Anesthesia machine malfunction simulating spontaneous respiratory effort

We present a case in which a dip in the capnogram of an anesthetized patient, which may indicate clinical complications such as spontaneous respiratory effort, was caused by a malfunction of the anesthesia delivery system. The rubber diaphragm of the ventilator relief valve was found to be coated with a sticky substance which may have caused adhesion at the valve seat. This adhesion blocked the flow ot excess gas to the scavenging system during exhalation. It was demonstrated that a pressure of 5 cm H₂O was needed to overcome this adhesion.