Circulation 37,100 • Volume 19, No. 2 • Summer 2004   Issue PDF

O2 Blender Causes Concern

Randy Ralston, MD; Tom Green

Is it fair to assume that a gas outlet fitting indexed to the oxygen diameter index safety system (DISS) should guarantee that 100% high pressure oxygen is indeed delivered at its output? Is it acceptable that an air-oxygen blender output is indexed to this oxygen standard, yet 21% oxygen can be delivered by this connection?

I ask these questions because we have recently had an incident at my hospital in Kettering, Ohio, where an anesthesia machine was connected to such a blender via a standard oxygen hose. This happened in an MRI unit that recently acquired an anesthesia machine to initiate the provision of general anesthesia. The blender was set at 21%. Of course, the low oxygen alarm within the anesthesia machine sounded shortly after the start of the anesthetic, and fairly quickly the problem was discovered.

I’m alarmed that the position statement of other departmental administrators is that we just pay attention to what we hook up (granted). They see no concern about the bigger issue of why the DISS is not protecting us against the delivery of unintended gas. Have the manufacturers of oxygen blenders subverted the system, or is this just a glitch in the standard? Should this go out as a sentinel event? I’d appreciate your input.

Randy Ralston, MD
Chief of Anesthesia
Kettering Memorial Hospital
Kettering, OH

Editor’s Note: Responses from several blender distributors acknowledged that these blenders do have male DISS oxygen outlet fittings. Below we reproduce a particularly helpful response from Mr. Tom Green of Paragon Service, a distributor of oxygen blenders. He speaks in reference to his particular SmartBlend blender, but the points are valid for this discussion.


  1. Fittings on anesthesia machines and blenders are diameter index safety system (DISS). On the rear of an anesthesia machine or blender are fittings specified to medical air, oxygen, etc. All anesthesia machines have a male DISS on the rear. Our SmartBlend blender has a male air DISS and a female oxygen DISS. Crossing gas connections would be difficult. The opposite end of our hoses would also be specific to air or oxygen.
  2. The SmartBlend uses pipeline medical air, not room air. The setting on the SmartBlend is dialed in to 21-100% oxygen output. An integral digital oxygen monitor (with high and low alarms) is part of the unit. Should there be no air or oxygen available into the unit, it can only give either 21% or 100% oxygen, and will be confirmed by the oxygen monitor.
  3. The outlet fittings on the SmartBlend are Oxygen DISS. We have attached a 1-15 LPM oxygen flowmeter. We could and would be willing to install an air flowmeter. There are no flowmeters available for air/oxygen mixture. I was told that “enriched” air is considered oxygen, which may be debatable and arguable. We will supply either at the customer’s request. There are air flowmeters available with DISS fittings. We remove the male oxygen DISS outlet fitting and put a male/male 1/8” NPT fitting into the flowmeter.

Subsequent information from Tom Green follows:

The issue that has arisen is which fittings/connectors should be used on a blender. The factory fittings are oxygen male DISS with check valve. We removed one of the oxygen DISS fittings and installed an oxygen flowmeter. I had a discussion with Bruce Brierley, President of Maxtec (Salt Lake City, UT), who said that an enriched (>21%) mixture is considered oxygen in the respiratory therapy environment.

The opposite could be argued. We could install an air flowmeter, but air is 21%. So that is not completely accurate either. Since there is not a flowmeter for a mixture of 21-100% oxygen, one of the two needs to be chosen. Both are correct, and both are wrong. We have chosen the oxygen flowmeter for the SmartBlend at this time.

I believe the much more important discussion needs to revolve around the SmartBlend itself and the safety features of the SmartBlend. It is the first device that allows anesthesia providers the capability of delivering an auxiliary mixture of 21-100% oxygen of their choosing (e.g., during MAC with a nasal cannula). Currently, most use only the 100% auxiliary oxygen flowmeter attached to the anesthesia machine. One hundred percent oxygen is not required for most patients via a nasal cannula and is very hazardous in case of a fire.

Tom Green
Paragon Service

Editor’s Note: The APSF Newsletter and the APSF Committee on Technology wish to solicit input from readers, specifically asking if others have had similar problems, experiences, or concerns. Do you think we need a new standard DISS fitting, and thus new blended flowmeters for oxygen/air mixtures?

Oxygen blender showing Oxygen DISS outlet fitting (left), and machine mounted blender showing auxilliary flowmeter (right).


The following information may be useful in the discussion regarding outlet fittings on air/oxygen blenders. You may want to preface the information by indicating it comes from the world’s largest manufacturer and original patent holder of medical gas blenders, Bird Products Corporation, a subsidiary of VIASYS Health care.

Air/oxygen blenders were developed more than 30 years ago for use predominantly in conjunction with mechanical respirators. When developed, the decision to utilize oxygen DISS outlets seemed logical based on the fact that the gas being delivered from the blender would, in most cases, have a higher concentration than 21%. Since there is no DISS standard for a “mixed” gas, it was reasoned that using an air fitting would be more misleading than using the oxygen fitting, and thus a standard was established.

The oxygen DISS outlet fitting is used on all medical gas blenders worldwide, including blenders used for nitrous oxide/oxygen mixing. With an estimated installed base of more than 250,000 gas blenders in the respiratory care market, the use of the oxygen DISS fitting has become accepted as the standard and is used by all known blender manufacturers.

Kris Ukkestad
Director, Sales Operations
VIASYS Critical Care
Palm Springs, CA



The Smartblend system incorporates the use of the Maxblend low flow blender which has a 0-30 LPM maximum flow range. Since the gas machine O2 inlet requires 50 psi, it would have warnings that would indicate to the user that the O2 supply is not adequate.

Therefore, I would recommend to people interested in blenders for anesthesia that low flow is preferred.

The reasons a clinician would want to have a blending device in the OR are

  1. The ability to give less than 100% oxygen easily through an auxillary flowmeter, which is important for neonates or fire prevention protocols for some procedures.
  2. Perfusion equipment.

Let me know if I can provide anything else for you.

Bruce Brierley
MAXTec, Inc.

Editor’s Note: While these letters make several valid and important points, it is important to NOT withhold supplemental oxygen from those patients for whom it is indicated by clinical judgment. The APSF Newsletter invites readers’ thoughts and comments on this topic.