Heart murmurs and arrhythmias in horses. When to worry?
Heart murmurs are caused by changes in the blood flow within the heart and can be physiologic (normal) or pathologic.
Due to the big size of the equine heart, heart murmurs are often physiologic and caused by a turbulent rapid blood flow through the heart and the big arteries.
Changes in viscosity (thickness of the blood) can also cause temporary heart murmurs in sick horses due to hypovolemia (dehydration) or anaemia. This explains why your vet might mention a heart murmur during a colic episode or when your horse is unwell. The heart murmur will not be present once the disease is resolved.
Heart murmurs can occur during the systole (the contraction phase of the heart and ejection of the blood into the big arteries) or during diastole (the period of relaxation and filling with blood)
The origin of the heart murmur can be established according to the point of the highest intensity of the heart murmur.
Pathologic heart murmurs can be caused by leaks at the valves between the chambers of the heart (atrium and ventricle) or between the chambers of the heart and the big blood vessels (aorta and pulmonary artery).
The leaks can have congenital (genetic) aetiology or can be caused by degenerative changes due to advanced age which means that they are likely to progress.
The intensity of a heart murmurs is graded on a scale of 1 to 6. Grade 1 can be faintly heard and grade 6 is the loudest, accompanied by a thrill. Heart murmurs louder than grade 2/6 associated with increased resting heart rate should be investigated in horses used for ridden exercise.
Often heart murmurs are an incidental finding at pre-purchase examinations or routine visits for vaccines.
Sometimes clinical signs can be present: poor performance, oedema of the abdomen or limbs, distended jugular veins and abnormally increased resting heart rate.
Arrhythmias of the heart can be physiologic or pathologic. The rhythm of the heart can be regular or irregular. If it is irregular the vet has to establish whether it is regularly or irregularly irregular. The former are usually physiologic arrythmias and disappear with stress and exercise and show a normal heart rate during exercise.
The latter is pathologic and characteristic for atrial fibrillation. In this case the electrical and contractile mechanism of the heart have lost their normal coordination. This is the most common performance limiting arrythmia and can be confirmed by electrocardiogram.
Pathologic heart murmurs and arrythmias left uninvestigated can cause enlargement of the heart chambers and heart failure.
It is very difficult to establish the clinical significance of heart murmurs and arrythmia for safety and performance. Therefore, further investigations are recommended:
• Echocardiography allows the assessment of the heart chambers, valves, main arteries and blood flow.
• Resting and exercising electrocardiograms allow the assessment of the electrical activity and rhythm of the heart at rest and during exercise.
• Cardiac biomarkers can be analysed in a blood sample.
Heart murmurs cannot be treated medically. Depending on the severity of the heart murmurs and associated cardiac changes, it might be necessary to reduce the level of performance or to retire the horse for safety reasons.
Treatment for atrial fibrillation can be pharmacological using antiarrhythmic medication in early stages. If this method is not successful, transvenous electrical cardioversion can be performed, where electric shocks are generated under general anaesthesia in order to restore the normal cardiac electric activity.
This method is often efficient, but the prognosis is worsened if underlying cardiac disease is present.
Your vet will check your horses heart at appointments as a routine procedure but if you are concerned, please don’t hesitate to contact the practice.