Although heart disease is rare, heart murmurs and arrhythmias (irregular heart rhythm) are commonly detected in horses, and their significance must be determined. In many cases a thorough examination of the cardiovascular system is all that is required to assess the relevance of an abnormal finding, but sometimes more advanced diagnostic procedures such as electrocardiography (ECG) or echocardiography may be necessary.
The horse is equipped with an enormous cardiac reserve so it is very common to discover a heart murmur or arrhythmia as an incidental finding when your vet examines your horse for an unrelated problem. However, some signs may alert you that there may be something wrong with your horses heart. The first thing that you may notice is that your horse gets tired more easily than normal or that it takes him/her longer to recover following exercise.
It is important to know however that the probability of heart disease is relatively low on the list of causes of poor performance, compared to respiratory or orthopaedic problems. As the disease progresses you may also see dramatic weight loss, ventral oedema or accumulation of fluid under the chest and abdomen or on the limbs, and distension of the jugular veins.
The first thing your vet will do is to perform a complete clinical examination to rule out other problems and to confirm that your horse is suffering from a heart problem. Specific things that your vet will do to assess the cardiovascular system include examination of the mucous membranes, particularly the gums and the eyes, and measure the capillary refill time (how long it takes an area of the gums to go pink again after it is pressed with a finger for a few seconds).
Your vet will also examine the jugular veins for any signs of distension or pulsing, and will assess how long it takes for the vein to fill after it is blocked with a finger. It is also important for your vet to assess the pulse rate and the quality of the pulse; this can be achieved by feeling a peripheral artery; this is most frequently done under the jaw or just behind the eye.
Your vet will then listen to the heart on both sides of the chest, this will allow detection of heart murmurs which are abnormal noises that occur during the cardiac cycle of contraction (systole) and relaxation (diastole) of each heart beat, which represents a turbulent flow of blood through the heart chambers and main vessels. Abnormalities of the hearts normal rhythm may also be detected.
If considered safe, your vet may also want to exercise your horse to evaluate their response and recovery rate.
Cardiac murmurs are common in horses – around 60% of horses have murmurs; however, most are functional murmurs which are unrelated to heart disease.
The critical part of the clinical examination is to identify those murmurs which are caused by cardiac disease, which may affect the athletic performance or even the riding safety of the animal, from functional murmurs which are of no clinical significance. In most cases the importance and origin of the murmur can be established with careful auscultation of the heart.
If in any doubt further investigation may be necessary by echocardiography or ultrasound scan of the heart. This will allow direct visualisation of the heart chambers and will show any abnormalities in any of the hearts structures. With the use of Doppler studies the actual flow of blood through the hearts valves and chambers can be assessed. Echocardiography is a non-invasive technique; all that will be required is for a small patch of hair to be clipped on both sides of the chest just behind the elbow, if your horse’s fur is too thick.
If your vet detects an arrhythmia they may want to perform an electrocardiogram (ECG). It is also important to notice at this stage that horses can have normal arrhythmias (for example they may drop a beat every several normal contractions) that are of no clinical significance or that may even indicate a healthy cardiac function.
An electrocardiogram is a recording of the changes in the electrical activity of the heart plotted against time. This is also a non-invasive technique that only involves placement of some electrodes on the body surface, particularly on the neck and chest. The ECG is recorded on a paper strip and will help identify the type and origin of the arrhythmia and its significance.