An insight Into Cardiology, by Ben Wakeling, MRCVS

The object of this article is to give the reader a basic understanding of how the heart pumps blood

around the body, and then describes some of the conditions which may affect it. As the equine heart rarely shows pathology, respiratory disease, lameness or simply poor levels of fitness are far more

likely to be the cause of poor performance than cardiac disease. The healthy equine heart’s output can increase as much as six times its resting output to cope with the demands of exercise. In general

this far exceeds our demands as riders upon our horses and therefore their hearts, even if they are suffering mild effects of heart disease. In reality only horses used for sports such as endurance, racing, eventing and polo potentially work at a level where mild cardiac pathology may cause loss of

performance.

Approximate Heart Rates In Large Fit Horses

Fitness can be judged more accurately by how quickly the horse’s heart rate

returns to its approximate resting level. In general the fitter the horse, the more rapidly the heart rate should return to normal

Approx beats per minute

Rest 26-45

Trot 80

Canter 110

Fast Canter 180

Gallop 240

Knowing how to take your horse’s pulse is obviously a helpful skill to acquire. The easiest arterial pulse to feel is just below the jaw. Only very gentle

pressure should be applied with the fingers in order to feel the rate and quality of the pulse. Another easily accessible point is just below and behind the eye, although this is usually more difficult to feel.

The capillary refill time measures the ability of the heart to provide blood to the peripheral tissues. It should be approximately 2 seconds in a healthy horse. This test requires practice to perfect. Healthy horses can have very pale gums and this should not be regarded as an abnormality. Owners often

confuse healthy pale gums as a sign of anaemia or heart disease which although possible is rarely the case. To carry out this test press your thumb onto the gum as shown below. Next remove your thumb and count the number of seconds it takes the blood to return to the blanched area of tissue.

Listening to, or auscultating, the heart using a stethoscope can be a relatively easy way to take a heart rate if a pulse is difficult to find. Due to the position of the horse’s heart in the chest, the stethoscope needs to be placed

relatively well behind and just above the level of the elbow as shown below.

The equine heart, in common with all mammals consists of two basic chambers each divided into atria (collecting chambers for blood returning to the heart) and ventricles (chambers used to propel the blood onwards around the body). The heart is situated within the body as shown in the

pictures below, with most of its volume covered by the muscles of the foreleg and shoulder. Rather than being on the right, the right ventricle actually wraps around in front of the left ventricle, making it slightly harder to hear with a stethoscope.

The RIGHT ATRIUM receives blood from the great veins (Vena Cavae). This blood has been exhausted of oxygen which it has supplied to the tissues of the body and gained carbon dioxide from these tissues as a waste product .

The atrium propels this blood through the TRICUSPID VALVE into the RIGHT VENTRICLE, which then pumps the blood through the PULMONIC VALVE to the lungs via the pulmonary arteries. Here the blood is replenished with oxygen and cleared of carbon dioxide. The blood returns to the LEFT ATRIUM

via the pulmonary veins, which then supplies the LEFT VENTRICLE through the MITRAL VALVE. The left ventricle propels oxygenated blood through the AORTIC VALVE and via the aorta around the body. The circuit is then complete. As the heart is the vascular system’s pump, the valves within this

organ are very important to ensure the driving forces created by the heart muscle push the blood in the right direction around the body. This movement of blood through the heart needs to be carried out in a co-ordinated manner as shown in the diagrams below.

In order for this cardiac cycle to work correctly, there is a complex mechanism of electrical stimulation causing the heart muscle to contract. The conduction pathway commences at the SINO-ATRIAL NODE located at the base of the right atrium. The electrical impulse created here travels across

both the atria causing contraction, the right atrium starting to contract a fraction before the left. The impulse then reaches the ATRIO-VENTRICULAR node which is situated at the base of the septum (wall) between the two ventricles. This clever piece of natural design only allows the AV node to

respond to the electrical impulse which has passed across the atria. The rest of the ventricular muscle is insulated from the atrial muscle and must be stimulated by the AV node only. The AV node slows the passage of the impulse allowing the atria to contract and discharge the blood they contain

into the ventricles, prior to stimulating the ventricles to contract.

