The horse’s heart is a fantastically effective organ, and a relatively uncommon source of any problems. Nonetheless heart problems do occur, and can range in significance from being mildly performance-limiting, to life threatening, to having no significance at all.
Auscultation/Examining The Heart
Auscultation (listening to) the horse’s heart is a quick and simple way of assessing the heart, although not without its problems. Here is a list of what your vet is assessing when he listens to your horse’s heart:
- Heart Rate
- This is expressed in beats-per-minute, usually worked out by counting the number of beats in 15 seconds, then multiplying by four.
- A normal heart rate is less than 40bpm, although excitement and stress of a vet visit means that we may not see a true resting heart rate. The larger and fitter the horse, the lower the heart rate tends to be.
- Increased heart rates have many causes, including excitement, stress, pain, infection, respiratory and metabolic problems, and primary heart problems.
- Heart Rhythm
- A normal horse’s heart should beat evenly, as if in time with a metronome.
- There may be a slight increase in heart rate during inspiration, a normal finding known as sinus arrhythmia.
- Fit and relaxed horses may occasionally “drop” a heartbeat, due to the efficiency of their heart. This is a normal finding known as 2nd-degree atrio-ventricular block, and again is a normal finding. It should disappear with exercise.
- Abnormal rhythms occur due to electrical disturbances in the heart, most commonly atrial fibrillation. They are best examined with an electrocardiogram.
- Heart Murmurs
- In a normal horse between 2 and 4 heart sounds can be heard. A heart murmur is defined as any sound within the cardiac cycle that should not normally be there.
- Murmurs can often only be heard while listening in specific locations, and these locations can help decide what the murmur represents.
- Leaky valves, heart defects and even anaemia can all cause heart murmurs.
- The root cause is best determined using an echocardiogram.
There are several other indicators that can show a potential heart problem:
- The regularity and strength of a pulse can be felt.
- Venous filling
- If the heart is working inefficiently then blood will back up into the veins, filling them up.
- This is most clearly seen in the jugular veins.
- Ventral Oedema
- This is accumulated fluid under the skin, at the bottom of the chest and abdomen.
- Again, the cause is backed up fluid.
- Increased respiratory rate can be caused by backed up fluid entering the lungs.
There are two major diagnostic tests that can be performed on horses with suspected heart problems, electrocardiography and echocardiography.
The electrical activity of the heart is measured by connecting the ECG machine to specific points on the horse using electrodes. A “trace” is produced that shows the net movement of electrical current at any given time. A normal trace is shown below:
An example ECG trace
The letters on the trace are the names of the different waves seen, and they represent specific events in the cardiac cycle.
P Wave – The atria depolarise and contract
QRS Complex – The ventricles depolarise and contract
T Wave – Both atria and ventricles repolarise
Timings are also measured, for example the P-R interval, the R-R interval and the S-T interval.
Rhythm is also assessed on the ECG trace, for example, making sure that every P wave has a QRS complex afterwards.
Heights of the different waves are measured, although this is less specific in the horse than for humans and other animals.
Echocardiography is simply a posh word for ultrasound of the heart, so if you are unfamiliar with how ultrasound works then take a look at our ultrasound page. Unlike ultrasound of tendons and ligaments, we will be looking at a moving picture as the different chambers of the heart contract. Taking measurements of the heart during different stages of the cardiac cycle tells us about the size of the heart and how well it is contracting. We can also use a technique known as colour-flow Doppler. This relies on the fact that the frequency of sound changes if the source of the sound is moving (the same reason a siren changes sound as it drives past you!). Using this technique we can look at blood flow within the heart, and see any leaks or abnormal flow.
Assessing Heart Problems
We most commonly assess heart problems after heart murmurs have been heard, either at a pre-purchase examination (vetting), or on a routine examination. Once a murmur has been heard we need to answer two questions:
- Will this murmur impact on the safety of riding the horse?
- Is the problem likely to progress to a point where it will affect either the horse, or the safety of the rider?
These questions can often be answered using a combination of the above techniques. Usually we will recommend repeat echocardiography at 6 month to 2 year intervals to monitor the condition, and assess any progression.
Although heart problems are not common in the horse, there are some conditions that occur more frequently than others:
- Aortic Regurgitation
- The aortic valve stops blood that has been pumped into the aorta returning to the heart. Aortic regurgitation occurs when this valve is leaking.
- This is a very common finding in older horses due to aging changes (stiffening of the valve leaflets), and does not usually cause problems.
- It causes a typical heart murmur that can usually be easily auscultated.
- Persistent Ductus Arteriosus (PDA)
- The ductus arteriosus is a blood vessel that is present in the foetus before birth. Its function is to let blood bypass the lungs, as the foal cannot breathe while inside the mare.
- The duct should close functionally at birth, and totally with 24 hours of life.
- A PDA means the duct remains open longer than it should, and occasionally permanently. This diverts blood away from the lungs, which can have potentially serious consequences.
- A very characteristic murmur is heard, often described as “machinery-like.”
- Atrial Fibrillation
- Normal electrical activity of the heart follows a strict pattern, with current flowing from the atria to the ventricles, and then stopping.
- In atrial fibrillation, current flows in continuous tiny circles in the atria, which never contract properly.
- It causes an extremely irregular heart rhythm, and can be diagnosed using ECG.
- The most common presenting sign is poor performance.
- Other heart problems can lead to this condition.
- Treatment is possible, but can be risky, and is unlikely to be successful if underlying conditions are present.
The prognosis for heart conditions can vary hugely depending on the exact underlying cause, which is why it is vital that tests are carried out to determine this. Although few treatments are available for serious conditions, we are fortunate that they occur extremely rarely.