To the Editor:
High positive end-expiratory pressure (PEEP) buildup during mechanical ventilation is a known complication that may arise from pressure relief valve or scavenging system malfunction.1,–,5In Draeger Apollo® (Draeger Medical Inc., Telford, PA), the preuse checkout is performed in two phases. First, the initial Check List Screen displayed by the workstation guides a series of manual check steps. This is followed by an automatic self-test. When successfully completed, the self-test displays a green light indicating proper functioning of the tested components, including the scavenging system. To complete the preuse assessment of the scavenging system, the manufacturer's manual recommends visual inspection of connecting hoses and verification that the float is within the reference range. We present a case of high PEEP buildup because of an internal obstruction of the scavenger system that has not been reported before. We did not detect failure by applying the workstation preuse self-test and visual inspection of the external scavenger system.
A 75-yr-old woman presented for left carotid endarterectomy under general anesthesia. Following uneventful induction and intubation with a 7.0-mm endotracheal tube, volume control ventilation was initiated. Settings were: tidal volume 500 ml; rate 12 bpm; PEEP, 5 cm H2O; 50% oxygen/air and sevoflurane; and fresh gas flow of 2 l/min. Shortly after initiation of mechanical ventilation, PEEP increased to 10 cm H2O within few administered breaths, triggering the high PEEP alarm. The reservoir bag, noticed to be under tension, was disconnected, temporarily restoring the PEEP to the set level. Chest compliance was appropriate with manual ventilation. The adjustable pressure relief valve knob was moved through the full range and confirmed to be unobstructed and open. The endotracheal tube was inspected showing no obstruction or kinking, and the correct position was confirmed. The scavenger float was observed to be within the recommended range. PEEP was then set to zero. When positive pressure ventilation resumed, PEEP buildup recurred, triggering the high PEEP alarm. Troubleshooting continued with manual ventilation, using an Ambu-bag® (eMED America Inc, Little Rock, AR) and supplemental intravenous anesthesia.
The surgeons were notified of the equipment failure, and surgery was paused. The breathing circuit was replaced with no effect. Biomedical engineering technicians assisted in changing the breathing system (which houses flow sensors and inspiratory, expiratory, and scavenging system valves) without resolution. A replacement workstation was then delivered to the operating room, checked, and connected to the patient, providing normal ventilation without excess PEEP. The remainder of the surgery and recovery were uneventful.
The malfunctioning workstation was removed from service and examined by the biomedical engineering technicians. The failure was caused by a small piece of plastic film (12.5 × 4 cm) (fig. 1) partially obstructing the anesthetic gas scavenger system cone located behind the breathing system drawer (fig. 2). The exact origin of the plastic film could not be determined.
A simple experiment replicating the intraoperative scenario was designed in order to: (1) confirm that the above-referenced obstruction could cause the buildup of PEEP, and (2) verify that the workstation-recommended checks would not indicate obstruction.
After occluding the anesthetic gas scavenger system cone with the same plastic film, the workstation was powered on and the manual steps according to Check List Screen were followed. For the last manual step, the adjustable pressure limiting (APL) valve was set to 30 cm H2O. Prolonged activation of the oxygen flush hyperdistended the reservoir bag, but the pressures remained below the upper reference limit of 45 cm H2O. Upon discontinuing the oxygen flush, the pressure remained above the reference level of 15 cm H2O. A self-test was then run and the obstruction was not detected; the green light was displayed for the scavenger system and the external scavenger float remained in appropriate position. Positive pressure ventilation was initiated with a reservoir bag connected to the breathing circuit, and buildup of PEEP was again observed. When the plastic piece was removed, normal PEEP function was restored. It appears that following the manufacturer's preuse checkout procedures does not detect this type of obstruction.
Upon expert advice, we then tested three Draeger Apollo-specific checkout protocols available on the American Society of Anesthesiologists website.*†1Of the three guidelines, Wake Forest or Massachusetts General Hospital guidelines would have detected this fault, as either increased or put persistent circuit pressure above 10 cm H2O with open APL valve. If the manufacturer would add an additional step to the Check List Screen (full opening of the APL valve) this step would detect a persistent positive circuit pressure with this type of obstruction.
The events resulting in the obstruction of the anesthetic gas scavenger system cone remain unknown. We hypothesize that an opportunity presented when the ventilator drawer was opened during changing of the reusable carbon dioxide absorbent canister. A space (0.7 cm) above the ventilator drawer (fig. 3) allows for small debris to pass through and fall behind the ventilator drawer. Opening and closing the drawer may allow debris to be forced into the anesthetic gas scavenger system cone. We have recently replaced the reusable absorbent canister with a disposable CLIC canister (Amsorb Plus Universal Bubble Can Absorber; Armstrong Medical Ltd., Coleraine, Northern Ireland). One advantage of the disposable canister is that its replacement does not require opening of the breathing system drawer.
The described internal obstruction of the scavenging system presents a new mechanism of anesthesia workstation malfunction resulting in potentially dangerous levels of PEEP. This obstruction was not detected by following manufacturer's recommended procedures. This case illustrates the shortcoming of relying only on the automatic check test, as underscored by the 2008 American Society of Anesthesiologists Recommendations for Pre-anesthesia Checkout Procedures.2This report should raise awareness of anesthesia providers and the manufacturer of this potential malfunction and reinforce the importance of adopting comprehensive workstation-specific checkout procedures.
The authors thank Biomedical Engineering Service, Barnes-Jewish Hospital, St. Louis, Missouri, for their professional support.