Fresh Gas Flows, Filters, HMEs and Humidification

I recently submitted a “Letter to the Editor” of the BJA in response to the clever study by Zhong and colleagues demonstrating that the use of high fresh gas flows (FGF) during TIVA produces a cost-saving with, importantly, no adverse environmental consequences. My letter was responding to their conclusion regarding the impact of high FGFs on humidification of inhaled gases, and specifically the issue of heat and moisture exchangers (HMEs).

The letter was edited and shortened significantly during the review process, so much so that I felt some important discussion around a commonly misunderstood device was missing. The published letter is now available here,  but I felt it might be helpful to make the full text of the original letter (with some minor corrections) publicly available, so I have presented it below.

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A Purple Patch for a Power Problem

Part of my job as an Anaesthesia Quality and Safety Fellow is to monitor reports made though our Anaesthesia Safety Monitoring Project, which allows staff to report latent safety threats, near-misses, faulty or badly designed equipment and pretty much anything that might pose a threat to patient safety. Below is the text of a “Letter to the Editor” that I wrote explaining how we were able to quickly and cheaply respond to an issue highlighted by one of these reports. The letter was published in the Canadian Journal of Anesthesia, and it can be accessed here (login required) or viewed online here by anyone.

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Valvular Insufficiency

Take a look at the image below and see if you can spot the differences between these two IV giving sets:

Maybe the packaging will help?

How did you go? There are actually a few differences between them, but there is one in particular that I am interested in, and you should be too. Can you guess which one it is? Yes, that little white bit circled in red is what’s called a “one-way”, “back check” or just “check” valve. As the name suggests it ensures fluid in the line can only move one-way i.e. from the IV fluid bag and into the patient. This valve is there so that if you are running a secondary infusion it will flow into the patient, rather than back up the primary line and into the fluid bag. A visual representation of what this valve does can be seen in this video:

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Breathing System Filters

The following is a report that I wrote for our Anaesthesia Equipment Committee at John Hunter Hospital in June 2017, after concerns were raised about differences in performance of different filter types, and the possible cross-contamination risks resulting from this.


It has long been known that breathing circuits are a potential source of cross infection between patients . Of particular concern are hepatitis C and tuberculosis .  Eliminating this risk altogether seems unlikely, but there is ongoing debate about how best to reduce the risk, while also being conscious of financial and environmental concerns . The use of breathing system filters (BSF) is one approach that has tried to address some of the concerns, and the presumption that they protect the anaesthetic circuit from contamination by the patient and vice versa has led to the widespread adoption of BSFs as a way to reduce cost and waste by facilitating the reuse of the same circuit for multiple patients . However the practice is not universal , and no BSF is 100% effective at preventing contamination . Furthermore, as with any piece of medical equipment, the risks associated with the use of BSFs need to be considered along with their benefits . Many manufacturers now produce circuits  that are approved for use for up to 1 week as long as a new BSF is used for each patient . This is the current policy at John Hunter, with the proviso that the circuit is changed immediately if it is visibly soiled, has been used for a patient with a multidrug-resistant organism, or is excessively wet. If we assume this practice will continue, then the question that needs to be answered is which filter will best protect the circuit from contamination, and that question is the main focus of this report.

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