Physiologic Flow Murmurs (PFM) are created in normal horses because of the large heart size and the large amount of blood that is pumped out when the ventricles contract, causing large volumes of blood to flow out into the aorta and pulmonary artery. These murmurs are often heard early in systole and are very short in their duration as they arise when the aortic and pulmonary arteries initially open as the ventricles collapse and blood is ejected into the large blood vessels (similar to the gates on a dam being opened and water surging through). PFM can also be heard early in diastole and may be due to the initial flow of blood back into the ventricles from the atria as the ventricles relax.
Mitral Regurgitation (MR) is perhaps the most common murmur auscultated in the horse and this can be caused by mitral valve prolapse, thickening of the valve, malformation of the valve or rupture of the connective tissue that supports the valve. Many horses with MR can continue to compete successfully as an athlete but this will be dictated by the severity of the valve damage and highlights the importance of assessing the valve with cardiac ultrasound.
Aortic Regurgitation (AR) occurs more commonly in adult horses and is often associated with degeneration of the valve. In mild cases, athletic performance is not affected but as the horse ages, the AR is likely to deteriorate and annual check-ups to monitor the horse’s heart are recommended.
Tricuspid Regurgitation is not an uncommon finding in thoroughbred racehorses with no effect on performance detected. Usually there is no pathology of the valve causing the blood to flow back into the R ventricle, but instead the blood is able to flow back through the valve because the right ventricle has become enlarged simply by exercise producing normal muscle enlargement in that side of the heart. There are however, some instances where the valve can be deformed or the connective tissue supporting the valve can rupture, so it cannot be assumed that all cases of TR are irrelevant to the health of the horse.
Pulmonary Valve disorders are rare and are more likely to be associated with other cardiac abnormalities.
Endocarditis is a bacterial infection of the endocardium or inner lining of the heart, but mainly presents as a bacterial growth on the valve referred to as a vegetative lesion. This vegetative growth affects the valve’s function and reduces the valve’s ability to close and stop backflow of blood into the chamber. The murmur detected on auscultation will depend on where the growth is located (which valve) and the size of the vegetative growth. It is usually diagnosed using an echocardiogram, as the growth can be visualised directly. It can occur in foals that have had septicaemia but is also seen in mid- to old-age horses without any cause identified. In older horses, the aortic valve is more commonly affected, but cases can occur on both the mitral and tricuspid valves. Vegetative lesions on the tricuspid valve are usually the result of an infection in the jugular vein from an intravenous needle or catheter.
A “hole in the heart” will cause a murmur that is detected on cardiac auscultation and is the most common congenital heart abnormality that can occur. A hole can occur between the two atria or between the two ventricles, however, it occurs more frequently between the left and right ventricle. A hole between the ventricles is referred to as a Ventricular Septal Defect (VSD) and forms during foetal development. The size of the hole determines the effect the defect will have on athletic performance and life expectancy, with small defects having minimal effect on performance or life expectancy and much larger holes unable to support any athletic pursuits and result in a premature death. These holes can also be associated with other congenital defects in the heart, so if a murmur is suggestive of a VSD, a thorough cardiac evaluation is recommended.
Patent Ductus Arteriosis (PDA) is a rare cause of cardiac murmur when detected after a couple of days of age and produces what has been described as a machinery murmur (a continuous murmur that changes intensity throughout the cardiac cycle). A PDA can be heard in healthy day-old foals sometimes, but its persistence past a day or two is pathological and signals the existence of a complex congenital defect that is usually incompatible with life.
There are several other murmurs that can be generated in the heart and most will require a full cardiac workup to identify the origin and the significance of them. Unfortunately, with the possible exception of some bacterial infections on the valves, most pathological murmurs cannot be cured, and the aim is to differentiate the murmurs that are compatible with athletic performance from those that affect performance but are compatible with life and those that are not compatible with life. EQ
Heart imaging courtesy of Laura Nath: BVSc (Hons), MVSc, CertEM, FANZCVS. Registered specialist in Equine Medicine. PhD candidate University of Adelaide.