This article, which follows on from Hallowell (2018), will focus on tips and tricks regarding decision-making and next steps upon hearing a heart murmur in a horse. Commonly asked questions will be answered and supported with some case examples.
What to do if you hear a heart murmur
Heart murmurs are a common finding in horses. Physiological aortic flow murmurs are the most common murmur, found in 60-80% of horses. One of the key steps is to determine whether the murmur is physiological or pathological, which is often easier said than done!
The first important investigative process is two-fold. If the horse has a normal resting heart rate, it is not in heart failure so this will not be the cause of any comorbidities that may need evaluating, such as weight loss, ventral oedema, or poor performance. The next important factor to identify is whether the horse has a regular rhythm; if it does not, determine whether it is regularly irregular (making it more likely to be second degree atrioventricular blockade), or irregular, making atrial fibrillation more likely. This can helps with further investigation, as atrial fibrillation is often found with severe mitral (and tricuspid) regurgitation.
For a full cardiac evaluation, auscultation should be performed over both the left and right sides. Ensure that the heart is auscultated over as large an area as possible - particularly cranially and dorsally. This technique allows for the point of maximal intensity to be identified. It is recommended that murmurs louder than grade 3 (same intensity or louder than the heart sounds) are further evaluated in horses still in ridden work.
As a recap, grade 1 murmurs are very quiet and can only be heard in a very quiet environment; grade 2 murmurs are audible in normal conditions and are quieter than the heart sounds; grade 3 murmurs are the same intensity as the heart sounds; grade 4 are louder than the heart sounds; grade 5 are louder than the heart sounds and accompanied by a pre-cordial thrill (vibration) and grade 6 can be heard with the stethoscope off the chest wall (these are very rare).
It is important to palpate the peripheral pulses. If the horse has bounding peripheral pulses, it increases the likelihood of increased stroke volume, which is suggestive of an enlarged left ventricle. These bounding pulses are more commonly associated with moderate-to-severe aortic regurgitation. If the client does not want further cardiac investigation, but the horse is still in ridden work, then it is prudent to recommend that the horse no longer be ridden, as the enlarged left ventricle can predispose ventricular dysrhythmias during exercise, which can be associated with poor performance, collapse, and sudden death.
Is the murmur systolic or diastolic?
Techniques that can be used to help with the differentiation of systolic and diastolic murmers include:
- Using the length of the murmur (diastole is a longer phase of the cardiac cycle than systole, although murmurs do not always fill the entire period)
- Feeling the peripheral pulse while listening to the murmur (if the murmur and pulse occur simultaneously, then it is systolic)
- Watching jugular fill while listening to the murmur (if the jugular fills while the murmur is auscultated, then it is systolic ).
The character of the murmur can also help. Harsh, plateaushaped murmurs are usually systolic and are associated with mitral and tricuspid regurgitation, or the right-sided murmur of the ventricular septal defects, whereas blowing or musical decrescendo murmurs are classically found in animals with aortic regurgitation.
Following this, if the murmur still cannot be differentiated and the murmur is louder than the heart sounds, then recommend a cardiac evaluation, rather than committing to what you think it might be. Explain to the owner that the horse may need both an echocardiographie evaluation and exercising electrocardiogram. This can avoid the challenges that arise when you tell an owner what the murmur type is, which they then go and research, only to be told it is a different one by the specialist. In these cases, it becomes difficult for the owner to know who to believe and trust - which should always be their regular vet!
What does it mean if the heart murmur (or rhythm abnormality) disappears after exercise?
Succinctly, the answer to this is nothing. It was once believed that auscultation after exercise could help discern between physiological and pathological murmurs - the insignificant ones disappeared, and the significant ones became more evident - we now know that this is not true. Murmurs are caused by turbulent blood flow; with increasing heart rates, blood flow may become more laminar (thus the murmur becomes quieter) or more turbulent (resulting in a louder murmur). It is also important to appreciate that some murmurs heard only after exercise are unlikely to be significant.
