Superior caval flow during positive pressure mechanical ventilation and spontaneous breathing was investigated by Doppler echocardiography in a neonate with a coexisting superior cavopulmonary shunt and an aortopulmonary shunt. During positive pressure ventilation, retrograde systolic flow in the superior vena cava was recorded, with low velocity anterograde flow. This pattern was reversed during spontaneous respiration. Low intrathoracic pressure plays an important role in maintaining anterograde pulmonary blood flow in patients with this physiology.