I WAS recently asked by one of the staff here at Mark Johnston Racing whether racehorses could have heart attacks and, if so, could they be a cause of sudden death during exercise. The scientific answer is: No.

The term “heart attack” has incorrectly evolved into referring to sudden death associated with heart disease. In human medicine, the term refers to myocardial infarction (coronary heart disease) caused by clogged blood vessels which reduces blood flow to the heart muscle itself. The result is damage to, or death of, the heart muscle, which can cause functional failure of the heart and death of the individual. Because horses are herbivores and, unlike humans, do not suffer from coronary artery disease, heart attacks in their true definition rarely occur.

Heart disease, however, and cardiovascular failure can be associated with sudden death in exercising horses. The equine heart is an amazing organ. A fit horse’s heart rate can vary from 25 beats per minute at rest to 240 beats per minute at maximum exercise.

The job of the heart is to pump blood round the body, providing oxygen to all the tissues of the body. Up to 50 litres of blood are constantly pumped round the body by the heart. The size of the heart varies between horses, and a large heart has been associated with superior performance in racehorses. The normal weight range is between 3.4 and 8kg. The great American racehorse Secretariat was said to have had a heart weighing 10kg.

The heart is a large, four-chambered muscular pump that undergoes co-ordinated electrical stimulation allowing its chambers to work in a synchronised fashion, pumping blood first to the lungs to be oxygenated and then around the body. In racing thoroughbreds, sudden death is rare and our understanding of the causes of sudden death is limited as often the investigation has to rely on post-mortem findings, because death has occurred before any investigation can take place.

Acute equine cardiovascular disease typically results from direct insult to the heart muscles, blood vessels or the electrical signalling system that controls the heartbeat. Examples are arrhythmias (irregular heart beat), rupture of the great vessels carrying blood from the heart (aorta or pulmonary artery), and bacterial or viral disease of the heart muscle or valves. Because the heart is incapable of regenerating new heart muscle, any acute insult can lead to functional failure and death.

CARDIAC ARRHYTHMIA

Cardiac arrhythmia is defined as abnormal electrical activity in the heart which causes an irregular heartbeat. The electrical stimulation for each heart contraction is initiated in the sino-atrial node, a special group of cells in the heart which act like a pacemaker and determine the rate at which the heart beats.

The electrical impulse created by the sino-atrial node spreads across the atria, the small chambers of the heart, creating a contraction which helps fill and prime the larger ventricles. The impulse then moves through the atrioventricular node which sends a wave of impulses across the ventricles, making them contract and force blood out at high pressure, either to the lungs from the right ventricle or to the body through the aorta from the left ventricle. An irregular beat of the heart decreases its efficiency and reduces performance.

In severe circumstances, the heart rhythm can become increased (tachycardia), or the impulses can be blocked through faulty conduction leading to a decreased heart rate (bradycardia). At peak performance, acute arrhythmia can lead to sudden collapse and death as the heart’s efficiency is massively impaired and oxygen circulation in the blood around the body fails.

COMMON CAUSE OF DEATH

Vascular rupture is the most common cause of sudden death of horses during exercise. Pulmonary artery rupture can be spontaneous or associated with hypertension (high pressure) in the blood vessels of the lungs. Aortic root aneurysm is a very rare but serious condition. The typical aneurysm is an abnormal bulging of the aorta, which is at risk of rupturing.

Rupture of the aneurism within the aorta can lead to collapse or sudden death, usually when the horse is exercising, as heart rate and blood pressure increase with the pressures of exercise, putting more strain on the vessel walls. Acute rupture of the chordate tendinae, the supporting structures of the valves of the heart, can lead to sudden collapse and death during exercise due to acute heart failure.

Endocarditis is an inflammation usually caused by bacterial infection of the valves of the heart and the endocardial tissue.

It is hypothesised that damage to the valves or endocardium due to repetitive strain allows bacteria to colonise the damaged region, in turn creating more damage. Often horses with endocarditis may show signs of fever, depression high respiratory rate and new onset murmurs.

Myocarditis is inflammation of the heart muscle. Inflammatory cells infiltrate the tissue and the cardiac muscle cells can become damaged and die. It can be infectious, caused by bacteria or viral pathogens, but can also be non-infectious inflammation. In acute cases, cardiac failure due to reduced cardiac muscle function leading to collapse and death can be seen. Thankfully, acute cardiac failure in horses is a rare occurrence.

A graduate of the University of London, Neil Mechie is a veterinary surgeon with Mark Johnston Racing. This article first appeared in the Kingsley Klarion.