What is a shunt? Concepts of central shunts and effective flow and a method for graphical presentation of circulatory quantities and their interrelations.

The terminologies commonly used in descriptions of abnormal patterns of blood flow in patients with congenital heart defects are valid only in the case of simple abnormal communications in an otherwise essentially normal cardiac anatomy. In more complex cases the expressions are unambiguous and elucidatory only when qualified further, but then the terminology becomes cumbersome. Much might be gained in clarity by considering the combined ventricular output as having four components: two are essential, and equal in size; they constitute the effective flows to the systemic and pulmonary arterial trees of blood from the pulmonary and systemic veins, respectively. The other two are recirculated systemic and pulmonary flows and may or may not exist; they may be referred to as blue and red shunts, respectively. Here, 'shunt' is defined as a by-pass of the capillary system to which a given blood flow is destined. The red and blue shunts are both central--as distinct from peripheral ones, where the by-pass links arteries and veins in one of the two circulations. Adoption of the concepts of central shunts and effective flow as defined here would facilitate a rational and consistent quantitative analysis of the circulation in individual cases irrespective of variations in cardiac anatomy. It would also make it possible to present in instantly understandable diagrams the magnitudes and interrelations of such variables as the oxygen consumption, the blood's oxygen capacity and the oxygen contents of the blood in the great vessels connecting the heart and the two circulations, the combined ventricular output and its components, the mean pressure difference across the pulmonary circulation and the pulmonary vascular resistance. A graph blank for such a presentation is proposed, and its application in cases with and without central shunts is exemplified.