Accurate data on the cardiac output in various types and intensities of exercise are not available, due largely to the very great technical difficulties of measurement. Such data as are available must be accepted with some reservation for the same reason. A great deal is known about the changes in heart rate which accompany exercise, but little is known about the equally important adjustments in the stroke volume.
Even without direct measurements, this could be postulated from the enormous increase in oxygen consumption which occurs. Krogh and Lindhard 3 using an indirect Fick method, found that the cardiac output may exceed 20 liters per minute during heavy exercise. In spite of calculations which indicated a possible total blood flow of 30 to 40 liters per minute during running when the oxygen consumption was 4 liters per minute, it is probable that in the average man the cardiac output during work seldom exceeds 20 liters per minute. An example of the enormous increase in cardiac output which can be achieved by exceptional athletes.
The increase in cardiac output which may be attained in exercise is limited by several factors. First, it is obvious that the heart cannot, for more than a few beats, eject more blood than is returned to it by the systemic veins; in other words, cardiac output cannot exceed the venous return. In numerous conditions, such as shock, hemorrhage, the erect posture with no movement, extreme heat, prolonged bed rest, etc., the cardiac output is reduced below normal by an inadequate venous return. Following strenuous exercise, the sudden withdrawal of the pumping effect of skeletal muscle contractions on venous blood flow results in a sharp fall in cardiac output. On the other hand, the available evidence indicates that the normal healthy adult heart is always able to increase its output sufficiently to handle the greatest amount of blood which can be returned to it in maximal exertion. Additional factors which may limit the increase in cardiac output in exercise are the maximal capacity of the heart for dilating and the reduction in diastolic filling of the heart which results from extremely rapid heart rates.
Finally, such adverse conditions as fatigue, lack of sleep, malnutrition and acute infections may seriously reduce the maximal cardiac output of which the subject would otherwise be capable.
Wednesday, February 27, 2008
Basal Values, The Effect of Posture
Basal Values
Under basal conditions, the cardiac output in man averages about 64 ml. per kilogram per minute. 2 It is higher in the adolescent than in the adult and decreases in subjects over forty-five years of age. Sub-basal values are found in the early hours of the morning, during standing and probably also during recovery from heavy exercise. The normal basal cardiac output has an absolute value of 4 liters per minute for an individual of average size and if the pulse rate is 70, the stroke volume is 57 ml.
The Effect of Posture
The cardiac output is usually reduced when the subject stands. Since the pulse rate is usually increased, his must indicate a reduction in stroke volume. This is especially true in prolonged standing without movement and is common after a long period of hot weather. The reduction in stroke volume is due apparently to a decrease in the venous return from the lower part of the body. In the sitting posture, the output is slightly less than when the subject is reclining, but peculiarly enough, the output is not reduced when the subject is in the erect position but leaning against a support.
Miscellaneous Influences
The cardiac output is increased by the taking of food and water and, temporarily, by high altitude. Exposure to cold slows the heart rate but increases the stroke volume, leaving the cardiac output unchanged, unless shivering occurs, when the cardiac output is increased. Exposure to warmth increases resting cardiac output very considerably. The heart rate is increased with a slight reduction in stroke volume. If the heat is severe enough to cause dehydration, cardiac output returns to the basal level. It may fall below the basal level in heat prostration in spite of a rapid heart rate, indicating a reduction in stroke volume due to inadequate venous return to the heart. Cardiac output is diminished by many types of cardiovascular abnormalties.
Under basal conditions, the cardiac output in man averages about 64 ml. per kilogram per minute. 2 It is higher in the adolescent than in the adult and decreases in subjects over forty-five years of age. Sub-basal values are found in the early hours of the morning, during standing and probably also during recovery from heavy exercise. The normal basal cardiac output has an absolute value of 4 liters per minute for an individual of average size and if the pulse rate is 70, the stroke volume is 57 ml.
The Effect of Posture
The cardiac output is usually reduced when the subject stands. Since the pulse rate is usually increased, his must indicate a reduction in stroke volume. This is especially true in prolonged standing without movement and is common after a long period of hot weather. The reduction in stroke volume is due apparently to a decrease in the venous return from the lower part of the body. In the sitting posture, the output is slightly less than when the subject is reclining, but peculiarly enough, the output is not reduced when the subject is in the erect position but leaning against a support.
Miscellaneous Influences
The cardiac output is increased by the taking of food and water and, temporarily, by high altitude. Exposure to cold slows the heart rate but increases the stroke volume, leaving the cardiac output unchanged, unless shivering occurs, when the cardiac output is increased. Exposure to warmth increases resting cardiac output very considerably. The heart rate is increased with a slight reduction in stroke volume. If the heat is severe enough to cause dehydration, cardiac output returns to the basal level. It may fall below the basal level in heat prostration in spite of a rapid heart rate, indicating a reduction in stroke volume due to inadequate venous return to the heart. Cardiac output is diminished by many types of cardiovascular abnormalties.
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.
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.
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