#13358 - How Do Heart Murmurs Relate To Cardiac Anatomy - Organisation of the Body

How do heart murmurs relate to cardiac anatomy?

Heart murmurs are a series of vibrations that can be heard at the chest wall and radiates from the chest wall from the heart or great vessels. The murmurs are created by disturbances in the laminar blood flow. The type of heart murmur detected can be used to decide if there is any structural abnormality in the heart and tells us where the damage is. Often murmurs are caused by abnormal blood flow through damaged valves or septational defects.

Functional anatomy of valves

The generation of heart murmurs is often due to abnormal blood flow which can be caused by structural abnormalities in the valves. There are two types of valves: inlet valves (atrioventricular valves) and outlet valves (semilunar valves). There are two atrioventricular valves and these control blood flow from the atria to the ventricles. The tricuspid valve is found between the right atrium and the right ventricle and consists of three cusps (leaflets) which are inserted in the anterior, posterior and septal margins. The cusps of these valves, which are made of fibrous tissue and are covered by a thin layer of endothelium, are connected to an annular fibrous skeleton which is formed from atrioventricular endocardial cushions. The end of the cusps are attached to thin strands of connective tissue called chordate tendineae and these are connected to papillary muscles which arise from the base of the ventricles. Similarly the left atrioventricular valve has two leaflets, anterior and posterior, which are attached to a fibrous skeleton, chordate tendinae and papillary muscles.

The second type of valves are semi-lunar valves which consist of pulmonary and aortic valves. The pulmonary valve lies between the infundibulum and pulmonary artery; whereas the aortic valve is between the left ventricle and the aorta. Both valves have three semilunar concave cusps. During ventricular contraction the leaflets of the semilunar valves are pressed against the wall of the outflow tracts but when interventricular pressure falls blood in the artery returns to the ventricles and the filling action results in the leaflets being pressed against each other to form a closed valve.

Heart valves can be impaired in two ways; it can either be stenosed or it can be incompetent. Both these effects which can be due to errors in embryonic development or due to diseases in adult life reduces the pumping efficiency of the heart and causes turbulence of blood flow. The turbulence of the blood flow can be heard using a stethoscope and depending on the characteristic of the heart murmur the type of abnormality can be diagnosed and treated. Before discussing why murmurs occur, the formation of normal heart sounds will first be discussed.

Normal heart sounds

When a stethoscope is placed over certain positions of the chest wall 2 heart sounds ‘lubb-dupp’ are heard in every individual which is caused by the closure of valve leaflets. When the cardiac valve closes tension is created and results in a vibration that is transmitted through the heart tissue into the chest wall and can be recorded using a microphone. In non pathological conditions only the closure of the valve is heard and the opening of the valve is silent. The first heart sound, S1, occurs at the beginning of ventricular systole. When the interventricular pressure exceeds atrial pressure there is almost synchronous closure of the tricuspid and mitral valves. The elastic property of the valve causes vibrations when it closes and these vibrations are transmitted through the heart tissue to the chest wall. The mitral valve closure is heard best in the intersection of the midclavicular line with the 5^th intercostals space. In contrast the tricuspid valve is best heard in the 5^th intercostals space at the left of the sternal edge. The second heart sound is due to the closure of the aortic and pulmonary valves which happens during ventricular diastole. During this phase the interventricular pressure decreases below aortic pressure causing the valves to close. The closure of the aortic valve is clearly heard in the 2^nd intercostals space at the right sternal edge whereas the pulmonary valve closure is heard in the 2^nd intercostal space on the left sternal edge. However unlike the closure of the atrioventricular valves, the closure of the semilunar valves can be split into two sounds ‘lubb-terrupp’. This is because the aortic valve closure occurs fractionally earlier the pulmonary component. This splitting is more obvious when breathing in.

There are two key differences between the two sounds. The first heart sound created by the closure of the inlet valves is prolonged and has a low frequency. Whereas the second heart sound produces a high frequency.

The third heart sound is caused by the rush of blood into the relaxing ventricles during diastole and this sound is often heard in under 35 year olds. The fourth heart sound which occurs before the first heart sound is caused by atrial systole which is considered to be normal if the person is more than 40 years old.

Abnormal blood flow: murmurs

Murmurs are caused by abnormal blood flow which can caused by two reasons. Functional murmurs are caused by severe anaemia which causes the blood to thin. This results in the blood flowing rapidly through the aortic valve and the high velocity of the blood flow resonates and is heard as a murmur. The second cause for murmurs is organic which can be due to structural abnormalities in the valves, heart chambers. Heart murmurs can be classified according to their characteristics: time (systolic, diastolic), shape, location, radiation and intensity of the heart murmur.

Systolic murmurs are heard during or after the first heart sound and ends before or during the second heart sound. Midsystolic murmurs are heard between S1 and S2 heart sounds and are often caused by an outflow obstruction. Aortic valve stenosis is a common cause and the narrowing of the valve means a higher pressure is needed to eject blood from the left ventricle into the aorta through the narrowed open valve. Due to this common signs include, an increase in systolic pressure, left ventricular hypertrophy, decrease in aortic pressure that supplies the...