An unusual cause with a simple solution for failure of oxygen sensor in a Dräger Fabius GS ventilator

Abstract

Sir, Oxygen sensors present in the anesthesia ventilators are a very important safety device to detect hypoxic delivery of gases. Modern anesthesia ventilators use various techniques such as infrared analysis, paramagnetic oxygen analysis and electrochemical oxygen analysis to measure oxygen concentration delivered to the patients. The Dräger Fabius GS premium ventilator uses an electrochemical oxygen analyzer to measure oxygen concentration. Here, we describe an unusual user error for the failure of oxygen sensor in the intra operative period. This event occurred after induction of an adult patient for craniotomy. The anesthesia machine was checked according to standard guidelines, the oxygen sensor was calibrated before the start of case.[1] However after positioning of the patient, ventilator was shifted down to make way for the surgeon to operate from the head end. The galvanic cell where the chemical reaction takes place is placed in a cartridge, the upper cartridge of the oxygen analyzer has a wire, which was twisted and entangled leading to rotation and stretching of the cartridges and so we rotated it and untangled the loops of wire. However after this, we noted that suddenly oxygen sensor failure alarm appeared on the screen. There was no drop in supply pressure of oxygen and hemodynamic was stable. We disconnected the analyzer and recalibrated, but it was unsuccessful. At this stage, we noted that the cartridges which are made up of two parts enclosing the oxygen cell had loosened. We tightened it and calibration was successful after that. The Dräger Fabius GS ventilator uses electrochemical (Galvanic cell) type of oxygen analyzer which generates a current proportional to the quantity of dissolved oxygen is generated, when oxygen is dissolved through the diaphragm in an electrolytic solution in which an anode (base metal) and cathode (noble metal) are adjacent to each other, The amount of oxygen passing through the diaphragm is proportional to the partial oxygen pressure of the sample gas, therefore, the oxygen concentration can be determined by measuring the current. The main advantages of the Galvanic cell type oxygen analyzer are that they are dependable, compact, reliable and economical. The main disadvantages are that they have a slow response time, need for calibration before each use and at least every 8 h and the cell life is limited.[2 3] In Dräger Fabius GS ventilator this galvanic cell is housed inside a plastic cartridge (upper and lower) [Figure 1a]. The design of the cartridge is such that even if we rotate it, it turns as a whole. However probably in our case, the cartridge had slightly loosened and was missed during pre-use check since it passed the calibration. But once the entangled wire was noted during the intra operative period [Figure 1b] we rotated the cartridge, when probably only the upper cartridge rotated further loosening the cartridge and the spring loaded oxygen measuring cell inside the cartridge lost its contacts to the metallic contacts of the cartridge. So, due to this opened O2 sensor housing, Fabius correctly alarmed. Furthermore, calibration was not possible, because the real root cause of the problem (no connection of the oxygen cell to the contacts in the housing), was not solved. Though in the ventilator user manual disconnection of O2 sensor or faulty cable is mentioned as a cause for O2 sensor failure, there is no mention of the loosening of the cartridge as a cause. Figure 1 (a) Pictorial description of the working principle of the oxygen sensor. (b) Snapshot showing the cartridge of the oxygen sensor with entangled wire Our report shows that simple and correctable causes for oxygen sensor malfunction. Care should be taken in handling this sensor and prevent loosening of the cartridges. Possible hazards like putting high pressure or traction on cables should be avoided while setting up the machine.