RAPID Response to questions from readers

(formerly Dear SIRS)

Reader Questions Why Some Anesthesia Machines Allow O2 Flow Below Basal Metabolic Needs

Dear SIRS:

I am very curious why new machines have a flow that is below “basal metabolic needs.” Here are specifications for a number of different anesthesia machines to make the point:

Kyle Jones, MD
Huntsville, AL

Spacelabs Blease Sirius 150 mL
GE Aestiva 50 mL ± 25 mL
GE Aespire 50 mL ± 25 mL (unless it is a single flow tube then it is 200 mL ± 25 mL)
GE Avance 150 mL
Dräger Fabius & Apollo 0.00 mL (there is an alarm that sounds if you start a case and there is no flow)
Older Machines
Dräger NM GS 150 mL to 200 mL
Dräger NM 2C 100 mL to 200 mL
Dräger NM 2B 100 mL to 200 mL
Dräger 6000 150 mL to 200 mL
Datex Ohmeda Excel and Modulus machines 175 mL to 225 mL


The comments below reflect different manufacturers’ rationale for minimum oxygen flows from the early development of hypoxic mixture guard systems to the currently available anesthesia machines. The goal was to contact those individuals who represented the expertise in this subject from the major anesthesia machine manufacturers starting at the time of the introduction of systems for the prevention of delivery of hypoxic mixtures. Recognize that the experts who contributed these opinions may have moved from one company to another blurring the rationale as presented by each manufacturer.

As indicated below, metabolic oxygen requirements have little to do with a manufacturers’ choice of minimum oxygen flow. Basal oxygen requirements vary over an extraordinary range. For example, Smith’s Anesthesia for Infants and Children1 claims that a neonate’s oxygen consumption may vary from 6 to 10 mL kg-1 min-1 and up to 20 mL kg-1 min-1 during heat stress. An adult typically requires 3 mL kg-1 min-1. The average maximum oxygen consumption during labor was measured to be 6.7 mL kg-1 min-1 with one parturient achieving a maximum oxygen consumption of 10.7 mL kg-1 min-1 during the first stage of labor. The maximum oxygen consumption in this study could be higher if coupled with other disease states. Generalizing, oxygen consumption for any given patient may vary from approximately 6-10 mL of O2 per minute to more than 800 mL O2/minute. Meeting metabolic oxygen needs of an individual patient was never the primary goal of basal oxygen flow.

Manufacturers’ Replies:

Response from Dräger


  • The nature of the minimal oxygen flow features in anesthesia machines originates from the demand to avoid hypoxic mixtures. Taking the typical metabolic rate of an adult, this was translated to a minimum oxygen flow of ~250 mL/minute.
  • With the emergence of Workstations optimized for low flow anesthesia techniques, the minimum oxygen flow feature was sometimes met with pushback by neonatal anesthesia clinicians because of concerns about the resulting higher then desired inspired oxygen concentrations.
  • The Narkomed machines included an “Air only mode” that disabled the minimum oxygen flow. Also, a minimum oxygen flow elimination kit was available.
  • The Fabius GS, Fabius GS Premium, Fabius MRI, Fabius Tiro and Apollo machines use a Sensitive Oxygen Ratio Controller (S-ORC). The fail-safe component shuts off nitrous oxide if the oxygen flow is less than 200 mL/min (Apollo) and 250 mL/min (Fabius), or if the oxygen fresh gas valve is closed.
  • The S-ORC is not active when Air is selected as the carrier gas and 100% Air can be metered throughout the entire flow range, in order to be able to meet the FiO2 requirements of pediatrics and neonates.
  • The standards, ASTM F1850-00 (clause 51.13.1) and EN60601-2-13 (clause 51.102.2), require avoiding oxygen concentrations below 21% (V/V) in the fresh gas measured at the common gas outlet in cases when nitrous oxide is used as carrier gas. A certain minimum oxygen flow is not required by these standards.

