3.5 Cardiac murmurs
There are two important steps in the evaluation of a cardiac murmur. The first is to identify the source of the murmur, the second is to judge its significance. The significance of the murmur cannot be established with any accuracy without a specific diagnosis. It is therefore very important to identify clearly and classify the murmur or murmurs heard and, if necessary, then to consider the use of diagaids. The situation in the horse is complicated by the common finding of murmurs associated with normal blood flow.
Murmurs are generated when there is turbulent blood flow which causes vibration of cardiovascular structures. Turbulence is generated in fluids under a number of circumstances. Low fluid viscosity, large diameter vessels, and high velocity flow are factors which increase the likelihood of turbulent flow. Turbuin blood flow is common in the horse because of the large stroke volume and large ventricles, atria and great vessels. These factors mean that high velocity blood flow occurs in the aortic and pulmonary artery roots in early-mid systole. Murmurs are frequently associated with this flow and are therefore called 'flow' murmurs. Because early-mid systolic flow murmurs are associated with ejection into the great arteries, they are also called 'ejection-type' murmurs. The term 'functional' is also used for murmurs associated with normal blood flow. In addition, a murmur can result from low blood viscosity, for example due to anaemia. This is termed a haemic murmur. Physiological situations which are associated with a relatively high cardiac output may also result in a high blood flow velocity. This is the case in anaemia, septicaemia, and peripheral arteriofistulae. These murmurs can be called physiological.
Abnormal communications within or around the heart often lead to high velocity flow and turbulence. The most common example is when murmurs result from blood leaking back through valves. If a small hole or leak between leaflets is present, a pressure gradient across the valve will result in a jet flowing through the communication, similar to a fountain. The intensity of the murmur generated by the turbulence will also be affected by the volume and velocity of the jet, so a number of factors may potentially affect murmur intensity. Valvular regurgitation is usually due to a valvular abnormality, but some leakage may occur in the normal heart at the moment when the valves close. Usually any turbulence associated with this is inaudible. Abnormal communications are also formed by structural defects in the myocardium, such as a ventricular septal defect, or by shunting vessels such as a patent ductus arteriosus.
In humans and small animals, murmurs are often associated with flow through abnormally narrow (stenotic) valves. This narrowing results in an increase in the velocity of blood flow through the valve and generation of turbulence. This situation is rare in the horse.
Auscultation of murmurs is easiest when the heart sounds are well conducted to the body surface. A small amount of turbulence occurring in the heart and great vessels may not produce sufficient vibration for them to be heard using a stethoscope. Intracardiac phonocardiography demonstrates that most horses have murmurs generated in the great arteries, but that they are insufficiently loud to reach the body surface.
3.5.1 Classification of cardiac murmurs
3.5.2 Functional cardiac murmurs
3.5.3 Cardiac murmurs associated with pathological conditions
3.5.4 Other abnormal heart sounds