Murmurs

Murmurs are sounds caused by turbulent blood flow through the heart and can be classified according to how loud the sound is, graded 1 to 6, the position at which the murmur can be heard most clearly and the place the murmur occupies in the heart cycle, i.e. between which beats the murmur occurs.

Where the murmur occurs between normal heart sounds is the most valuable tool as this can provide information as to what motion the heart is making when the turbulence is caused. For example if the turbulent flow produces a sound after the first heart sound then the murmur is most likely occur during systole, i.e. ventricles are contracting and the murmur could be associated with leaking mitral or tricuspid valves.

Causes

Murmurs can be caused by the following

  • Reduced blood viscosity (e.g. anaemia)
  • Increased cardiac output (exercise, excitement, high temperature)
  • Abnormal blood flow (valvular incompetence, shunting of blood within the heart)

Diagnosis

Murmurs are diagnosed initially via listening to the heart using a stethoscope (auscultation). As mentioned above this can provide information as to volume, position in relation to heart beats and side of the heart.

Electrocardiography (ECG) can be used to see the electrical impulses that move through the heart in order to co-ordinate its controlled contraction as a wave. A typical wave is shown below. Changes to this ECG trace helps the veterinary surgeon diagnose the cause of the arrhythmia and also provide information on the exact state of the heart.

Ultrasound scanning can be used to visualize the heart and therefore, assess the valves, take measurements of heart walls and chambers and even assess the direction and velocity of blood flow. Murmurs are usually only investigated Most murmurs are incidental and need only be monitored and then investigated further if the murmur worsens or there are signs of deterioration in the horse’s condition.

Symptoms

Heart murmurs are often an incidental finding and cause the horse no problem, however, there are a number of possible symptoms that may occur, these are listed below.

  • Poor performance
  • Collapse or fainting
  • Sudden death
  • Reduced cardiac output
  • Increased heart rate (tachycardia)
  • Pale mucous membranes
  • Generalised weakness
  • Weak pulses
  • Left-sided heart failure
  • Frothy nasal discharge
  • Moist cough/wheeze
  • Increased respiratory rate
  • Right-sided heart failure
  • Ventral oedema (swelling of brisket, hind-limbs and sheath)
  • Dull lung sounds
  • Distention and pulsing of veins
  • Increased temperature, depression and off food (bacterial endocarditis)

Types of Murmur

Mitral Insufficiency

This is the most common cause of the signs of heart failure in horses. This murmur is heard on the left hand side of the horse and occurs after the first heart sound (systolic) i.e. the left ventricle is contracting to pump blood to the body. The turbulence is caused by blood leaking through the mitral valve into the left atrium during this period of contraction of the ventricle. The causes of mitral valve insufficiency are valve degeneration, infection of the valve (bacterial endocarditis), changes in size of the left ventricle, valve dysplasia (reduced size of valve) and prolapse of the valve. Mitral valve insufficiency murmurs can also occur without any underlying disease in fit horses, therefore, further investigation is only necessary if the horse is showing any symptoms of heart disease or if they have been graded as grade 3 or more.

Tricuspid Insufficiency

This murmur is common in racehorses due to increase size in the heart muscle and is well tolerated.

The murmur is heard most loudly on the right hand side of the heart and occurs after the first heart sound (systolic) i.e. the right ventricle is contracting to pump blood to the lungs. The turbulence is caused by blood leaking through the tricuspid valve into the right atrium during this period of contraction of the ventricle.

The causes of tricuspid valve insufficiency are valve degeneration, infection of the valve (bacterial endocarditis), changes in size of the right ventricle, valve dysplasia (reduced size of valve) and prolapse of the valve. Tricuspid valve insufficiency murmurs can also occur without any underlying disease in fit horses, therefore, further investigation is only necessary if the horse is showing any symptoms of heart disease or if they have been graded as grade 3 or more.

Aortic Insufficiency

Aortic insufficiency is usually seen in older animals and is normally slowly progressive. This murmur is heard on the left hand side of the horse and occurs during diastole i.e. the ventricles are relaxed to allow them to fill with blood. The turbulence is caused by blood leaking through the aortic valve from the left atrium. The causes of aortic valve insufficiency are valve degeneration, infection of the valve (bacterial endocarditis), valve dysplasia (reduced size of valve) and prolapse of the valve. Aortic valve insufficiency murmurs can also occur without any underlying disease in fit horses, therefore, further investigation is only necessary if the horse is showing any symptoms of heart disease or if they have been graded as grade 3 or more.

Ventricular Filling Murmur

The third and fourth heart sounds caused by the filling of the ventricles can occasionally be heard and is a normal finding and of no significance.

Ejection Murmur

Turbulence can be heard as blood is pumped out of the heart this is usually of no clinical significance and must be differentiated from mitral valve insufficiency.

Ventricular Septal Defect

This condition is present at birth (congenital) and causes a right-sided, systolic murmur (ventricle contraction) due to a connection between the left and right ventricles. This allows blood to flow through to the right hand side when the heart contracts. Adult horses often show few problems but may have reduced performance.

Treatment

Permanent damage to valve structure cannot be treated. Bacterial endocarditis should be treated with antibiotics.