Figure 2. (A) The “phantom” capnograph. (upper trace): Airway pressure recorded at the Y piece when ventilating a patient with the Siemens Servo 900c ventilator before and after disconnection. Minute volume was 5 l/min, respiratory rate was 8 breaths/min, positive end-expiratory pressure (PEEP) was 0 cm H²O, trigger sensitivity was -20 cm H²O, with a Siemens Servo Evac 180 evacuation system connected (25 l/min on evacuation flowmeter). (lower trace): Sidestream capnography recorded at the Y piece. “Phantom” capnograph tracings after disconnection closely resemble the square wave capnograph tracing before disconnection, both in amplitude and in shape. Note the slight blunting of the exhalation portion of the “phantom” capnograph. (B) Decay of the “phantom” capnograph when ventilation is maintained under the conditions as outlined in Figure 2(A). Note that, although the shape and amplitude of the tracings decline over time, a recognizable waveform persists for more than 3 min. (C) Connection of a false lung (1-l breathing bag) to the Y piece while maintaining ventilation after disconnection (note the positive deflections on the airway pressure trace when ventilating the false lung). Conditions otherwise as described earlier. The “phantom” capnograph trace disappeared immediately and did not reappear, even after the false lung was disconnected. Conclusion: The expiratory limb was washed out by fresh gas “exhaled” by the false lung. The expiratory limb is the source of carbon dioxide for the “phantom” capnograph (see also Figure 2[C]). (D) Disconnecting the evacuation system caused the “phantom” capnograph to disappear immediately. If the evacuation system had been covered and not allowed to void to the atmosphere, then reconnection led to an immediate return of the “phantom” waveform. No interruption in the “phantom” capnograph tracing was observed if the evacuation system was left connected to the exhaust outlet but the suction source was disconnected. Conclusion: A weak positive pressure gradient between the evacuation bag and the atmosphere creates the force for reverse flow in the expiratory system; remove the bag, eliminate the reverse flow. (E) PEEP setting and the “phantom” capnograph are shown. Setting PEEP (note the baseline airway pressure trace) before disconnection prevented the “phantom” capnograph. In addition, if, after an interval, the PEEP dial was then turned to 0 again, the “phantom” capnograph appeared. Conclusion: For reverse flow in the expiratory limb, the expiratory servo valve must be open, at least intermittently. Setting PEEP will force this valve to close in the presence of disconnection and, therefore, no “phantom” capnograph tracing will be seen.