Obstruction to scavenging system tubing.

Sir,

Obstruction to the scavenging system might result in serious complications including risk of pneumothorax. Several causes for obstruction had been highlighted in the literature.[1-5] We report a rare cause of obstruction to the scavenging system.

A 30-kg, 12-year-old girl was scheduled for open reduction and internal fixation of fracture both bones forearm. Anesthesia machine with circle system (Signet-615, Av.ulco Healthcare Pvt. Limited, Pondicherry, India) and anesthesia ventilator (eV-700, Av.ulco Healthcare Pvt. Limited, Pondicherry, India) were checked as per standard protocol. General anesthesia was induced by standard technique using midazolam, fentanyl, thiopentone, and vecuronium. After confirming endotracheal intubation, lungs were mechanically ventilated (tidal volume 250 ml, respiratory rate 14 per min, and inspiratory to expiratory ratio 1:2) with no positive end-expiratory pressure (PEEP). While observing the peak airway pressure in ventilator display to adjust tidal volume, we noticed PEEP developed and increased gradually to 10 cm of water with increasing peak inspiratory pressure. Ventilation was switched over to manual and normal airway pressure was re-established. Patient's vital signs were normal and auscultation revealed no evidence of pneumothorax. Manual ventilation was continued throughout the intraoperative period and the remaining surgical period was unremarkable. Postoperatively, there was no clinical or radiological evidence of pneumothorax.

No visible obstruction to the breathing circuit and scavenging system transfer tubing were found; ventilator malfunction was suspected. Using a test lung, the ventilator was tested on similar ventilator settings. The PEEP gradually increased to 20 cm of water. The scavenging system transfer tubing was disconnected to examine the ventilator exhaust port because any obstruction to the ventilator exhaust port would result in such a problem, and it was found to be normal. However, the PEEP became zero on disconnection, which helped us to locate the obstruction to the scavenging system transfer tubing. There was no visible obstruction or kinking in the scavenging system transfer tubing, but it was connected to a dummy connector instead of a regular connector to the scavenging system interface. This interface has two ports to receive the exhaust gas, both ports have similar diameter [Figure 1]. The presence of two ports facilitates connection of scavenging system transfer tubings, both from breathing system exhaust and ventilator exhaust. In this machine, the scavenging system transfer tubing is usually connected to one port through a regular connector. The other port is plugged with a dummy connector (without any lumen) to prevent the leakage of scavenged gas [Figure 2]. The operation theater technician had connected the scavenging system transfer tubing to dummy connector by mistake, after the machine was checked. The misconnection happened as the outer diameter of both the regular connector and dummy connector are same. Modification of this dummy connector with different outer diameter would avoid this complication in future.

Figure 1

The scavenging system interface with dummy (1) and regular connector (2) with similar outer diameter

Figure 2

The dummy connector without any lumen (1) and regular connector with lumen (2)

A standard scavenging system consists of a collecting assembly, transfer tubings, receiving system, and disposal system for waste gases. Depending upon the disposal system used, the scavenging system is classified into active or passive system. Based on the interface, it is classified into open or closed system. Positive and negative pressure relief valves are also an integral part of the scavenging system. In our case, the scavenging system is of active system with open interface. The open interface acts as both negative and positive pressure relief valves. The positive pressure relief valve (open interface) vents the waste gases only if the obstruction is downstream of the interface. Obstruction or kinking of the scavenging system transfer tubing will result in the development of PEEP.

It was suggested that a standard scavenging system transfer tubing should have a 30-mm female connector with positive pressure relief valve connected to breathing system or ventilator exhaust.[6] These standards are incorporated in newly manufactured anesthesia machines. To avoid this complication in the existing anesthesia machines, the outer diameter of the dummy connector should be modified.