Understanding heart disease in the horse by Gayle D Hallowell

Horses rarely present in heart failure; but when they do, they often have end-stage disease and are frequently difficult, if not impossible, to treat.

Murmurs (associated with turbulent blood flow) and dysrhythmias are, however, common findings in the horse. The challenge with these cases is deciding which of these murmurs and dysrhythmias is likely to be performance- limiting or life-threatening and likely to increase anaesthetic risk. The key to being able to distinguish the innocuous physiological problems from potentially serious pathological ones relies upon accurate localisation and descriptions of any murmurs or dysrhythmias present, and then correct interpretation of any electrocardiograms obtained. Table 1 illustrates how murmurs are described conventionally.

The aims of this article are to discuss some of the common physiological and pathological cardiac conditions encountered in the horse.

Common physiological and pathological cardiac findings

Systolic ejection murmurs, diastolic squeaks and second degree atrioventricular blockade (2° AVB) are common findings in normal horses. Potentially significant murmurs are associated with regurgitation at the mitral and aortic valves and ones associated with congenital cardiac disease. Potentially significant dysrhythmias include atrial fibrillation and atrial and ventricular premature depolarisations, and ventricular tachycardia.

Physiological murmurs and dysrhythmias

Physiological ejection murmurs

Physiological ejection murmurs are very common in horses and are associated with turbulent blood flow in the heart and great vessels (aorta and pulmonary artery) when the valves are open in systole. Ones associated with the aortic valve are particularly common. These are usually focal murmurs, are loudest further forwards and usually – although not always – less than grade III/VI, are systolic and usually decrescendo in shape.

Diastolic squeaks

Early diastolic squeaks are regularly heard in young horses, are often intermittent, and are frequently dependent upon heart rate – they are usually more obvious at higher heart rates. They are heard between S2 and S3 (if present) and are completely benign, although they can be a surprise when they are discovered.

Second degree atrio-ventricular block (2° AVB)

Second degree atrio-ventricular blockage occurs at low heart rates and is associated with high parasympathetic tone. It is often particularly evident in fit horses. It is a regular irregular dysrhythmia and usually horses ‘drop’ beats at similar intervals – every 2, 4, 10 beats, for example. On ECG, there are normal complexes followed by a P wave without a QRS and T following it (Figure 1).

Figure 1: ECG from a horse with second degree atrio-ventricular blockade (2oAVB). There are normal complexes followed by a P wave without a conducted QRS complex and T wave. (Image courtesy of Mark Bowen)

Most common pathological findings

Mitral regurgitation

Mitral regurgitation is associated with a systolic murmur that is coarse and plateau- or band-shaped, and is loudest on the left-hand side over the apex (LICS5). It often radiates forwards and upwards (cranio-dorsally). This condition can lead to enlargement of the left atrium (and predispose horses to developing atrial fibrillation) and also to enlargement of the pulmonary artery caused by backflow of blood into the lungs, which can predispose to pulmonary artery rupture.

Horses found to have this murmur should undergo echocardiography in order to determine risks and current changes in chamber size. They require monitoring to assess progression.

Aortic regurgitation

Aortic regurgitation is often associated with water-hammer-shaped pulses (strong at the beginning quickly, becoming weak) and with a diastolic decrescendo murmur – which can sound ‘blowing’ or ‘musical’ – whilst being loudest on the left side over the base (forwards and upwards under the triceps muscle, from the apex beat; LICS4).

This condition can lead to enlargement of the left ventricle (Figure 2) and predispose horses to pathological ventricular dysrhythmias at exercise or rest. In addition, enlargement of the left ventricle can affect how the mitral valve cusps close together, predisposing horses to developing secondary mitral regurgitation and all of the problems discussed above under mitral regurgitation.

Figure 2: ECG from a horse with severe aortic regurgitation. The colour-flow Doppler is used to assess the extent of the regurgitant jet (large in this case with a wide base) and the left ventricle (which can be assessed qualitatively) is enlarged.

Horses found to have murmurs should undergo echocardiography and, ideally, ECGs at rest and under exercise.

Ventricular septal defects

The most common congenital abnormality seen in the horse is ventricular septal defect (VSD), which is a hole or defect in between the left and right ventricles. This is displayed in an echocardiographic image using colour- flow Doppler in Figure 3.

