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.