[Acute obstruction of an anesthetic gas evacuation system. Ventilation with a Servo Ventilator 900D]

During aorto-coronary bypass surgery acute expiratory airway obstruction occurred in two patients during controlled ventilation with a Servo D ventilator (Siemens Elema) in combination with a Servo EVAC 180 gas evacuation system. In this system expiratory volume passes from the ventilator to the reservoir bag. Distension of the bag will open the valve to the receiving unit by vertical dislocation of the valve spring. The mechanism relies on free mobility of the valve spring within the reservoir bag. We observed an increase in mean expiratory and inspiratory airway pressure above 40 mmHg due to blockage of the expiratory gas outlet by external lateral dislocation of the valve spring. In conclusion, while free mobility of the valve spring within the hanging Evac bag has to be ascertained at all times for safe application of the EVAC 180 system, the manufacturer should provide some appropriate mechanical shelter around the bag.