Scavenging system obstruction: A cause of raised PEEP during a magnetic resonance-guided neurosurgical procedure.

A 48-year-old man was scheduled for a temporal lobe tumor resection. The use of intraoperative magnetic resonance imaging (MRI) was required, needing special equipment including the anesthetic machine (Aestiva 5 MRI, Datex-Ohmeda, GE Healthcare, Madison, USA). After a total intravenous anesthesia induction and orotraqueal intubation the patient was ventilated with volume control modality (tidal volume 550 mL, respiratory rate 12, PEEP 6 and 1:2 inspiration/expiration ratio) ensuring normocapnia and normoxygenation controlled by arterial blood gas. Before opening the dura mater, the PEEP level suddenly rose up to 10 cm H2O (980.6 Pa), with an increase in both peak and mean airway pressures (Fig. 1, left). Simultaneously an increase of brain tension was observed in the surgical field. We ruled out a respiratory or endotracheal tube incidence because there was a lack of abnormalities in pulmonary auscultation, capnogram, pulse oximetry or hemodynamics. Our main suspect at that moment was a dysfunction in the anesthetic station. We excluded an inspiratory or expiratory valve malfunction because of the absence of rebreathing and other abnormalities in the shape of the capnography curve. Decreasing the fresh gas flow from 6 to 3 L min and changing to manual ventilation solved the air trapping. After checking all the connections between the patient and the machine and a correct aspiration from the central system, we observed a partial obstruction in one of the holes of the AGSS tube close to the anesthetic machine. The disconnection of the tube solved the problem (Fig. 1, right), confirming our hypothesis of an incident with the scavenging system. The brain tension was relieved and a successful tumor resection performed. The patient was extubated in the operation room and discharged home 5 days after the surgery without neurologic sequelae. All anesthetic machines require a security system to allow a proper scavenging system. In Aestiva 5 MRI machine the security system for avoiding an over-pressure consists on two orifices in the AGSS tube to make sure that a proper gas aspiration happens. In this case the combination of a relative high fresh gas flow and a damaged AGSS tube may have played a role in the critical incident.