Circulation 118,032 • Volume 30, No. 2 • October 2015   Issue PDF

Medical Air Supply Causes Rapid Inflow of Water Into Flow Meters

Alvaro Segura-Vasi, MD; A. William Paulsen, PhD, AA-C

Dear Q&A,

We were using oxygen and air with sevoflurane for a case in the cystoscopy suite. During the maintenance phase of the case, we found what we assumed to be water inside the air flowmeter, with its float rising and dropping rapidly and the liquid spilling into the next flowmeter (nitrous oxide). We promptly turned off the air flow. We went through a machine inspection and were prepared to take patient off the machine at the first sign of fluid getting into the oxygen flowmeter. We disconnected the air pipeline supply and the connectors were discovered to be wet. We immediately contacted plant operations, and they informed us that there was a failure of the dehumidifier in the medical air pipeline system. We then proceeded to disconnect all of the machines from the air pipeline. There were no alarms for this problem. We disconnected the air supply before any of the other machines could be affected. Fortunately, there were no clinical issues with the patient.

I think there is something to be said about the safety of being able to see your flowmeters—as opposed to the digital interface in the newer anesthesia machines.

As a corollary, fixing the machine will cost about $8,000 and it will be out of service for an unknown period of time. It took a week to fix the pipeline air supply problem. Luckily, no harm came to patients out of this event.

Sincerely, Alvaro Segura-Vasi, MD Florence, Alabama

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Rapid inflow of water into air and nitrous oxide flow meters caused by water in medical air supply. Water rushing from medical air supply.

Dear Dr. Segura-Vasi,

This is a problem that is more common than one might realize. The last letter that we received about this same problem was approximately 5 months ago. When hospitals create their own medical air, there are multiple sources of water that can ultimately end up in the air pipeline supply. First, the use of oil-less compressors that may have water seals (rather than hydrocarbon lubricants) to help maintain purity standards for medical air can introduce water into the medical compressed air. Another source of water is the humidity of the outside ambient air. In Alabama, the humidity is relatively high and the water in the air that is taken into the compressor introduces a great deal of water into the medical air system. That water is typically eliminated by a dryer located within the system. There are also dew point sensors scattered throughout the air pipeline system to electronically monitor the water content of the medical air system to prevent the situation that you described. There are several potential points of failure, but the most common that causes the situation you described is the failure of the dryer. There is a very good review article on Medical Air that can be found in the summer 1996 issue of the APSF Newsletter, which can be accessed at

Your approach to this problem was exemplary. Turning off the air flowmeter and disconnecting the hose to the air pipeline is imperative to prevent water from entering other flowmeter tubes. The machine you described has the air flowmeter on the left side of the flowmeter bank with nitrous oxide as the second flow tube and oxygen on the far right. You mentioned that water was flowing into the nitrous flow tubes. Some machines have the air flowmeter in the middle, in which case the water would have entered the oxygen flowmeter. Potential failure of the rotameters due to water or residual debris requires that respiratory gas analysis be used for monitoring gas concentrations. Calling the anesthesia machine manufacturer and asking them to evaluate and repair the anesthesia machine before it can be used again is vital for subsequent safe use of the machine. Machines that were disconnected and thought to be contaminant-free should be checked for proper needle valve and flow tube accuracy before their next use. All anesthesia machines deemed safe for use should use air tanks (E cylinders) until Plant Operations informs you that the pipeline is dry and safe to use, and that the dryers are fully functional and providing the proper dew point.

You also made a very important point when you said, “I think there is something to be said about the safety of being able to see your flowmeters—as opposed to the digital interface in the newer anesthesia machines.” Again, the importance of a respiratory gas analyzer cannot be overstated, especially when you don’t know what the electronic flow controllers and measuring devices will do under such circumstances.

A. William Paulsen, PhD, AA-C Chair, APSF Committee on Technology

The APSF sometimes receives questions that are not suitable for the Dear SIRS column. This Q and A column allows the APSF to forward these questions to knowledgeable committee members or designated consultants. 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 the APSF. It is not the intention of the 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 the 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.