Malfunction of an adjustable pressure limit valve.

EDITOR: We report the malfunction of an adjustable pressure limit (APL) valve on a KION (Siemens, SiemensElema AB, Solna, Stockholm, Sweden) anaesthesia workstation which occurred twice. The first time, the malfunction was detected during the pre-use manual check, but not by the automatic self-check. The second time, the malfunction occurred intermittently during ventilation of a patient but it was rapidly discovered by the anaesthesiologist and no harm came to the patient. After the second incident, visible rust was found on the exit of the APL valve as the cause of the malfunction. The KION anaesthesia machine is an electronic anaesthesia workstation, which incorporates an anaesthesia circle breathing system with a bag-inbottle ventilator and an integrated patient and machine monitor. In the morning before the beginning of the operating list the anaesthesia machine was switched on and the pre-use automatic function check was faultlessly performed. Afterwards the anaesthesiologist performed a manual check according to departmental protocol (check for leakage in the breathing system and check for correct function of the APL valve). For that purpose, the circle system was short-circuited by connecting the Y-piece to a manual breathing bag, the APL valve was set at 30 mbar and fresh gas flow was turned on until the pressure in the circuit reached 30 mbar. Then the fresh gas flow was turned down to the lowest possible flow this anaesthesia machine can allow, i.e. 0.1 L min. According to European Standard EN 740, the maximum acceptable fresh gas flow during the leakage test to sustain a pressure of 30 mbar must be 0.15 L min [1]. With these settings, if the APL valve functions correctly, a pressure of 30 mbar should not be exceeded. However, the pressure in the breathing system increased to 70 mbar. Further pressure rise was limited by the additional electronic safety control, which was set (upper pressure limit) at 70 mbar. The anaesthesia machine was switched off and then switched on again. The pre-use automatic function check was performed again without a failure indication. The manual function check was repeated with the previous settings and again showed the same problem. In addition, the standard test for the correct function of the APL valve was performed with settings of the APL valve at 0 or 20 mbar, and with high fresh gas flows of 10 L min. This test also did not result in release of the system pressure. The anaesthesia machine was rejected and replaced by another to continue with the operating list. Afterwards, the APL valve was removed and replaced by a different one and this new one functioned as expected. Although the originally malfunctioning APL valve could be turned, without any obvious difficulty, to any position between 90 and 0 mbar, it displayed the same malfunction when it was connected to the anaesthesia machine. Finally, it was removed from the anaesthesia machine to be checked by a Siemens service technician. In the meantime, the spring mechanism at the lower end of the valve was pressed several times by another anaesthesiologist without apparent problems. The valve was again placed into the anaesthesia machine to be photographed, and, surprisingly, it worked correctly. The Siemens technicians could not find any malfunction of the valve in the subsequent testing. Three months later, the same malfunction of the same APL valve occurred. This time, both the pre-use automatic and the manual function checks were passed correctly. When the patients’ lungs were ventilated manually with the APL valve set at 20 mbar, the airway pressure rose quickly to 55 mbar. Further pressure rise was prevented by the electronic safety control function. Although the APL valve was immediately rotated to the fully open (spontaneous) position, the valve did not release the system pressure. Without delay, the patient’ circuit was disconnected from the patient and the anaesthesia machine was exchanged with Correspondence to: Xenia Souvatzis, Department of Anaesthesiology, University Hospital of Heraklion, PO Box 1352, Heraklion, GR 71110, Greece. E-mail: [email protected]; Tel: 130 281 039 2414; Fax: 130 281 039 2409