Reply from GE Healthcare


  • From our GE perspective the history of minimum O2 flows started when we implemented the Link-25 hypoxic guard (U.S. Patent 4,266,573 filed Oct 1979). Originally it was implemented with a 200 mL/min minimum O2 flow rate simply because it was believed at that time that nobody would want to go below that flow rate.
  • Gradually we began to receive requests to lower this minimum flow, primarily because of the issue of having this constant flow going whenever the machine was turned on.
  • In a few cases lower flows were also requested by clinicians who wanted to practice with a true closed circuit and with very small patients, but this was not common.
  • Initially an option was offered to allow 50 mL/min minimum O2 flows, and this option became popular enough that when the Aestiva and later the Aespire were designed they provided this 50 mL/min minimum flow limit as a standard feature.
  • Eventually we moved on to electronic gas mixing with the introduction of the Avance product. This allows us to automatically turn the O2 flow off when a case ends, and back on again at the start of the next case. Therefore the minimum O2 flow during a case was raised to 150 mL/min, and we found that there were minimal if any complaints that this was too high.
  • When the Aisys product was introduced, the minimum O2 flow rate during a case was raised further to the original limit of 200 mL/min, again with good acceptance.

Reply from Oricare


  1. Minimum O2 Flow (Basal O2 Flow) was added to anesthesia systems as a safety feature given that some users would make the error of turning off the oxygen flow by mistake.
  2. As time went on some customers asked for finer control to allow lower O2 concentration levels to be reached for certain special cases. In Europe it was undesirable to have a minimum oxygen flow in a typical configuration of an anesthesia machine.
  3. The No Minimum O2 Flow configuration also saved on O2 gas supply use when the anesthesia machine sits idle but at the ready.
  4. This led to interest in removing the Minimum O2 Flow feature so the user could totally control the O2 level­—sometimes using Air only, or allowing rebreathing on Air to reach a lower O2 level for specialized cardiac infant surgery and for other special cases.
  5. With a major focus on cost of use, avoiding absorbent desiccation, and to allow for machine use flexibility when machines offer reliable oxygen monitoring systems and alarms, many feel the best current machine design is without minimum O2 flow.

Reply from Spacelabs Healthcare


  1. Older anesthesia systems did not have hypoxic protection built in. There used to be no link between N2O and O2, and the clinician could easily deliver hypoxic mixtures. Oxygen could also be turned completely off.
  2. In order to maintain a hypoxic link properly with a fully pneumatic system (needle valves), setting a minimum O2 flow is necessary. Otherwise the link at low flows becomes impossibly difficult to maintain due to error stack-up within components.
  3. Regarding safety, it makes sense that there would be a minimum O2 flow of about 250 mL/min. However, as closed system anesthesia has been used by some clinicians, low flow has become a competitive issue and manufacturers have gradually lowered their specifications.
  4. One disadvantage of a constant O2 flow when the machine is powered up is that the CO2 absorbent can become desiccated. Some institutions leave their machines on all the time; this can become a problem with carbon monoxide production.3 Again, this has pushed manufacturers to lower their specification for the lowest O2 flow.
  5. Newer electronic mixers permit the O2 flow to be turned off. This could potentially present a problem. Most manufacturers are still enforcing a minimum flow while the system is in clinical use.
  6. The new Spacelabs anesthesia machine has an electronic mixer as offered by other manufacturers. This new machine has a low limit of 100 mL/min for oxygen similar to other manufacturers. In the standby state, the flows may be turned to zero to prevent desiccation of the absorbent.


  1. Davis PJ, Cladis FP, Motoyama EK. Smith’s anesthesia for infants and children, 8th edition. Philadelphia: Elsevier Mosby, 2011.
  2. Eliasson AH, Phillips YY, Stajduhar KC, Carome MA, Cowsar JD. Oxygen consumption and ventilation during normal labor. Chest 1992;102:467-71.
  3. Fang ZX, Eger EI II. Source of toxic CO explained: -CHF2 anesthetic + dry absorbent. APSF Newsletter 1994;9:25,28-30.

The APSF Committee on Technology
The information provided is 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.