Rare is the anesthesiologist who has not encountered problems with the adjustable pressure limiting (APL) valve commonly known as the popoff valve on the anesthesia machine. Instead of allowing variable adjustments to the positive pressure attained within the patient breathing circuit and alteration of the gas volume contained within the rebreathing bag, the malfunctioning valve can offer only fully closed or a fully open position. This can lead either to barotrauma or inadequate ventilation.
This problem recently arose in the APL valve of an Ohio Modulus I machine that was serviced regularly under a preventive maintenance program from the manufacturer. On disassembly of the APL, the problem was readily apparent. Soda lime dust blown back from the absorber into the APL had escaped around the diaphragm seat and entered the adjustable mechanism via a small hole in the spring sleeve. This mixture of dust and moisture eventually caked and hardened, preventing the free movement of the pin inside the spring sleeve. The accompanying photograph shows the effect of this caking inside the APL valve housing. (Readers are referred to either the manufacturer’s manual or The Anesthesia Machine: Essentials for Understanding by Bowie and Huffinan, Ohmeda, Madison, Wisconsin for a schematic representation.)
When this problem occurs, it is easily connected. The valve needs to be disassembled and inspected. The individual components need to be cleaned and rinsed of all deposits and reassembled. To check for a proper functioning APL valve, the assembly should rattle when shaken, indicating free movement of the pin within the spring and sleeve. Needless to say, once the cleansed APL has been reattached to the anesthesia machine, a full machine check-out should be performed. Often contract maintenance pro8rarns do not examine or disassemble the APL, but this should be added to the quarterly or biannual inspection routine.
Topic prepared by: Alvaro R. De LA Rocha, M.D., Instructor, New York Medical College; Attending Physician, Lincoln Hospital, Bronx, N.Y. Edited by David E. Lm, M.D.
DUST encrusted interior of an adjustable pressure limiting (pop-off) valve showing how soda lime dust can foul the mechanism and created a potential problem.