An ASA 3 patient undergoing a cochlear implant procedure underwent induction of anesthesia and intubation without incident. The OR table was rotated 180 degrees to facilitate surgery. However, the anesthesia gas machine started alarming “ PEEP High /Blockage?” Mechanical ventilation was stopped and hand ventilation initiated and the patient was found to have normal lung compliance. No kinks or obstructions were identified in the ventilator tubing or endotracheal tube, and suctioning of the endotracheal tube was unremarkable. Subsequently, it was noted that the scavenger bag was full despite the needle valve being almost completely open. The scavenging bag was removed, allowing waste gases to release into the OR and decreasing the PEEP to normal. It was then noted that the evacuation pipeline became dislodged from the attachment to the bottom of the machine. It was reconnected, and the alarm message and audible alarms resolved.

Shashank Saxena, MD, is Clinical Assistant Professor at the University of Pittsburgh School of Medicine and Staff Anesthesiologist, VA Pittsburgh Health Care System, Pittsburgh, PA.

Maria Laykish is CRNA at the VA Pittsburgh Health Care System, Pittsburgh , PA.

Michael P Mangione, MD is Associate Professor at the University of Pittsburgh School of Medicine and Chief of Anesthesia, VA Pittsburgh Health Care System, Pittsburgh , PA.

GE Healthcare Response

The gas scavenging 10 cmH₂O positive pressure relief valve within the Advanced Breathing System of Aespire, Avance, and Aisys anesthesia systems was designed in 2000 to comply with 2 standards: EN 740:1999 Anaesthetic workstations and their modules—Particular requirements; and ISO 8835-3:1997 Inhalational anaesthesia systems—Part 3: Anaesthetic gas scavenging systems—Transfer and receiving systems. Clause 111.1.2c of EN 740:1999 requires the pressure rise at the patient connection port of a breathing system shall not exceed 1 kPa (10 cm H₂O) at a continuous flow of 75 L/min under single fault conditions, and Clause 4.2.1 of ISO 8835-3:1997 requires the pressure rise at the inlet of the AGSS shall not exceed 1.5 kPa (15 cmH₂O) at a flowrate of 75 L/min, having introduced single faults one at a time.

There are multiple Anesthesia Gas Scavenging System (AGSS) options for the Aespire, Avance, and Aisys. The 2 most common options are the high vacuum active AGSS with adjustable flow and the high vacuum active AGSS with low flow. The high vacuum active AGSS with adjustable flow is a closed system with a needle valve and a waste gas reservoir bag.

If the Aespire, Avance, or Aisys closed system AGSS becomes occluded or if the hospital vacuum source is lost, back pressure will build in the anesthesia system and expand the waste gas reservoir bag until gas is relieved to atmosphere by the 10 cmH₂O positive pressure relief valve. The anesthesia system will annunciate a high priority alarm, “PEEP high. Blockage?,” when an elevated pressure is sustained for 15 seconds. The threshold for the sustained airway pressure alarm depends on the PEEP setting and the airway pressure high limit (Pmax) alarm setting. The default sustained airway pressure threshold during mechanical ventilation is 8 cmH₂O when Pmax is 40 cmH₂O and PEEP is Off. The default threshold will allow the “PEEP high. Blockage?” alarm to occur before waste gas is relieved to atmosphere. The clinician can identify an AGSS blockage or a loss of vacuum to the AGSS by seeing the fully inflated waste gas reservoir bag.

Prior to Aespire, Avance, and Aisys, the Aestiva anesthesia machine was designed with a similar 10 cmH2O positive pressure relief valve; however, the Aestiva relief valve is within the integrated AGSS. The Aestiva closed system AGSS option differs from Aespire, Avance, and Aisys by requiring an external add-on kit with the needle valve and waste gas reservoir bag. The add-on scavenging kit contains a positive pressure relief valve specified to relieve at 4.5–7 cmH₂O at a flow of 60 L/min. However, if an occlusion were to occur in the Aestiva AGSS or between the Aestiva AGSS and the add-on scavenging kit, back pressure will build in the Aestiva until gas is relieved to atmosphere by the 10 cmH₂O positive pressure relief valve.

As an alternative to the closed system AGSS, there are open system AGSS options for the Aespire, Avance, and Aisys that do not build back pressure in the anesthesia system if there is insufficient extraction flow. The open system manages peak waste gas flow that exceeds the extraction flow by using a 2 L holding volume within the Advanced Breathing System. Waste gas will spill from the holding volume into the room only if the waste gas flow exceeds the extraction flow for an extended period of time. When extraction flow exceeds the waste gas flow, the extraction will remove waste gas from the holding volume. Since the open system AGSS is designed for the average extraction flow to exceed the average waste gas flow, the hospital vacuum source will entrain room air.

No matter which scavenging system you use, we thank you for bringing attention to the importance of verifying correct operation of the scavenging system prior to each use.

Sincerely, Karl Knauf Lead System Designer GE Healthcare