Figure 1. The Siemens Servo 900c ventilator and the Siemens Servo Evac 180 evacuation system and the genesis of “phantom” capnography (see text for explanation) are shown. The dark shaded area is the carbon dioxide-containing gas within the expiratory limb and the evacuation system; the light shaded area is the carbon dioxide-containing gas within the portion of the inspiratory limb adjacent to the Y piece. The arrows show the flow of fresh and exhaled gas.