The appearance of the heart shadow on the x-ray film or screen is determined partly by the structure of the heart and partly by what William Harvey called "the movements of the heart and blood." The dynamic events which accompany the pumping of blood influence the size and shape of the shadow, the excursion of its borders, the pulsation of the great vessels and even the volume of blood in the adjacent lungs. Much has been learned about the dynamics of the heart through the medium of the x-rays; it is possible that something may be learned about x-ray signs by a study of circulatory dynamics. The brilliant operation described by Gross1provided the opportunity for making measurements of the circulation of patients with patent ductus arteriosus. Figure 1 illustrates the flow of blood in a normal heart. In a resting individual about 4 liters of blood per minute