Simulation analysis of interatrial transposition of venous return (Mustard's operation)

Transposition of the great arteries is functionally corrected by Mustard's operation, an operation in which the atrial septum is removed and the resulting common atrial chamber repartitioned by a baffle to transpose venous return to the heart. To better understand the new physiology, a physically-based mathematical model of the infant circulation following Mustard's operation was developed and studied with the aid of computer simulation. The model reproduces certain clinical observations, including the tendency for mean pressure in both atria to be equal early postoperatively and for the pressure waveform to exhibit a steep y-descent in the systemic venous atrium. Simulation studies suggest that the mechanism for the former is transbaffle pressure coupling resulting from dynamic motion of the baffle; the mechanism for the latter is limitation of the extent of such baffle excursions. Dynamic volume of the two atria is found in the model to change according to the relative performance of the two ventricles, and stiffening the baffle leads to pressure waveforms characteristic of a small, noncompliant atrium. Mechanisms for venous "obstruction" and decompression were also studied. The baffle and its movements, however, have little effect upon cardiac output in the model, leaving unexplained the clinical observation of low output early postoperatively.