised. The phrenic nerve that runs across the heart to the diaphragm muscle begins to fire in time with the heart beat, stimulating the diaphragm muscles to contract and causing the distinctive, thumping flutter.
Thumps should be regarded as a warning sign that the horse is suffering from acute dehydration and electrolyte imbalance. Immediately halt exercise, administer water and electrolytes, and call your vet for treatment instructions. If ignored, the consequences include founder, colic, heat exhaustion, or collapse. Recognizing this sign of impending metabolic failure can help to avoid much more problematic consequences.