Figure 3: ECG from a horse with a large ventricular septal defect. The colour-flow Doppler is used to assess the extent of the defect and confirm blood flow through this hole in the interventricular septum. The defect is through the interventricular septum just below the level of the aortic valve.

Ventricular septal defect is associated with two murmurs – one systolic murmur on the left, heard loudest over the heart base. This murmur is associated with extra blood flowing through the pulmonic valve (relative pulmonic stenosis). In addition, there is a rightsided pansystolic murmur (which covers the heart sounds) associated with blood flowing through the VSD.

These murmurs are often very loud, particularly on the right. The loudness does not correlate with severity; in fact, the largest holes often have quieter murmurs as there is less turbulent blood flow, but they are usually more severe. Many of them are insignificant, but require echocardiography to look at changes in chamber size and assess the size of the defect, and to investigate that there is a good pressure difference between the left and right ventricles. If the pressure difference between the two ventricles falls, the jet direction can reverse, going from right to left, such that blood bypasses the lungs.

Atrial fibrillation

Atrial fibrillation is the most common pathological dysrhythmia in the horse. This is where normal conduction of electrical impulses through the atria is impaired. On auscultation, these horses have a normal to slow heart rate with an irregular rhythm with no S4. They often have a loud S3.

Although these animals have no P waves, they do show a normal QRS complex on an electrocardiogram, but these latter are not at regular intervals. Instead of P waves, many fibrillation waves (or F waves) are seen on the ECG (Figure 4). This condition can occur simply as a consequence of the large mass of the equine atria. However, it can also occur secondarily to mitral (and occasionally tricuspid) regurgitation caused by a pathological enlargement of the left (or right) atrium. Echocardiography is required to determine whether this is primary or secondary to underlying cardiac disease and should be performed prior to treatment.

Figure 4: ECG taken from a horse with atrial fibrillation. Variable R-R intervals can be seen, as can the absence of P waves and an undulating baseline (F waves)

Treatment with quinidine sulphate or electroconversion is most successful in those horses that have not had atrial fibrillation too long (<3 months) and that do not have underlying disease or that are in heart failure. Horses being treated with quinidine sulphate should have a continuous ECG that is monitored because the drug itself can lead to fatal dysrhythmias.

Atrial and ventricular premature depolarisations and ventricular tachycardia

It is beyond the scope of this article to discuss these dysrhythmias in detail, but they can be associated with cardiac chamber enlargement, myocarditis (or at least myocardial irritation) and toxicities. Examples include atrial ventricular depolarisations (Figure 5), ventricular premature depolarisations (Figure 6) and ventricular tachycardia (Figure 7). A trace of ventricular fibrillation that followed from refractory ventricular tachycardia is shown in Figure 8.

Figure 5: ECG taken from a horse with atrial premature depolarisations. Here there is a normal QRS complex that comes too early, with a reduced R-R interval (indicated with an arrow)

Figure 6: ECG taken from a horse with ventricular premature depolarisations. Here there are two abnormal wide and bizarre QRS complexes, with a reduced R-R interval (indicated with an arrow) followed by a compensatory pause before the next normal QRS complex

Figure 7: ECG from a horse with ventricular tachycardia. Here there is a very high heart rate (approximately 90-100 beats per minute) with runs of ventricular premature depolarisations and the R wave of the next complex on top of the T wave from the previous one (called R-on-T phenomenon and predisposes horses to developing ventricular fibrillation [and ultimately dying])

Figure 8: ECG taken from a horse with ventricular fibrillation. There are no obvious normal complexes and this eventually progressed to asystole

If these are discovered on electrocardiograms, any drugs being administered should be stopped, electrolytes should be evaluated and corrected, any underlying cardiac disease identified and then appropriate anti-dysrhythmogenic drugs administered.

Author

Gayle D Hallowell

MA VetMB CertVA DipACVIM-LAIM MRCVS

Gayle graduated from Cambridge vet school in 2002 and then completed an internship in equine studies and a residency in equine medicine and critical care at the Royal Veterinary College, during which time she was awarded a Certificate in Veterinary Anaesthesia and a Large Animal Internal Medicine Diploma from the American College of Veterinary Internal Medicine. She is currently undertaking a PhD in equine cardiology, funded by the Horserace Betting Levy Board, and has just become a lecturer in large animal internal medicine at the University of Nottingham

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