An Unusual Cause of Carbon Dioxide Rebreathing

Abstract

In Response: Drs. Possamai and Corbanese are correct in pointing out that the Ohmeda model 5400 (as well as the later models 5410 and 5420) volume monitor (Ohmeda, Louisville, CO) has the capability to detect reverse flow past the flow transducer Figure 1. This function is controlled by an OFF/ON switch Figure 1. When the REVERSE FLOW DETECT switch is in the ON position, reverse flow volumes greater than 50 mL are indicated by an intermittent low-tone audible alarm, a flashing yellow light-emitting diode, and the visual message "REV FLOW" on the liquid crystal display. When the REVERSE FLOW DETECT switch is in the OFF position, the liquid crystal display visual advisory message "REV OFF" is displayed.Figure 1: A Ohmeda volume monitor (model 5400) with a vane-type flow transducer. Note that the door of the battery compartment is open and that a REVERSE FLOW DETECT ON/OFF slide switch is located in the upper left region of the housing above the battery pack.Reverse flow occurs when previously exhaled gas from the reservoir bag is inspired through an incompetent exhalation check valve and exhalation limb. In a circle breathing circuit, the manufacturer recommends monitoring exhaled volume at either of two locations: 1) at the exhalation check valve in the exhalation limb, or 2) at the common airway connected to the tracheal tube. When the transducer is located in the exhalation limb, the volume monitor should signal when it detects reverse flow occurring with each inspiration. By contacting the manufacturer of the device (Ohmeda), we learned that the switch feature for turning off reverse flow detection was added to allow the positioning the flow transducer at the tracheal tube connector during use with a circle absorber or a Bain circuit. When the flow transducer is positioned at the tracheal tube, reverse flow through the transducer will occur with each inspiration, thus necessitating that this alarm be disabled. During the procedure described in our case report [1], an adult circle absorber system was used and exhaled tidal volume was monitored in the exhalation check valve in the exhalation limb of the anesthesia circuit. Because no audible or visual alarms from the volume monitor were noted at the time of the event, we must assume that the REVERSE FLOW DETECT switch was set in the OFF position. Had this alarm been properly functioning, it would have indicated, immediately and unequivocally, that the cause of the rebreathing was a faulty exhalation valve, thus confirming the mass spectrometry findings. We have now added checking the status of the REVERSE FLOW DETECT switch on the volume monitor to our routine preoperative anesthesia machine checklist. Ninos J. Joseph, BS M. Ramez Salem, MD Adamina G. Podraza, MD Stanley M. Aung, MD Department of Anesthesiology, Illinois Masonic Medical Center, Chicago, IL 60657