The Relation Between Cardiac Output and Body Size

A prerequisite in the study of abnormal body function is the ability to establish the limits of normal. In the case of parameters such as cardiac output that vary with the size of the subject, it has become an accepted practice to standardize values in relation to the body surface area. Thus the cardiac index describes the cardiac output per square metre of body surface area, and the stroke index describes the volume of blood per heart beat per square metre of body surface area. The validity of these expressions depends on the premise that there is a constant or straight line relation between body surface area on the one hand, and cardiac output and stroke volume on the other, over the whole range of body size to be studied, and that the relation can be described by a simple regression equation cutting the intercept at zero. Only where this is so is it meaningful to refer to "the normal" cardiac index or stroke index. Though this premise is backed by observations in the case of adult or adolescent subjects, it was until recently entirely unsupported in the case of children and is still unsupported by any data for infants. This report is an attempt to supply the necessary data.