In Vitro Evaluation of Flow Distribution in All-Region Perfusion during the Norwood Operation

Continuous all-region perfusion has the potential to reduce total body ischemia during the Norwood procedure. This technique involves placing cannulas in the innominate artery, descending aorta, and native aortic root, thus providing continuous flow to the whole body at mild hypothermia (32-34°C) during the entire operation. However, the cannulation strategy in this approach must be optimized to achieve adequate flow rates to all vascular beds. To ensure appropriate blood flow rates to three cannulas of different caliber supplied by two inflow pumps, a benchtop evaluation of comparative flow rates through branched cannulas with variable independent perfusion pressures was conducted. Two isolated vertical columns with outlets at 10 mmHg intervals to allow for adjustable perfusion pressures were filled with expired banked packed red blood cells and fresh frozen plasma. The main arterial line was bifurcated with an 8-Fr arterial cannula inserted at the base of one column and a 4-mm (outside diameter [OD]) olive tip cannula inserted at the base of the other column. Flow rates were measured on each branch of the ¼″ arterial lines. Under experimental cardiopulmonary bypass (CPB) perfusion pressures of 30-50 mmHg on the 8-Fr arterial cannula column, the branched olive tip cannula accommodated less than 25% of total flow at total flow rates of 50-700 mL/min. This fraction is insufficient to sustain adequate lower body perfusion. However, the olive tip flow fraction was found to be sufficient for coronary blood flow (5-20 mL/kg/min) when olive tip perfusion pressures close to average neonatal diastolic blood pressures (20-60 mmHg) were tested. For all-region perfusion during the Norwood operation, primary CPB arterial flow should be divided to the head and coronary circulation with an independent pump delivering flow to the descending aorta. This should avoid cerebral over-circulation and insufficient flow to the lower body.