A recent study shows that only 10% of horses with heart murmurs have reduced performance. Nevertheless, heart murmurs detected at pre-purchase examinations have a significant effect on the sale price.
Murmurs are caused by turbulence in the blood stream. Sometimes they are the result of changes in the blood - such as anaemia or hypoproteinaemia (low blood protein level). Or they may be caused by structural changes in the heart - such as insufficiency (leaking), or stenosis of a valve. Occasionally, congenital problems such as a septal defect ("hole in the heart") may be responsible.
Heart murmurs may be detected in horses undergoing prepurchase examinations. Even careful examination of the heart with a stethoscope, may not be able to identify the source of the murmur. This makes it difficult to interpret the significance of the finding for future athletic usefulness.
The advent of echocardiography (ultrasound examination of the heart) has allowed a much more detailed examination of the heart than has previously been possible. In particular, colour Doppler ultrasound highlights the presence of abnormal jets of blood and so can be used to assess the patency of the heart valves.
Between 1997 and 2002, Dr Marianne Sloet and her colleagues at the University of Utrecht in the Netherlands, assessed the significance of murmurs that were found during prepurchase examinations or at long distance rides.
They found murmurs in sixty two horses at prepurchase examination, and in fifteen at long distance rides. The horses were mostly warmbloods (64). In addition, there were nine arabs, two ponies, one Belgian draught, and one standardbred. Twenty four horses were aged between one and five years and had not worked much previously. Fifty three horses were older than five years and were in regular work.
Horses that had murmurs were given a full cardiological examination. An ECG (electrocardiogram) was used to check heart rate and rhythm. Echocardiography allowed the researchers to assess valvular function , and to take various measurements of the heart such as the diameters of the aorta and the pulmonary artery, the main arteries leaving the heart. They also recorded the left atrial dimensions.
The researchers recorded whether the horses were sold or not. And if sold, whether they sold for the asking price or at a reduced price. They also recorded whether the abnormalities had any effect on performance.
They found an abnormal rhythm in seven horses. Six had atrial fibrillation, in which there is a completely irregular heart rhythm associated with very rapid contractions of the atria. In five of those horses the murmur was caused by insufficiency (leaking) of the valve between the left atrium and the left ventricle. Another horse had premature ventricular contractions.
Seven percent of the murmurs were loud; the rest were less obvious. In 68 horses there was good correlation between the findings on listening to the heart with a stethoscope and those of the echocardiograph. Sometimes additional leaky valves were found on echocardiography that had not been detected with the stethoscope. In three horses no insufficiency could be found despite there being a murmur. Using the colour Doppler ultrasound scan, the researchers found leaking of the mitral (left atrio-ventricular (AV)) valves in 63 cases and of the tricuspid (right AV) valve in 40. They classed the insufficiency as severe in 17% of cases.
"The left atrial diameter (LAD) was the most important feature to predict future performance"reports Sloet. The left atrium increases in size in response to the pressure of blood leaking back through the mitral valve between the ventricle and the atrium. The LAD was greater than 14 cm in 15 cases. Six of these horses had severe mitral insufficiency. The other nine had moderate mitral insufficiency.
"Only 10% of the warmblood horses in our study, mostly used for dressage and show jumping, showed a depressed performance" Dr Sloet reports. "But the presence of a heart murmur had a significant effect on the sale price."
Forty three percent of the horses sold for the asking price, despite having a murmur. Twenty one percent were sold for a lower price. The sale did not proceed because of the murmur in 36% of the horses.
One horse which had a murmur but no valvular abnormality did not sell despite a report saying that there was nothing wrong. Some expensive horses sold even though they had a left atrial diameter greater than 14cm. The vendor usually guaranteed them for two years. (This was probably because the vendor knew the murmur had been present for a while and had not affected performance.)\
"Echocardiography is a specialist job. You need to do hundreds before you can do it properly." points out Dr Sloet." In anything other than a very cheap horse you should refer purchase exams with murmurs for echocardiography. "
source: Conference on Equine Sports Medicine and Science, Saumur. 2002