Ventilator Alarm Failure Due to Modification of the Scavenging System

A 4-month-old, 4.1-kg infant with ventricular and atrial septal defects was scheduled to undergo closure of both defects by means of cardiopulmonary bypass. After anesthetic induction, a 4.0-ID cuffed endotracheal tube was placed orally, and the child’s lungs were mechanically ventilated with use of a Siemens Servo 900c ventilator (Siemens-Elema AB, Solna, Sweden). Fresh gas flow to the ventilator was supplied by an Ohmeda Modulus II anesthesia machine (Ohmeda, Madison, WI). Ventilator settings included minute ventilation (MV) of 2.5 l, respiratory rate of 35 breaths/min, inspiratory time of 25%, inspiratory pause of 5%, and peak airway pressure of 20 cm H²O. The endotracheal tube was disconnected from the breathing system temporarily to position the infant on the operating table. Twenty seconds after disconnection, the lower expiratory MV alarm had not sounded as expected. The pulse oximeter continued to show 99% saturation. The lower MV expiratory alarm was set at 0.5 l. Although the child had been disconnected for approximately 60 s, both the needle gauge and the digital readout showed an MV of 2.0–2.5 l. The lungs were ventilated manually without significant desaturation. The expiratory MV value never decreased below 2 l.

Ten days later, a 6-day-old, 3.1-kg, cyanotic infant with transposition of the great arterieswas scheduled for anatomic correction. After anesthetic induction and treatment similar to that in case 1, a different Siemens servo 900c ventilator was set with an MV of 1.2 l, a respiratory rate of 30 breaths/min, an inspiratory time of 25%, and an inspiratory pause of 5%. As before, the infant was disconnected from the breathing circuit to ensure safe positioning on the operating table. Again, the ventilator lower expiratory MV alarm did not sound, and both the needle gauge and the digital readout showed an expired MV of 1.0–1.5 l. The oxygen saturation measured by pulse oximetry (Spo²) was 85%.

With further investigation, it seemed that the problem was related to the use of the Ohmeda scavenging system, which was connected to both the Siemens Servo 900c ventilator and the Ohmeda Modulus II anesthesia machine ventilator (fig. 1). Waste gas from either ventilator enters the evacuation system at point A. Hospital suction is connected to point B, and an adjustable needle valve varies the suction flow from 0 to a maximum draw of 15 l/min. The positive-pressure relief valve vents waste gases into the room if the pressure in the scavenging interface reservoir bag exceeds 5 cm H²O. This occurs when waste gas flows exceed the adjusted evacuation flow rate. The positive-pressure relief valve prevents excessive airway pressure and possibly prevents cumulative gas trapping from being presented to the patient’s lungs. The negative-pressure relief valve opens to allow room air into the evacuation system and then through the hospital suction system when pressure in the scavenging system is less than −0.5 cm H²O. This occurs when the scavenging system is adjusted to remove more gas volume/min than the anesthesia administration system is presenting to the scavenging system. The reservoir bag will collapse, generating negative pressure. The negative-pressure relief valve prevents significant negative pressure from being transmitted to the patient’s lungs.

When the Siemens Servo 900c ventilator was used with an Ohmeda Modulus II anesthesia machine, to simplify the system, the Siemens evacuation component was removed, and the evacuation tubing of the Siemens Servo 900c was connected to point D of the Ohmeda scavenging system, and point CD was obstructed. With the Ohmeda scavenging system adjustment knob (fig. 1) open, the Ohmeda negative-pressure relief valve opens at −0.5 cm H²O. At this negative pressure, the scavenging system draws approximately 1.0–1.5 l/min room air into the Siemens Servo 900c ventilator through the expiratory limb. In an adult, a flow of 1.0–1.5 l/min would be inconsequential because the lower expiratory MV alarm would be set at 4–5 l. With the small MV of an infant, the lower expiratory MV alarm is not activated. The expired MV never reaches a value of 0 l/min, even with the endotracheal tube disconnected from the breathing system.

A solution to this problem is the attachment of a long reservoir tube, open to room air, to an additional arm of the intake port of the Ohmeda closed-reservoir scavenging interface point C. This effectively modifies the scavenging system into an open system, and neither positive nor negative pressure is possible within the scavenging apparatus. It eliminates the negative-pressure–generated flow through the expiratory system of the Siemens Servo 900c ventilator. If the reservoir tube total volume exceeds 3 l, the same as the volume of the reservoir bag, it vents waste gas into the operating room to the same degree as the unmodified semiclosed Ohmeda scavenging system.

In conclusion, this report shows the potential consequences of user error and use of anesthesia equipment. Inadequate knowledge of anesthesia equipment and ventilators may compromise important monitoring devices and alarms, placing patients at increased risk for complications and death.

The authors thank Gerald J. Rach, Supervisor, Biomedical Department/Anesthesia Maintenance, Mayo Clinic and Foundation, Rochester, Minnesota, for technical expertise in anesthesia equipment in the preparation of this manuscript.