Draeger Medical GmbH (Lübeck, Germany) would like to thank the authors Vannucci et al. for bringing this to the attention of the anesthesia community.
The authors describe a situation where the internal connection between the breathing system and the outlet to the scavenger system of the Draeger Apollo was blocked. This occlusion caused a high positive end-expiratory pressure buildup in the breathing system and at the patient.
In the schematic of the breathing system (fig. 1), you can see how the gas mixture is circulating through the breathing system. The excessive gas, which is not necessary for ventilating the patient, is led out of the breathing system via the anesthetic gas scavenging system connector (it is important to note that this outlet is used in both manual and spontaneous ventilation modes and in all mechanical modes).
Fig. 1. A schematic of the Apollo's breathing system. The green line on the schematic depicts the path through which gas is scavenged during automatic ventilation. The orange line on the schematic depicts the path through which gas is scavenged during manual and spontaneous ventilation. APL = adjustable pressure limitation; MAN = manual; PAW = airway pressure; PEEP = positive end-expiratory pressure; SPONT = spontaneous.
Fig. 1. A schematic of the Apollo's breathing system. The green line on the schematic depicts the path through which gas is scavenged during automatic ventilation. The orange line on the schematic depicts the path through which gas is scavenged during manual and spontaneous ventilation. APL = adjustable pressure limitation; MAN = manual; PAW = airway pressure; PEEP = positive end-expiratory pressure; SPONT = spontaneous.
In the situation of a complete occlusion in the scavenging line, both manual and spontaneous ventilation modes and mechanical modes would not be possible. As soon as excessive gas cannot be let out of the breathing system and the internal breathing system pressure as well as the airway pressure of the connected patient starts to increase, the Apollo's safety system alerts the user via acoustic and visual alarms (table 1). After 6 s, the alarms “continuous pressure” and “high airway pressure” appear, and, after the opening of the MV2 internal safety valve, the “pressure relief” alarm appears. In addition, the user will see the manual breathing bag expansion in both manual and spontaneous ventilation and in mechanical ventilation.
As a backup and safety function, the Apollo is equipped with the MV2 internal safety valve integrated into the breathing system interface, which opens to ambient air in case of high pressure or negative pressure.
The check-out procedure on the Apollo anesthesia workstation is designed in two parts:
The first part is the manual test performed by the user according to the device checklist displayed on the screen after startup. In the manual check-out procedure, the user is asked to set the adjustable pressure limitation valve to 30 mbar and press the oxygen flush button to pressurize the breathing system. Under normal working conditions (open anesthetic gas receiving system outlet), the oxygen flow will fill the breathing bag, the breathing system, and ventilator piston. All excessive gas will be released by the adjustable pressure limitation valve and anesthetic gas receiving system outlet as soon as a pressure according to the adjustable pressure limitation valve setting (30 mbar) is reached. Because of flow resistances, the pressure stabilizes at a level of about 35–40 mbars. After releasing the oxygen flush button, the airway pressure should drop and stabilize after a couple of seconds at a level above 15 mbars, typically 25–30 mbars. The breathing system pressure can be monitored either on the screen of the Apollo by a bar graph display or via the mechanical pressure gauge, which is integrated into the left handle of the Apollo anesthesia workstation.
The second part is the automatic test. During the automatic test, the Apollo checks the internal safety valves for pressure release to the ambient air and the anesthetic gas receiving system check valve in the breathing system. The automated self test of the Apollo is designed to test the internal safety mechanisms of the passive anesthetic gas scavenging system valve and safety valves. An occlusion of the internal anesthetic gas scavenging system connection would therefore not be detected at that point.
With these safety measures, we have implemented a design which allows the user to quickly recognize the situation during operation of the machine, and react accordingly.
In the article, the authors describe a situation where first manual ventilation was performed uneventfully, and when switching to mandatory ventilation mode, the positive end-expiratory pressure increased. The only way this could happen is that after manual ventilation, the ventilator drawer of the Apollo was pulled out by the clinician, and the piece of foreign plastic debris fell behind the drawer. When the user pushed the ventilator drawer back to the closed position, the plastic debris became trapped in the scavenger port, occluding the exhaust port.
Should the ventilator drawer be opened at any time during use, an integrated infrared light barrier triggers an acoustic and visible high priority alarm alerting the user that the ventilator drawer is unlocked (table 2).
Based on the findings provided by the submitting author, Draeger believes that it is very unlikely that the occlusion was missed by the automated self test and by the clinician's manual self test. Since the clinician was able to manually ventilate before the reported event during mechanical ventilation, repositioning of the breathing system (most likely by opening the ventilator drawer) enabled the plastic debris to be introduced or repositioned to occlude the scavenger port. Upon this occurring, the anesthesia machine correctly warned the clinician of the continuous pressure and/or increased positive end-expiratory pressure.
In summary, Draeger would like to thank the authors for sharing this unique scenario with the anesthesia community. Although this is the first time an event of this nature has been reported to Draeger, it underscores the importance of changes that can occur during anesthesia delivery (after self test), and the need for clinicians to remain vigilant to the anesthesia machine's alarm indicators and to act in accordance to the manufacturer's operating instructions.
