There is a patient safety hazard associated with the use of unidirectional valves that are inserted into the anesthesia breathing system to generate positive-end expiratory-pressure (PEEP). (1,2,3)
One common design for a PEEP valve is shown schematically in the figure correctly inserted in the expiratory limb of a circle breathing system. The potential exists to insert the valve incorrectly, causing a total or near total obstruction of the breathing system. Some believe such devices to be unacceptable for anesthesia? At least one manufacturer warns against using its machines with any”. . components that establish a flow direction.” Yet, Most Other alternatives for applying PEEP in an anesthesia breathing system present hazards of their own.
It is reasonable to ask: “Shouldn’t any competent anesthetist recognize when a unidirectional device has been inserted in the wrong direction?” Yes, but, under some circumstances, that might not happen. Such would be the kind of circumstances in which accidents typically occur. Consider that PEEP is often chosen intraoperatively at a time when some problem has already surfaced. In an attempt to combat hypoxemia, the clinician is already somewhat distracted by trying to sort out the clinical picture. Consider also that most breathing systems do not clearly differentiate between the inspiratory and expiratory limbs of the breathing system. In the haste of the moment with a patient who may be in trouble, it isn’t too difficult to imagine placing the device on the wrong port usually at or near the CO, absorber head. Such events have happened more than once. The same potential exists with older models of humidifiers, which also may be unidirectional.
This problem is not an easy one to solve. A bi-directional valve is one approach. It has a second flow channel with its own one-way valve. If inserted correctly, flow passes through the PEEP-generating mechanism. If inserted in the reverse direction, flow bypasses the PEEP-generating mechanism. PEEP is not delivered, but the breathing system is not obstructed. One problem with such a bi-directional valve can arise if it is used with an older model breathing system like the one illustrated in the figure Unlike newer circle breathing systems, airway pressure is monitored on the machine side of the unidirectional valves. Because PEEP is not transmitted past the valves, it will not be indicated on the gauge. The clinician is dependent on the accuracy of the crude valve markings to know the level of PEEP. It is possible to place such a valve between the breathing system and the ventilator. In that case PEEP should be observed on the breathing system pressure gauge. The valve must still be placed in the correct orientation. If it is placed in the wrong direction, PEEP will not be delivered, but again, there will not be a circuit obstruction.
The greater hazard with the common bidirectional valve or any type of PEEP valve that has adjustable settings is that, if left in the circuit, the valve can easily be set to accidentally deliver PEEP in a patient who would not tolerate it well. Again, with older breathing system, this would not be observed on the airway pressure gauge.
A PEEP mechanism may be optionally incorporated into the CO, absorber assembly or ventilator of newer versions of some anesthesia machines. This effectively prevents incorrect placement of an add-on valve and the consequent danger of obstruction. In one design, an adapter is mounted on the exhalation unidirectional valve of the absorber. This has the disadvantage of having a similar appearance to the APL (pop-off valve and can accidentally be left on in the same manner as the bi-directional described above. In another manufacturer’s version, there is a more distinguishable on/off switch. In both cases, integral pressure alarms sensing pressure on the patient side of the breathing system inspiratory unidirectional valve add considerably to safety.
There are other alternatives to address the very real PEEP hazard. In our own institution, we implemented an intense training program for all attendings and staff and adopted a rule that PEEP valves cannot be used without the attending anesthesiologist being present. We also stress the danger of this specific hazard during the in-service training which is required for all new residents. To help prevent the accidental placement of the valve without the elbow adaptor, an adaptor has been permanently attached with permanent adhesive to every PEEP valve. Needless to say, all patients are monitored with pulse oximetry and almost all with capnography.
Some hospitals have adopted a stricter rule, requiring that two people be present when a PEEP valve is inserted, regardless of the training of the senior person.
THE BOTTOM LINE: If you are using a unidirectional PEEP valve, be aware of the hazards. If inserted before the procedure begins, do an appropriate pre-use inspection to be sure that the system is not obstructed. If inserted in mid-procedure, be extremely cautious about where it is placed and check for appropriate indications of ventilation after insertion.
In any event, establish a protocol or guideline that enhances the safe use of PEEP valves in your practice setting. This should include the steps outlined above or the equivalent. It can be hoped that the new anesthesia machines will address this problem and eliminate a hazard for which there is no simple, comprehensive remedy.
Dr. Cooper, Director, Anesthesia Technology, Department of Anesthesia, Massachusetts General Hospital is a member of the APSF Executive Committee.
1. -: Hazard – PEEP valves in anesthesia circuits, Health Devices, 13:24, 1983 (published by ECRI, Plymouth Meeting, PA 19462).
2. —-: Evaluation – PEEP valves. Health Devices, 14:379-394, 1985 (published by ECRI, Plymouth Meeting, PA 19462).
3. Arellano R, Ross D and Lee K: Inappropriate attachment of PEEP valve acusing total obstruction of ventilation bag. Anesth Analf 66:1049-60, 198 7.
4. North American Drager: Operator’s Instruction manual, Markomed 3 Anesthesia System, Telford, PA, 1988.