Abstract: A restrictor fitting is provided at the discharge side of an infusion pump to substantially reduce or eliminate the presence of undissolved gas such as air within the detection field of an air-in-line sensor. The restrictor fitting defines a flow orifice of reduced cross-sectional size disposed along an infusion line in close proximity with a pump discharge port, and at a position slightly downstream from the air-in-line sensor. The restrictor fitting maintains an infusion liquid under substantially constant pressure conditions, with minimal pressure drop across the sensor detection field, and thereby minimizes the occurrence of undissolved gas attributable to outgassing at that location. In addition, the flow orifice acts as a bubble trap to prevent backflow of gas bubbles to the sensor detection field.