Shunt dynamics in experimental atrial septal defects.

Inorder to study the hemodynamic variables involving the magnitude, direction, and timing of phasic shunt flow, both the interatrial pressure gradient and blood flow along with other pertinent hemodynamic variables were measured instantaneously across a surgically created atrial septal defect (ASD) in seven awake dogs. Atrial and ventricular pacing and infusion of phenylephrine and isoproterenol were used to alter hemodynamic conditions. The wave form of phasic ASD flow was similar both in configuration and timing to the interatrial pressure gradient. During the cardiac cycle, both left-to-right (L-R) and right-to-left (R-L) shunting occurred: atrial contraction augmented L-R flow; the onset of ventricular contraction was associated with R-L flow; during the latter part of ventricular contraction, flow returned to L-R with the maximum L-R shunting occurring in early diastole. Tachycardia, infusion of phenylephrine and isoproterenol did not alter the phasic flow pattern. Both spontaneous and positive pressure respiration decreased net L-R shunting.