Dr Laura Nath is a veterinary specialist in equine medicine and is currently researching heart rhythm abnormalities as part of her PhD studies through the University of Adelaide. These studies are supported by Racing Victoria and Agrifutures Australia and the research group is interested to find out whether the changes to the heart that occur in response to training increase abnormalities of the heart rhythm. Dr Nath shares some of her findings with us.
We are all familiar with the legacy of Phar Lap, who dominated racing in Australia through the Great Depression and whose success continued overseas. Tragically, Phar Lap died of acute enteritis shortly after winning North America’s richest race, the Agua Caliente. Scientists at the time were impressed by the size of Phar Lap’s heart, which weighed 6.35kg and is now an icon of the collection in the National Museum of Australia. Other legendary racehorses known to have large hearts include Key to the Mint, Mill Reef, Sham and Secretariat.
Heart size is particularly important to performance in horses. The resting heartbeat of a horse is around 28 beats per minute. This increases to 220–240 beats per minute during a race. The size of the heart determines the volume of blood that can be ejected with each heartbeat. So, the total amount of blood delivered to the muscles depends on both the size of the heart and the heart rate. Incredibly, horses have a unique ability to give themselves a blood transfusion at the onset of exercise. Blood is stored in the spleen, a particularly blood-rich organ, and this squeezes out an extra 10 litres of blood into the system during exercise. The muscles use the oxygen in the blood for fuel to create the energy required to propel the horse forward, moving 500kg at 60 kph.
The heart is a muscle, and just like the muscles of the skeleton, different horses will be born with bigger or smaller hearts. Also, just like other muscles, the heart will increase in size in response to training. This is also the case in human athletes and is known as ‘Athletic Heart Syndrome’. In human athletes, endurance sports in particular such as cycling and distance running are known to greatly increase heart size to approximately three times the size of non-athletes. In addition to an increase in heart size, trained horses have a lower resting heart rate and are more likely to have small leaks of the heart valves. These changes are generally associated with fitness and will promote oxygen delivery to the muscles and improve performance.
During exercise, horses’ hearts pump a phenomenal 200 litres of blood per minute which is similar to that delivered by a commercial fire hose. A household shower delivers 9 litres of water per minute. To achieve this, horses generate immense pressures in the heart, aorta, and pulmonary artery.
Heart rhythm abnormalities are usually very short-lived, lasting from seconds to minutes. They impair athletic performance because the heart is unable to pump efficiently. However, on rare occasions, heart rhythm abnormalities can also cause cardiac arrest, as was the case of Admire Rakti. Cardiac arrest is an electrical event within the heart, therefore autopsies of horses with cardiac arrest generally have no abnormal findings. In human athletes, death from cardiac arrest is exceptionally rare, approximately 1 in every 53,000 competing athletes per year. In humans, the causes of cardiac arrest are broadly distinct in young athletes less than 35 years compared to older athletes. In young athletes, inherited abnormalities affecting the shape of the heart, the position of the coronary arteries, and conduction of the heartbeat are most commonly involved. Cardiac disease in general is less common in fit people than in unfit people. However, similar to the general population, coronary artery disease is the main cause of cardiac arrest in older human athletes. In a small subset of athletes performing regular long hours of training over many years, scarring of the heart can also be implicated in cardiac arrest. Death from cardiac arrest in thoroughbred horses is approximately 50 times more common than in human athletes, but the underlying causes are largely unknown.
For more than a decade, Racing Victoria has collaborated with The University of Melbourne to perform comprehensive autopsies on all thoroughbreds that die during racing or training in metropolitan Melbourne. This program has provided fundamental knowledge about the nature and causes of thoroughbred fatalities of both musculoskeletal and cardiovascular origin. My co-workers and I have found that young horses early in their racing careers are particularly vulnerable to fatal heart rhythm abnormalities. Applying this to what is known about fatal rhythm abnormalities in human athletes, we are looking at developing screening tools to identify young horses that are most at risk. Such screening tools could include electrocardiography (ECG), echocardiography (heart ultrasound) and genetic testing.