Dear SIRS refers to the Safety Information Response System. The purpose of this column is to allow expeditious communication of technology-related safety concerns raised by our readers, with input and responses from manufacturers and industry representatives. This process was developed by Drs. Michael Olympio, Chair of the Committee on Technology, and Robert Morell, Editor of this newsletter. Dr. Olympio is overseeing the column and coordinating the readers' inquiries and the responses from industry. Dear SIRS made its debut in the Spring 2004 issue.

The information in this column is provided for safety-related educational purposes only, and does not constitute medical or legal advice. Individual or group responses are only commentary, provided for purposes of education or discussion, and are neither statements of advice nor the opinions of APSF. It is not the intention of APSF to provide specific medical or legal advice or to endorse any specific views or recommendations in response to the inquiries posted. In no event shall APSF be responsible or liable, directly or indirectly, for any damage or loss caused or alleged to be caused by or in connection with the reliance on any such information.

Go With the Flow (of O₂)

Dear SIRS:

A free-standing ASC where I work has recently begun using the Datascope AS 3000 gas machines. I am concerned about the gas flow controls.

I am concerned that when one turns on nitrous oxide, both nitrous oxide and oxygen flows increase. When N₂O flow is decreased, the O₂ flow correspondingly decreases to its original setting.

However, when I perform a pediatric general inhalational induction, I begin with N₂O and O₂ in a 70-30 admixture. If the N₂O is turned on first, the O₂ flow rises. When the N₂O flow is decreased, so too is the O₂ flow. If both gases begin at 0 L/m, decreasing N₂O flow will reduce O₂ flow to 0 L/m. To illustrate, N₂O flow is increased to 7 L/m and O₂ flow rises to about 4 L/m. Using sevoflurane, when an adequate level of anesthesia is attained, flows are adjusted to a 50-50 admixture. This results in O₂ flow decreasing to near zero, or 200 ml/min. This necessitates resetting the O₂ flow to equal that of N₂O.

Yesterday, in the pediatric ENT room, this questionable mechanism resulted in no fewer than 8 flow control knob adjustments. I feel it is inherently unsafe that O₂ flow can decrease to near zero independent of someone physically turning the flow control knob. Where is the reciprocating gear mechanism that links the flow control knobs in this system? It is as if the gear system is reversed.

Again, I feel that oxygen flow should only decrease when one physically turns the O₂ flow control knob. I feel this apparatus is minimally safe and maximally impractical. Further, I feel that these gas flow control knob adjustments greatly increase the workload in an already busy setting.

Thank you so much for your time and attention in this matter.
Sincerely,
Shayne Miller, CRNA
Abilene, TX

In Response:

Datascope Patient Monitoring welcomes the opportunity to discuss the differences in the AS3000’s implementation of the O₂/ N₂O ratio device. Anesthesia machine brands on the U.S. market today have subtle differences in their designs, and all were determined to be safe and effective devices.

The AS3000 Anesthesia System incorporates an O₂/N₂O ratio device that assures that at least 21% O₂ is supplied in the Fresh Gas mixture when N₂O is flowing.

The AS3000 design allows a user who first adjusts N₂O flow to see the O₂ flow rise up to an appropriate level. The AS3000 accomplishes this via a gear drive system that is activated when the user turns the N₂O flow control. Competitive machines commonly use either a pneumatic control system that prevents N₂O flow until sufficient O₂ flow is present or a linked gear system that controls the O₂/N₂O ratio. The gear system in the AS3000 is designed to maintain a safe O₂/N₂O ratio by adding O₂ flow as the user increases the N₂O flow, not to maintain O₂ flow at the highest flow level achieved when N₂O is decreased.

When the user is adjusting N₂O and O₂ flow rates, redundant steps can be avoided by always adjusting the O₂ flow after adjusting N₂O flow if changing the desired ratio. This will assure that the desired O₂ flow is achieved. We see this focus on the O₂ flow control as being consistent with the clinician’s primary desire to always provide appropriate O₂ flow to the patient.

Datascope Patient Monitoring frequently reviews its AS3000 Operating Instructions and In-service training materials. We will use this as an opportunity to make sure that we are informing our new users to always adjust the O₂ flow after adjusting N₂O flow.

We thank the reader and look forward to input from the clinician community that helps us continuously improve the AS3000.

Sincerely,
David T. Jamison
Director of Engineering, Anesthesia Systems
Datascope Patient Monitoring
Mahwah, NJ, USA