Flow sensor fault causing ventilator malfunction

Abstract

A fault with a flow sensor in a Datex Ohmeda 7900 ventilator (Datex Ohmeda, Hatfield, UK) was detected following malfunction of the ventilator in the volume control mode. The anaesthetic machine (Datex Ohmeda Modulus SE) was checked using standard guidelines [1] and no faults were detected and the patient was started on volume control ventilation. Near the end of the procedure we encountered a ventilator alarm showing ‘low tidal volume’ and ‘system air leak’. There was no chest expansion or capnograph trace; the excursion of the ventilator bellows was minimal. However, we were able to manually ventilate the patient using the circle system. No fault could be identified for the ‘system air leak’ alarm displayed on the ventilator screen. On restarting mechanical ventilation in the volume control mode, the ‘system air leak’ alarm and inability to ventilate persisted. Subsequently, the ventilator was switched to ‘pressure control/pressure limited ventilation’ and we were able to ventilate the patient, although the ‘low tidal volume’ alarm persisted. By this time we had identified a fault in the flow sensor in the expiratory limb. There was an incomplete stress fracture of the blue limb of tubing to the flow sensor (Fig. 6). We have deliberately cut the translucent limb to show that the lumen size is about 2 mm, which could not cause a system air leak. Flow module sensor showing incomplete fracture of blue limb. The pale clear tubing has been deliberately cut to show the size of the lumen (arrow). In the Ohmeda machine there are two flow-pressure sensor devices, one in each limb of a circle. The flow sensor in the expiratory limb measures patient exhalation volume. The flow sensor in the inspiratory limb supplies feedback for the volume control ventilation mode. The airway pressure is monitored on the patient side of the inspiratory ‘flow sensor’[2]. This explains our ability to ventilate in the ‘pressure control/pressure limited’ mode. Krock et al. reported stress fractures of the Ohmeda 7900 flow sensors [3]. Ohmeda subsequently changed the manufacturing process so that these fractures are minimised. Shogase et al. described a protective device for the flow sensors made from a 50 ml syringe [4]. We feel that clinicians using the Datex Ohmeda 7900 ventilator and flow sensor devices must be aware of this problem as the alarm displayed on the ventilator screen will not direct the anaesthetist to the actual fault site. Furthermore, the fault will not be picked up by the high pressure leak test. Ohmeda recommends ‘the calibration of the flow sensors at least monthly or as required according to procedures followed by your facility’[2]. As an isolated sensor integrity check requires disconnection of the common gas outlet, we feel that visual confirmation of the integrity of each limb of the flow sensors and the use of a dummy lung in the breathing system and running of the ventilator must form an integral part of the anaesthetic machine check. GE Healthcare appreciates the opportunity to respond to the letter regarding the ventilator malfunction that resulted from a flow sensor fault. The situation the authors describe has been seen before. The early design of both the inspiratory and expiratory flow sensors involved the use of an adhesive to attach the flow sensor tubing to the body of the flow sensor itself. To address this issue, GE Healthcare, then known as Datex-Ohmeda, produced both a protective cover for the sensors and redesigned the adhesive connection to remove the hardened sharp surface. At that time we also discontinued the manufacture and sale of the flow sensor affected. To help prevent this and other issues related to the orientation of the flow sensor tubing, Datex-Ohmeda released a Marketing Newsletter in March, 2000 describing a Flow Sensor Manifold (Fig. 7) that users may use to locate the tubing correctly and to provide protection against strain or damage to the tubing. Flow sensor manifold for Datex Ohmeda 7900. The authors are correct in stating that the pre-operative checkout procedure may not detect such a leak. The checkout procedure permits a breathing circuit leak of 300 ml at a pressure of 40 cmH2O. The leak produced with the lacerated tubing may hide within this 300 ml allowance. GE Healthcare agrees with the authors that the error message does not immediately point the user to the site of the problem, although the Owner's Manual does suggest the flow sensors be examined as one measure to address the situation. GE Healthcare commends the authors for understanding that pressure control ventilation remained available and they were able to continue the case without any untoward affects.