Interruption of the breathing gas to a ventilated anaesthetizedBreathing circuit disconnections and mechanical malfunclions of the anaesthesia syslem are among the most common of preventable anaesthesia mishaps.' Most commercially available "patient disconnect" monitors based on breathing circuit pressure fail to detect several common breathing gas interruption (BGI) conditions, although technology exists to make anaesthesia significantly safer. 2 A standard series of tests which could be performed on anaesthesia systems configured for clinical use is needed to allow hospitals to evaluate the performance of their BGI monitors over the full range of foreseeable problems. It i~ the objective of this article to describe such a scries and the results obtained by applying the tests to typical BGI monitors.A review of the literature relating to anaes'chesia mishaps revealed more than 90 reports of patient anaes~ thesia incidents unrelated to gas composition or improper drug administration for the five-year period between 1982 and 1986. The incidents fell into ten categories. Disconnections at the endotraeheal tube or patient connection,4-2' kinks or partial obslructions of the b~ea~ing circuit tubing, 22-27 teaks in the circuit, 2g-43 disconnection of the vaporizer inlet or outlet, '~-46 disconnections of the fresh gas hose, 47 so excessive positive pressure in the scavenging system, ~-s~ excessive negative pressure in the scavenging system, 59-63 blocked fresh gas flow,64-73 and continuing high-breathing circuit pressure 7a-s6 Comprise CAN J ANAESTH 1988 / 35:6 ! ppS49-61