Some breed predispositions have been reported. Complex congenital heart disease has been noted to have a higher incidence in the Arabian than in other breeds. In the USA, Standardbreds have been reported to have a higher inciof congenital heart disease, acquired valvular disease and arrhythmias than Thoroughbreds. In the UK, atrial fibrillation has been found to be relatively common in heavy-hunter type animals. This arrhythmia is seldom recorded in ponies. To some extent these findings are biased by the particular populations in the hospitals from which the studies were reported. In the horse, breed is a much less significant factor in the relative prevalence of cardiac disease than in small animals. The influence of breed on a horse's use as an athlete or as a breeding animal is a much more significant factor.