The same is true for dysrhythmias - as heart rate increases, the R-R interval becomes shorter, and rhythms can sound regular even when they are not. However, the technique may be valuable for identifying intermittent rhythm irregularities, although it can commonly yield false negatives - respiratory sinus dysrhythmia is a common finding in the post-exercise phase. Changes following exercise are not important in cardiac decision-making.
Do murmurs cause poor performance?
The simple answer to this is no. Horses have a very large cardiac reserve and if the horse has a normal heart rate at rest (ie is not in heart failure) then any regurgitant fraction will be small and not impact cardiac output. However, the sequelae from murmurs - dysrhythmias - can certainly cause poor performance, particularly in animals exercising at maximal capacity such as hunters, eventers and racehorses. The most common dysrhythmia implicated in poor performance is atrial fibrillation. This can be primary or seen as a sequela to mitral (or tricuspid) regurgitation where the left (or right) atrium is enlarged or ventricular tachycardia secondary to left ventricular enlargement, which can be seen with both aortic and mitral regurgitation.
Are horses with heart murmurs safe to sedate?
The simple answer to this is yes, as long as the horse has a normal resting heart rate. As mentioned previously, the horse has a huge cardiac reserve so the impact of commonly used alpha-2 agonists, although they do impact on the cardiovascular system’s function, are unlikely to cause problems. The alpha-2 agonists can trigger dysrhythmias so they may need to be used more prudently in animals in heart failure or with ventricular dysrhythmias.
Are horses with heart murmurs safe to anaesthetise?
The simple answer to this is yes, with the same rationale outlined in the sedative section. For cases that require emergency surgery such as horses with abdominal pain, they require surgery and should be treated as any other horse would. It is also worth noting that hypovolaemic animals can develop ‘haemic’ murmurs - these occur as a result of the development of turbulent flow in the heart because of changes in blood viscosity - they are usually left-sided systolic murmurs and can be loud; they resolve when the haemoconcentration is corrected. When considering horses with murmurs that present for elective orthopaedic surgery, it may be prudent to fully evaluate the cardiac disease first; if the cardiac disease increases the risk of collapse and sudden death, the owners may not wish to spend several thousands of pounds on a surgery to treat the cause of lameness.
Do horses with ventricular septal defects need repeated monitoring?
This is not always true. Some animals with ventricular septal defects can be several years old before they present in heart failure (see Case study 2) so, although they may not need repeated monitoring, it is worth evaluating the ventricular septal defect when it is initially found to determine whether there are changes secondary to the defect that are likely to be progressive. In many cases, the evaluation will put everyone’s mind at rest that the defect is small and restrictive and unlikely to progress.
How many horses that have heart murmurs are unsafe to be ridden?
This is the million dollar question and depends somewhat on caseload. It is always worth seeking experienced specialists to evaluate cardiac cases. In the author’s caseload, where they see a high percentage of older horses with aortic regurgitation, this would be <5%. For those that see a higher proportion of younger racehorses and sport horses that percentage may be lower.
Let’s consider the reasons for horses to be at increased risk of collapse or death at exercise and the tests undertaken to identify these for each of the common causes of heart murmurs.
- Mitral regurgitation: An enlarged pulmonary artery secondary to pulmonary hypertension increases the risk of pulmonary artery rupture at exercise; this usually occurs in conjunction with a very large left atrium. Both changes will be identified with echocardiography. Additionally some horses with moderate to severe mitral regurgitation have an enlarged left atrium and concurrent atrial fibrillation. Atrial fibrillation is occasionally associated with idioventricular rhythms at exercise, which could predispose ventricular tachycardia. If the atrial fibrillation is successfully treated, these idioventricular rhythms also resolve.
- Ventricular septal defects: Significant ventricular septal defects can rarely be associated with various sequelae, which include enlarged left atria and ventricles (so there is a risk of atrial fibrillation and ventricular dysrhythmias respectively) as a result of left-sided overload. Sometimes, secondary aortic regurgitation can be identified as one or more aortic valve cusps prolapses into the defect, and can result in left ventricular enlargement and concurrent ventricular dysrhythmias Echocardiography to identify the size of the defect and any chamber enlargement is important.
