Heart Murmurs Cardiac Output

Heart Murmurs

Sometimes, due to congenital malformation or disease, the action of the heart valves is impaired. The valve orifices may be narrowed so that the normal flow of blood is impeded, and the valve leaflets may fail to close completely, allowing a leakage of blood in the reverse direction. This abnormal valve action results in distortion of the normal heart sounds or in the appearance of additional sounds. These abnormal heart sounds are called murmurs. The particular valve involved may be determined from the point on the chest wall at which the murmur is heard most clearly. In the case of so-called "functional" murmurs, there is no structural defect to account for the abnormal heart sounds.

Cardiac Output

From a functional standpoint the most important index of heart function is the cardiac output, that is, the volume of blood pumped by each ventricle per minute. Two factors, each of which may vary within wide limits, determine the size of the cardiac output. These are the heart rate and the stroke volume. The heart rate is very easily determined. The stroke volume, on the other hand, must be calculated from the values of the cardiac output and the heart rate. The fundamental importance of the cardiac output as an index of heart function has led to many attempts to devise a method for its measurement. The cardiac output could be calculated from the experimentally determined values of the oxygen content of the mixed venous blood (that is, the venous blood from all parts of the body after it has been mixed in the right heart), the oxygen content of the arterial blood and the oxygen consumption of the body. Suppose, for example, that the oxygen content of the mixed venous blood is 15 volumes per cent, that of the arterial blood 20 volumes per cent and the oxygen consumption is 250 ml. per minute. It is obvious that each 100 ml. of blood yields 5 ml. of oxygen to the tissues, so that of blood is required to furnish 250 ml. of oxygen to the tissues. Since this amount of blood must be pumped by each ventricle per minute, it represents the cardiac output. Until recently this method has not been used for studies on human subjects because of the difficulty in obtaining samples of mixed venous blood. Various procedures for determining the gaseous content of the mixed venous blood by indirect methods have been devised (for details, see textbooks of physiology) but all are open to criticism, especially when attempts are made to measure the cardiac output during exercise. A plastic catheter is inserted into an arm vein and carefully threaded up the vein and into the right heart. Samples of mixed venous blood are thus obtained and their oxygen content determined. The oxygen content of the arterial blood is determined by analysis of samples obtained by arterial puncture and the oxygen consumption of the body is measured with the ordinary clinical B.M.R. apparatus. From these data the cardiac output is calculated as illustrated above.