A recently released report on the follow-up of performance horses that had a cardiac abnormality identified as part of their pre-purchase exam (PPE) has highlighted the need to have horses fully evaluated when a murmur or arrhythmia is detected during a routine PPE.
A recent study found that relying on the horse’s breed, sex and age, together with assessing the heart with a stethoscope alone, were not always a reliable predictor of a horse’s future performance.
“Horses have a large heart with
a relatively slow heart rate…”
The study follows up on a multitude of horses that presented with a cardiac abnormality during a routine pre-purchase examination and assesses whether the abnormalities detected affected the level of future performance or longevity at their current level of performance. One of the aims of the study was to assess whether the full cardiac workup done on the horses as a result of an abnormality detected during a PPE was able to accurately predict future performance regarding this abnormality.
As we know, PPEs are very valuable for prospective owners to achieve peace of mind when buying a new horse. Often the horse they are looking to buy is that “one horse” they think can lead them to that goal they have been aiming at for a very long time. Prospective owners have saved their money and want some sort of guarantee that the horse they buy is going to do the job they are purchasing it to do for many years.
Unfortunately, when purchasing a new horse, there are no guarantees that the horse will perform the role it is expected to do, or that it will be able to continually perform at this level for many years after it is bought. But there are ways of improving the likelihood that the horse will perform at its expected level of performance, for a reasonable length of time, once it has been purchased. One of these ways is to have a PPE done by a reputable and experienced veterinarian.
The veterinarian’s role is not to tell you whether to buy the horse or not, or whether the horse has the skills to achieve the goals the prospective owner anticipates it has, it is their role to provide a reasonable assessment of the health of the horse and alert the prospective purchaser to any anomalies that may or may not affect the horse in the foreseeable future. Whilst it can be easy to detect anatomical and physiological abnormalities during an in-depth examination on the horse, deciding on the future impact of these abnormalities on the horse’s performance can be more difficult.
FURTHER DIAGNOSTICS
For horses that show lameness or swelling, radiographs or ultrasound examinations can be performed to inform the purchaser of the anatomical abnormalities present and help predict the ongoing effects any X-ray or ultrasound changes may have in the future. Most prospective purchasers are aware of the need to have X-rays performed if there is a suggestion of a joint not being perfect and accept the need to have these further diagnostic procedures performed.
Many purchasers, however, would be less familiar with the concept of performing more in-depth examinations for cardiac murmurs or arrhythmias detected during a routine PPE. The reason for this is difficult to determine, however, it may stem from the fact that many horses with cardiac murmurs detected on a routine examination do go on to perform without issue. Horses have a large heart with a relatively slow heart rate and therefore physiologic murmurs and some arrhythmias are common and occur without any underlying cardiac injury. Any turbulence in blood flow as the blood moves through the chambers and out through the large blood vessels (aorta and pulmonary arteries) can be detected as a murmur. By careful auscultation and identifying the location of the murmur and characterising the type and intensity, a veterinarian can identify physiologic murmurs and be confident to eliminate them as a course of future concern. However, when heart murmurs are not physiologic, prognosticating on their clinical importance and their potential to negatively affect performance is fraught with danger unless a full cardiac workup is performed.
One of the findings from this study was that relying on the horse’s breed, sex and age, together with assessing the heart with a stethoscope alone, were not always a reliable predictor of a horse’s future performance. The best predictor of whether a cardiac abnormality would affect future performance, put the rider or horse at risk, or shorten the lifespan of the horse, was to do a comprehensive cardiac examination.
An echocardiogram is currently the best diagnostic tool available to assess a heart murmur. This is an ultrasound assessment of the heart that allows direct visualisation of the functioning heart. During this procedure, the size of the atrial and ventricular chambers, as well as the size of the large blood vessels, can be assessed and measured, the valves between chambers can be examined, and the flow of blood through the heart can be followed.
Although most veterinarians do have an ultrasound machine in their cars, electrocardiography requires a dedicated program to interpret images and a small probe that can fit between the ribs to examine the heart, and is different to the ultrasound probes used for rectal or tendon scans. There are three types of modes used when assessing the heart:
- B-mode (2D) to look at the spatial arrangement, similar to how we look at tendons and other structures
- M-mode to assess the heart and chamber sizes when the heart is pumping (structures are in motion)
- Doppler to assess blood flow through the chambers and valves
Together these allow the veterinarian to look for thickenings in the heart wall, increased sizes of heart chambers, and disruptions to normal blood flow through the heart valves that can indicate pathology in the heart.
The equine cardiovascular system; horses have a large heart with a relatively slow heart rate.
TIME FOR AN ECG
When cardiac arrhythmias are detected, the best diagnostic approach to investigate them is to perform an electrocardiogram (ECG). An ECG measures the electrical activity generated by the heart and displays this as a characteristic number of waves in a repeating configuration (P wave, followed by a QRS complex, and lastly a T wave) as it moves along the screen. Any abnormalities in the electrical conduction are displayed as changes in the waveform or in the duration between waves; this can be utilised by the veterinarian to identify the site in the heart where the arrhythmia is generated from, and the type of arrhythmia present. For example, atrial fibrillation is diagnosed based on the lack of P waves and a string of QRS complexes that show no order or uniformity in their occurrence.
In cases where there is both an arrhythmia and a murmur, both an echocardiogram and an ECG are recommended.
Further tests such as an exercising ECG or a 24-hour continuous monitoring ECG are required with some horses that show intermittent heart abnormalities or show abnormalities when working. The appearance of abnormal wave forms during exercise can alert the veterinarian to an underlying issue that may limit performance in the future. In cases where a major heart abnormality is suspected, an exercising ECG should not be performed unless the veterinarian has performed an in-depth clinical assessment as well as an echocardiogram and resting ECG and is satisfied that there is no evidence the horse has congestive heart failure.
Cardiac troponin 1 is an enzyme that is released into the blood when myocardial cells are damaged. Levels rise with active cardiac disease, making it of value when conditions such as congestive heart failure or cardiomyopathy are suspected. There are many causes of cardiac murmurs and arrhythmias, and these can be discussed at another time. The important message to take home here is that, although the majority of murmurs and arrhythmias detected during a PPE are of minimal clinical significance, relying only on cardiac auscultation to prognosticate the future performance and longevity of a horse presenting with a cardiac anomaly is a risk. Taking that extra step to perform a full cardiac examination if there is any doubt as to the significance of a murmur or arrhythmia can prevent heartache and disappointment in the future. EQ
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