Dyspnea is one of the most constant findings in patients with heart disease. In these cases, Peabody and Wentworth1 found that there is a diminution in the vital capacity of the lungs, and this diminution varies directly with the degree of dyspnea. These authors, however, attribute this diminution in vital capacity to mechanical factors, such as rigidity of the bony framework of the chest, ankylosis of the costal joints, diminution in the elasticity of the lung tissue, accumulation of fluids in the pleura, enlargement of the heart, enlargement of the liver, etc. These conditions, when present, undoubtedly cause a decrease in the vital capacity of the lungs. There are cases, however, in which such conditions are absent, but still the vital capacity of the lungs is decreased. Peters and Barr2 report that the decrease in the vital capacity in heart disease does not depend primarily on anatomic factors, because of