In children with heart disease a knowledge of the capacity for physical activity is an essential part of the final diagnosis. There are numerous factors involved in the circulatory dynamics, however, which make an estimate of the functional capacity difficult; for this reason it has seemed desirable to make measurements of other functional processes.
The close interrelationship of the circulatory and respiratory functions in man is well shown by the marked disturbances in breathing exhibited by patients with failing cardiodynamics. Most of the studies on vital capacity are based on this fundamental relationship of function, and the attempt is made to give expression in a quantitative way of the degree of circulatory involvement by noting the altered ability on the part of the subject to take a deep breath and then to expel it completely. A number of studies1 have been made using the vital capacity as a criterion