Understanding heart murmurs

Murmurs are audible by a vet in up to 60% of horses. The vast majority can be considered normal, functional murmurs, and do not indicate any underlying heart or vascular disease (ie: disease of the blood vessels).

The difficulty lies in establishing which cases are functional murmurs and which are caused by a heart problem – pathological murmurs – which could affect the horse’s athletic ability and safety.

In addition, there is a third group of murmurs – physiological murmurs – which are caused by conditions not involving the heart, such as anaemia or pregnancy.

Pathological murmurs can be caused by:

• Leakage between two or more of the heart chambers, usually due to a defective valve. Valvular insufficiency or incompetence is most common.
• Disease of the heart wall muscle – myocardial disease.
• Abnormal narrowing of the heart valve – stenotic valve.

Assessing a murmur

If a horse is showing clear signs of congestive heart failure, such as appearing dull or lifeless, coughing or heaving, then the presence of a loud heart murmur is clinically significant.

However, if a problem arises while the horse appears healthy and well, the owner will want to know if the murmur is due to an underlying problem which is going to affect the horse’s performance.

The vet will take into account the horse’s age and recent performance when assessing a murmur. The vet will then carry out a clinical examination paying particular attention to:

• General condition
• Colour of mucous membranes
• Capillary refill time
• Strength and tone of the arterial pulse
• Filling of the jugular veins
• Nasal discharge
• Respiratory characteristics

The vet will also listen to the horse’s chest with a stethoscope. This will provide valuable information, but the horse must be in a relaxed state. Any true resting heart rate of more than 50 beats per minute could be considered abnormal.

This examination is done to evaluate the heart rhythm and classify the murmur according to:

• Timing and duration: where the murmur occurs in the cardiac cycle and for how long.
• Character: type, intensity and sound.
• Point of maximal intensity and radiation: where on the chest wall it can be heard best.

This classification can help establish the origins of a murmur and if it is likely to be associated with a cardiac abnormality or not. Many functional murmurs are localised, of short duration, “musical” and of low intensity, while pathological murmurs associated with serious cardiac problems tend to be louder, harsher and last longer.

Finally, the vet may refer your horse to a specialist centre for an echocardiography or ultrasound examination of the heart. This will provide more detailed information about the state of the horse’s heart in difficult or unusual cases.