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Ultrasound in the assessment of valvular heart disease in the horse

by Karl Loren

Ultrasound & Heart Disease

C01 Ultrasound in the assessment of valvular heart disease in the horse

Dr Peter Darke

Department of Veterinary Clinical Studies, Royal (Dick) School of Veterinary Studies, University of Edinburgh .

HOME OF REST FOR HORSES FUNDING:    £ 45,550 Equipment Grant 1989.

THE Vingmed CFM700 Doppler Echocardiograph was received in January 1989. The diagnostic potential of this noninvasive technique has exceeded our expectations, and it has proved invaluable in cardiac investigations in horses. The generous funding of the majority of this piece of equipment by the Home of Rest for Horses has already been acknowledged.

Essentially, the echocardiograph enables us to examine in detail the structures inside the heart of fully conscious horses without any invasive interference or pain. We have already detected congenital abnormalities such as ventricular defect (the commonest form of "hole in the heart") and valve deformities in horses with heart murmurs. In older horses, we have detected degenerative thickening of valves and pericardial disease can easily be identified. The effectiveness (contractility) and coordination of heart muscle contraction is easily defined and the size of cardiac chambers can be measured accurately, for example to show heart enlargement in cardiac failure. We have developed previously unreported views of the heart and there is even an indication that the mass of the heart may be related to training and possibly the performance of racehorses.

Some of this detail can be gained from less sophisticated scanners, but the most valuable feature of Doppler echocardiography is the ability to measure the velocity and direction of blood flow at most points within the heart. Abnormal flow is particularly easily identified in Doppler scanning. Correct alignment with flow is critical in the accurate measurement of blood flow velocity by Doppler and we have developed a systematic protocol for imaging the heart to provide alignment with each valve in turn. This protocol will be described in a future publication. The Vingmed 2.25MHz mechanical ultrasound transducer has proved to be particularly well suited to equine scanning. Flow velocities at the valves of a series of normal horses have proved to be higher than those previously reported. We doubt the validity of the technique employed in the previous study, from the USA.

For routine cardiac investigation, one of the valuable features of our machine is the facility for colour flow mapping. This is a form of noninvasive angiography, with which the flow of blood in the heart can be demonstrated by Doppler on a two-dimensional echocardiographic scan, with the flow being coded in colour for velocity and direction. This part of the machine is currently only on loan to us. Abnormal flow is readily portrayed by colour flow mapping, giving a clear indication of valve leakage or of abnormal flow through heart or valve defects.

With colour-flow Doppler the source of almost any cardiac murmur can be traced with confidence. We have already scanned some 150 horses and confirmed that many heart murmurs are relatively innocent. In a number of riding or performance horses referred to us with heart murmurs or with poor performance, and in which the safety of the rider or the future of the horse has been questioned, we have been able to reassure the owner with some confidence that the horse had no serious heart defect. This can also ensure the animal's survival in cases where the horse's performance has declined, or where for example a racehorse has pulled up. The performance of the heart can now be assessed with accuracy and confidence, often to enable the horse to continue to be used, or to allow us to give an accurate prognosis.

Conversely, some dozen horses have been withdrawn from work or condemned with severe heart defects or with heart failure as a result of our investigations. We may be able to dissuade an owner from working a horse with impending heart failure. This is particularly important in cases where a horse shows signs of respiratory distress. In this connection, we have studied the heart function in detail in more than a dozen of the many horses referred to this College with chronic obstructive pulmonary disease (COPD or 'broken wind'). We are monitoring the effect of this disease on heart function before and after treatment. Cardiac dysfunction may be of considerable importance to the well-being and work potential of horses with this common disease. We are able to assess pressures within the heart and to evaluate valve patency in COPD with no distress to the animal. Pulmonary hypertension appears to be common.

We have detected tricuspid valve regurgitation in several Thoroughbreds with right-sided heart murmurs. This disease, which sometimes seems to be associated with loss of performance, does not appear previously to have been reported. We are keen to follow the progress made by these animals. A fibre-optic endoscope has been adapted for our use as a transoesophagal ultrasound transducer (probe). We hope that this will provide us with fresh 'views' of the heart from the heart base for routine heart scanning and for Doppler blood flow studies. Normally, both lungs and ribs limit the use of ultrasound in cardiology. Initial investigations have been promising, but the probe is currently being modified to penetrate better the large heart in this species. Previously, oesophagal probes have been used only in man.

Briefly, Doppler echocardiography has extended enormously our ability to demonstrate the exact nature of heart disease in horses. Furthermore, we can define the severity of disease, monitor response to treatment, rest or a return to work, and we can trace the source of almost any significant heart murmur. This is achieved without surgical interference or stress to the animal and it enables us to be far more confident in making prognoses than previously.

Some of this work has been published in Equine Veterinary Education on the first cover of which are some excellent illustrations of the application of colour flow, and spectral doppler to horses. Another paper is in press in the New Zealand Veterinary Journal.

Gratitude for assistance in the purchase of equipment has also been expressed at several veterinary meetings, including those of the Veterinary Cardiovascular Society in October 1989, when a paper was presented on pulsed Doppler in horses, and in April 1990 when some case studies employing Doppler echocardiography were presented. Also in April 1990, the assistance of the Home of Rest for Horses was acknowledged at a meeting of the British Veterinary Radiological Association when a paper on the value of Doppler echocardiography in the diagnosis of congenital heart disease was presented, and at the annual meeting of the European Society for Veterinary Cardiology in Amsterdam, where a paper on the value of Doppler in the assessment of cardiac output was given.

At an Open Day organised by the Royal (Dick) School of Veterinary Studies for veterinary surgeons, our clients, and members of the public in September 1989, two stands displaying our work with Doppler echocardiography were presented in which the help of the Home of Rest for Horses was acknowl- edged, and a practical demonstration of scanning a horse was arranged. A similar exercise was organised again in July, 1990 and we prepared a demonstration for the Royal Highland Show, held during June 1990. Use of the Doppler echocardiograph has revolutionised our investigations of heart disease.

PUBLICATIONS

Long, K.J. (1990) Doppler echocardiography in the horse. Equine Vet. Educ. 2,15-17.

Long, K.J. (1990). Clinical assessment of heart murmurs in the Horse. NZ. Vet. J. In Press.

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