The principal of total internal reflection is utilized in sounding an alarm in a medical fluid infusion system when a downstream blockage occurs. A miniature prism is mounted in a cavity in a rigid exterior tube. The infusing fluid flows in a plastic tubing along the central channel of the rigid tube. The plastic tubing in the interior of this exterior rigid tube is diametrically expansive with an unpressurized outside diameter which is less than the inside diameter of said rigid exterior tube. A light source and light detector is positioned adjacent to the equilateral planes of the prism mounted in the rigid tube. When an overpressure caused by a downstream blockage in the infusion system occurs, the inner expansive tubing distends diametrically and contacts the hypotenuse plane of the prism causing the light incident on the hypotenuse plane to change from the reflective mode to the refractive mode which interrupts the signal received by the light detector which is located adjacent to one of the equilateral planes of the prism which would project the light ray out of the prism when the system operates in the reflective mode.