Cardiac failure or cardiovascular collapse in horses is a very rare event. If it does occur it is mainly isolated to the racehorse population almost exclusively. Most of these acute, devastating events happens shortly after exuberant exercise workout. This brings the heart rate up over 200 beats/minute and the pressures within the vessels in and surrounding the heart are extreme. Typically an aneurysm that has been undetected will rupture under these pressures and the horse will bleed out rapidly and collapse shortly after. Other times the pulmonary pressure will rupture vessels in the chest cavity and the the horse can have profuse bleeding into the lungs. Either way this is devastating and an event in which the horse will immediately expire. Please try and remember that these are very rare events and do not occur very often.