- Aortic regurgitation: Decision-making regarding safety for ridden exercise in these cases is usually dependent upon results from exercising electrocardiograms looking for ventricular premature complexes at peak exercise. The key thing to remember is that these cases are often older and have concurrent lameness issues. Lameness can result in much higher heart rates than would be expected in normal animals and this sympathetic stimulation could lead to ventricular premature complexes that are not associated with the cardiac disease, and may potentially resolve with the lameness (Van Erck, personal communication).
What does a heart murmur mean regarding saleability?
Most cases of mitral and tricuspid regurgitation progress slowly. However, both conditions can predispose atrial fibrillation secondary to left or right atrial enlargement, which can be performance and rarely safety limiting. As this is a progressive condition, buyers may want compensation for the repeated evaluations that will allow the condition to be monitored. Sometimes multiple evaluations over a period when the horse is in work allow some idea of progression to be forecast.
Aortic regurgitation is more likely to affect saleability. This is often a condition of the older horse and is variably progressive. It is more likely to impact on saleability and future performance if found in horses in their early to mid-teens or younger. As with mitral regurgitation a series of evaluations while in work may allow the likely speed of progression to be predicted.
It should also be noted that the heart and related signs (which can include collapse and sudden death) may be excluded from any insurance policy if pathological murmurs are identified.
Case studies
Case 1
A 6-year old Warmblood gelding being sold as a future showjumper. At the pre-purchase examination, the horse was quite anxious with a heart rate of 52 beats per minute, bounding peripheral pulses and a grade 5/6 pansystolic left-sided band-shaped harsh murmur. The horse was referred for a cardiac evaluation. The horse had a significantly enlarged left atrium and ventricle (Figure 1) and quite poor fractional shortening. The mitral valve was thickened and deformed which may suggest dysplasia (although previous endocarditis could not be ruled out). There was a significant chance this horse would not only develop atrial fibrillation but would also go into heart failure - both of which could limit its ability to be a showjumper.
Case 2
A 6-year old Welsh section A mare used as a child’s riding pony was presented for cardiac evaluation after a loud left and right systolic murmur was found at a routine vaccination. The pony was quite subdued at this evaluation with a resting heart rate of 60 beats per minute. On echocardiography, the pony had a very large ventricular septal defect with an overriding aorta, a very large left atrium and ventricle (Figure 2) with poor fractional shortening. The pony was deemed in heart failure and no further evaluations were performed. The pony was euthanised a few weeks after this evaluation.
Case 3
A 16-year old Thoroughbred gelding used for hunting presented with a grade 3/6 left-sided diastolic murmur at a prepurchase examination. The horse had a heart rate of 32 beats per minute and normal peripheral pulses. The horse was referred for a cardiac evaluation: on echocardiography, the horse had a normal sized heart with a nodule on the left coronary cusp of the aortic valve and moderate aortic regurgitation. Three exercising electrocardiograms were performed and no abnormalities were identified. Although this condition may progress, it was not currently performance- or safety-limiting and the horse was sold.
Summary
Hopefully this article has answered many of the common questions about heart murmurs with some tips and tricks to help with decision-making. It is important to remember that although worse case scenarios have been discussed, most heart murmurs do not mean the end to a horses career. However, frequent echocardiography and exercising electrocardiograms are required to help with decision-making and allow the careers of horses with cardiac disease to be safely extended.
KEY POINTS
- Heart murmurs are common; the art is deciding which ones need further investigation and which ones are physiological.
- Murmurs louder than grade 3 and diastolic murmurs warrant further investigation.
- In cases where further investigation is not possible, horses that have bounding peripheral pulses should be retired from ridden work as this finding is suggestive of increased stroke volume and an enlarged left ventricle.
- Sedation of horses with murmurs that have normal resting heart rates will usually lead to no untoward effects.
- Anaesthesia of horses with murmurs that have normal resting heart rates will usually lead to no untoward effects, although murmurs should be evaluated first to prevent unecessary strain on a client's finances.