Suitable shunt size for regulation of pulmonary blood flow in a canine model of univentricular parallel circulations

Abstract

Objective: We examined the influence of shunt size on regulation of the pulmonary blood flow in a canine model of a univentricular heart because specific guidelines regarding suitable shunt size in the Norwood operation remain undetermined. Methods: Beagle dogs (n = 8) 3 to 7 months old and weighing 3.0 to 5.0 kg were used. Atrial septectomy and patch closure of the tricuspid valve were performed, and a systemic-pulmonary arterial shunt was created by interposing a 3.5- or 4.0-mm graft between the right subclavian artery and main pulmonary artery. After cardiopulmonary bypass, hemodynamic variables including pulmonary and systemic blood flow were measured consecutively according to physiologically respiratory manipulations. The ratio of shunt size to body weight ranged from 0.80 to 1.33 mm/kg (1.08 ± 0.16 mm/kg). Results: Each dog with a ratio of shunt size to body weight of 0.8 to 1.1 showed significant negative correlation between the pulmonary/systemic blood flow ratio and arterial Pco2, but those with a ratio of shunt size to body weight of 1.1 to 1.4 did not. Consequently each dog with a ratio of shunt size to body weight of 0.8 to 1.0 got adequate systemic flow, whereas a ratio of 1.0 to 1.4 resulted in inadequate systemic flow and acidic status. Similar phenomena were shown with the grouped data on relationship between the pulmonary/systemic blood flow ratio and inspired oxygen fraction. Conclusions: These findings imply that when the ratio of shunt size to body weight is 0.8 to 1.1, the pulmonary/systemic blood flow ratio is controllable by physiologic respiratory manipulations. Larger shunts make pulmonary blood flow excessive and uncontrollable. We recommend that a ratio of shunt size to body weight of 0.9 to 1.0 be considered a useful index for suitable systemic-pulmonary arterial shunt in the Norwood operation.