The dyspnea of cardiac disease is a complex symptom occurring under a variety of circumstances and producing a multiplicity of clinical syndromes. Except for the dyspnea produced by exertion one rarely sees the various types in "pure" form because the same patient is likely to have at the same time several different kinds of dyspnea, and as the disease progresses new types develop while the old ones do not subside.
Elsewhere we have published studies in which attempts were made to achieve a more exact understanding of the pathogenesis of orthopnea1 and of the dyspnea produced by exertion.2 In this and in succeeding papers similar efforts will be directed toward an elucidation of nocturnal dyspnea. The literature on this subject is voluminous and is complicated by the fact that there is no general agreement on nomenclature. Because of this confusion in terms we prefer to use in the