In Reply:—
In their letter, Drs. Barahal and Sims describe a situation in which suction to the patient’s airway before bronchoscopy caused a problem with the Narkomed 6000 ventilator (Draeger Medical Inc., Telford, PA). Draeger Medical Inc. investigated this problem and would like to describe the findings and the results of the investigation.
When the clinical circumstances of this event were recreated, it was determined that a negative pressure of −375 mmHg was applied to the airway through a 14-French suction catheter drawing a flow of 30 l/min through the catheter. When the same suction level was applied through the bronchoscope suction port, only 4.6 l/min of flow was drawn through the port, and the problem could not be recreated. Therefore, it seems that the initial airway suctioning was the inciting event, not the suction through the bronchoscope.
The problem described by Drs. Barahal and Sims occurred when a relatively high negative pressure, resulting in a high degree of suction flow, was applied to the patient’s airway during ventilation. If the suction flow is high enough, it will exceed the capacity of the ventilator’s negative-pressure relief system. The resulting negative pressure in the breathing system holds the diaphragm control valves closed in such a way that gas cannot enter the breathing system and relieve the negative pressure. A similar situation was reported to Draeger Medical Inc. by another institution where a high degree of negative pressure was applied to the airway through a misplaced nasogastric tube.
Typical clinical guidelines for suction applied to the adult airway recommend that suction pressure be set as low as possible to clear secretions effectively. Desirable suction levels in the range of −100 to −150 mmHg applied for up to 15 s have been cited. 1American Society for Testing and Materials standard F960 governing Medical and Surgical Suction and Drainage Systems offers a maximum static vacuum level of −160 mmHg for adult tracheal suctioning as a guideline. 2When the suction regulator used by Drs. Barahal and Sims was adjusted to −120 mmHg, 18 l/m of flow was drawn through the 14-French suction catheter, and the ventilator problem could not be made to occur.
Draeger Medical has taken a number of steps to address this issue. Draeger recommends that regulated suction within the range of typical clinical guidelines be used either when applying suction to the airway or at the time of initial placement of a nasogastric tube before proper placement has been confirmed. We agree with Drs. Barahal and Sims that it is prudent to put the ventilator into “manual” mode and to fill the reservoir bag before applying suction to the airway (or to a gastric tube that may be in the airway). If the reservoir bag empties as suction is applied, suction should be stopped before the bag is completely empty to avoid exposing the breathing circuit to excessive negative pressure. Draeger has also incorporated a help feature into the Narkomed 6000, which appears automatically on the monitor display should this event occur. The help feature guides the user through the simple steps that must be taken to ensure the patient’s safety. Draeger has taken steps to enhance the training process to ensure that all users are made aware of the recommended degree of suction that should be applied to the airway. Finally, changes to the operator’s manual will more clearly describe this situation and the strategies for avoidance.
