2.1 Heart disease in horses compared with other species
A wide variety of clinical signs can result from cardiac disease in horses. The exact nature of clinical signs depends on the effects of homeostatic mechanisms and the extent of changes in the distribution of blood supply to tissues. The clinical signs which can be attributed to heart disease must also be distinguished from similar signs which can result from diseases of other body systems.
The term 'heart failure' refers to a situation in which the heart fails to circulate sufficient blood to meet the metabolic requirements of the body for nutrients. In some animals, clinical signs of heart failure are only seen at times of high metabolic demand such as exercise. In other animals, delivery of blood fails to meet even basal requirements and marked disability or death ensues. The clinical signs associated with heart disease in man have been graded on a scale of lA by the New York Heart Association (NYHA) This scale has also been applied to small animals and it is a useful concept in horses.
Class 1 Clinical signs associated with known cardiac disease are not observed or are limited to exercise intolerance during strenuous athletic activity. Class 2 Clinical signs including tiring and tachypnoea become evident with ordinary levels of exercise. Class 3 The animal is comfortable at rest, but cannot tolerate any exercise. Class 4 Clinical signs are severe even at rest.
Horses are usually presented for relatively minor clinical signs in comparison with many small animals. It is relatively uncommon for horses to be presented with overt cardiac failure; the majority of equine cardiac patients fall into classes 1 and 2 of the NYHA scale. There are two principal diagnostic problems for the equine clinician.
One is judging whether heart disease is responsible for signs of exercise intolerance, or if the problem results from abnormalities in other body systems.
The second is whether currently asymptomatic disease may deteriorate and become a problem in the future. The approach to equine cardiology is therefore very different from that in small animal and human cardiology, where the greatest problem is improving the quality of life for patients in class 3 and providing life-saving therapy for patients in class 4 heart failure.