A novel framework of circulatory equilibrium.

A novel framework of circulatory equilibrium was developed by extending Guyton's original concept. In this framework, venous return (CO(V)) for a given stressed volume (V) was characterized by a flat surface as a function of right atrial pressure (P(RA)) and left atrial pressure (P(LA)) as follows: CO(V) = V/W - G(S)P(RA) - G(P)P(LA), where W, G(S), and G(P) denote linear parameters. In seven dogs under total heart bypass, CO(V), P(RA), P(LA), and V were varied to determine the three parameters in each animal with use of multivariate analysis. The coefficient of determination (r(2) = 0.92-0.99) indicated the flatness of the venous return surface. The averaged surface was CO(V) = V/0.129 - 19.61P(RA) - 3.49P(LA). To examine the invariability of the surface parameters among animals, we predicted the circulatory equilibrium in response to changes in stressed volume in another 12 dogs under normal and heart failure conditions. This was achieved by equating the standard surface with the individually measured cardiac output (CO) curve. In this way, we could predict CO [y = 0.90x + 5.6, r(2) = 0.95, standard error of the estimate (SEE) = 8.7 ml.min(-1).kg(-1)], P(RA) (y = 0.96x, r(2) = 0.98, SEE = 0.2 mmHg), and P(LA) (y = 0.89x + 0.5, r(2) = 0.98, SEE = 0.8 mmHg) reasonably well. We conclude that the venous return surface accurately represents the venous return properties of the systemic and pulmonary circulations. The characteristics of the venous return surface are invariable enough among animals, making it possible to predict circulatory equilibrium, even if those characteristics are unknown in individual animals.