It is quite normal for a healthy equine heart to have a regularly irregular heartbeat, which is often seen in fit horses such as thoroughbreds with relatively large hearts and slow heartbeats at rest.

Beat-beat-beat--------beat-beat-beat---------beat-beat-beat.

Diagnostic Aids to Examine the Heart

Ultrasound is very useful to visualise the heart and make a diagnosis. The heart is best visualised from the right side.

An ECG is used to diagnose arrhythmias. The ECG picks up the electrical impulses caused by the contraction of the heart muscle in the atria and ventricles. This is printed out on a record sheet which can then be analysed.

Diseases of the Equine Heart.

Basic Principles:

1. Valvular heart disease is not as common a cause of poor performance as is widely thought. It is unlikely to cause collapse during exercise without first causing obvious loss of performance which would normally already have

been noticed by the owner.

2. Arrythmias (Irregularly irregular heartbeat), especially under exercise, are far more likely to be the cardiac cause of poor performance.

Valve Failure

Valvular Regurgitation occurs when the valve fails. The turbulence created by this back flow of blood creates a noise which is called a MURMUR. In general failure of the one way valves at the exits of the ventricles (Pulmonic and Aortic Valves), do not tend to cause significant effects on performance.

Tricuspid Regurgitation (right AV valve) is a common finding in fit National Hunt horses and potentially other large fit sport horses, when it is seldom a cause for loss of performance. The Tricuspid Valve in the right side of the heart is less important as it forms part of the relatively low pressure side of the

circulation. It can however progress to cause loss of performance and even congestive heart failure. Ultrasound examination is very useful in deciding the severity of the condition. It is more likely to be a problem if diagnosed in young horses or ponies which are usually not affected by this condition.

Mitral Regurgitation

The most important valve to fail is the Mitral Valve, as it forms the relatively high pressure side of the horse’s circulation. In terms of its significance, low grade focal murmurs are unlikely to affect performance in even the highest

performing horses. The key is to decide whether the regurgitation is likely to progress. The horse with a moderate widespread murmur may have evidence of loss of performance, but still be able to carry out work such as dressage, show-jumping and hacking. Horses that also have an arrhythmia or

increased resting heart rate have a poorer prognosis. As the condition progresses, then loss of performance becomes more evident, and in extreme and rare cases congestive heart failure can develop. This is because in severe valve failure the atria become enlarged and progressively thin walled.

The valves have tissue strings called CHORDAE TENDINEAE which act in a similar way to the lines attached to an open parachute to hold the valve in shape. These can rupture causing a rapid loss of heart function. Fluid from the

blood that the heart should be pumping onward then remains in the tissues of

the lungs. The right heart then becomes overloaded causing fluid to collect in the tissues of the body. This is called congestive heart failure, the clinical signs of which include a cough, filling in the legs and under the belly, increased heart rate, dullness and inappetance.

Congenital disease

Ventricular Septal Defects.

Ventricular Septal Defects (VSD) are caused by the failure of the septum (wall) between the ventricles to fuse early in foetal development. A hole is therefore present connecting the ventricles. Small VSDs are not likely to affect

performance but need to be scanned using ultrasound, as the severity of the murmur rarely matches the severity of the condition. These animals are not suitable for breeding and the condition may be excluded from insurance policies.

Arrythmias

Atrial fibrillation(AF)

Atrial fibrillation is a common cause for cardiac related loss of performance, and in such cases affects the horse’s ability to be sold as a performance animal. Horses can be perfectly suitable for less arduous activities such as

dressage, show-jumping and hacking. The heart should be examined for underlying heart disease, as this can often be the cause of the arrhythmia and affects the prognosis. In the longer term it should be understood that the condition is likely to progress.

Atrial Fibrillation is caused when the Sino-Atrial node no longer provides the only source of electrical stimulus for atrial muscle contraction. Consequently with multiple sites acting as stimuli, the atria fail to contract fully and regularly. The atria start to “rapidly quiver” or fibrillate producing many semi-

contractions. The A-V node does not get a correct and regular signal to cause the ventricles to contract. The end result is the atria fibrillating and the ventricles contracting irregularly. Sudden onset (paroxysmal) AF can occur under exercise, when it may cause collapse.