 In the case of horses that consistently demonstrate greater severity of EIPH this is most likely due to congenital factors, such as very high pulmonary vascular pressures.In horses that experience isolated episodes of increased severity of EIPH, possible contributing factors may include, amongst others, pulmonary infection or atrial fibrillation, inflammation, longer distances, longer duration of exercise, hard surfaces, steeplechasing/hurdling, increased length of career, breed (i.e.Thoroughbred greater than Standardbred), time in training/racing, genetics, and cold temperatures.