8.4.1 Causes of sudden death
There is no evidence that animals with mild-moderate valvular insufficiency are any more likely to drop dead than any other horse. Sudden death in animals with previously diagnosed valvular disease is only likely to result from rupture of a major chorda tendinea, rupture of the pulmonary artery, or a severe arrhythmia. Coronary artery disease is exceptionally rare in horses. Certainly they do not develop atheromatous plaques in the coronary arteries which result in occlusion and cause a heart attack'. This term is commonly used to account for sudden death in horses because it seems a plausible and understandable explanation to owners, but it is entirely inaccurate.
Acute onset tachydysrhythmias and bradydysrhythmias may be the cause of sudden death in animals in which no abnormalities are detected at routine examination. Unfortunately there is no way of proving this diagnosis without an ECG recorded at the time of death. Arrhythmias which are sufficiently severe to account for sudden death are very seldom encountered on clinical examination, even in sick animals. However, despite limited documentary evidence in animals or man, arrhythmias are still thought to cause sudden death in some cases.
Horses with congestive heart failure (CHF) may die suddenly. In animals with left-sided heart disease this is may be as a result of rupture of the pulmonary artery following pulmonary hypertension. Clinical signs usually develop from the point of poor athletic performance to CHF over a period of weeks to years. Horses with severe mitral regurgitation are particularly at risk and should be monitored echocardiographically if possible. For this reason animals with CHF should not be ridden under any circumstances. However, very few horses with valvular heart disease are in CHF.
Rupture of a major chorda tendinea may result in sudden left-sided cardiac failure. The clinical signs usually develop over a few hours to days, but occaresult in sudden death. If this cause is suspected, PM should be carried out with great care so that the chordae are not damaged in the course of opening the left ventricle.
Peripheral cardiovascular disease
Peripheral cardiovascular disease, in the broadest sense of the term, is a relacommon cause of sudden death. The most common cause of sudden death recognised at PM is rupture of a major artery. This may be traumatic or result from pre-existing disease. Conditions include rupture of the pulmonary artery, aortic root, a major pulmonary vessel, a carotid artery (due to mycotic infection of the guttural pouch), a major abdominal artery (likely to be associated with parasitic damage), and rupture of a uterine artery (usually in the near-term mare). Major arteries may also be damaged as a result of fractures, particularly pelvic fractures.
A summary of the potential causes of sudden death, including non-cardiodisease is shown in Table 8.9. Readers are advised to refer to papers in the list of further reading for more detailed discussions.