Low Flow Anesthesia & Patients Waking Up – Part 3

For Part 2 in this series, click on this link.

“Why would my patient wake up just because I reduced my oxygen flow rate?”

In this series we dive deeply into answering that question. We’re approaching the answers from three directions:

1. We looked at the anesthetic gas itself in Part 2
2. We’ll look at how a vaporizer works in this post
3. We’ll look at the flow of oxygen through a vaporizer in Part 4

In this post we explore the role of the vaporizer, and how it turns a liquid into a precise concentration of gas. The big concepts of the process are pretty simple to understand if you don’t get too tied up in the details. The picture below is used by one of our engineers to explain how vaporizers work. We’ll use it to follow the flow of oxygen through a vaporizer and see how it picks up anesthetic.

Wow. So many colored arrows going every which way. Let’s break it down and follow just the oxygen. First we’ll look at just the top half of the vaporizer.

I’ve stripped away everything but the oxygen flow in this top-half view. I have highlighted the “Concentration Control Dial” label and showed what turning the setting to OFF will do to the oxygen flow. Notice the oxygen just flows straight through the vaporizer with no oxygen diverted into the lower half of the vaporizer.

In this view, everything is the same except the highlighted “Concentration Control Dial” is turned ON. Notice a percentage of the flow is diverted into the lower half of the vaporizer, while the majority of the flow still passes directly through and out of the vaporizer. Now let’s slide the image a little lower and follow the percentage of the total flow of oxygen that was diverted into the bottom of the vaporizer.

The dial at the top of the vaporizer – the one we use to set our desired percentage of anesthetic gas – actually diverts a percentage of the oxygen flow downward, through the wick and into the sump holding the liquid anesthetic. There, it becomes saturated with volatilized anesthetic and travels through baffles and up the vaporizer to rejoin the main stream oxygen flow.

In this view we see the saturated oxygen moving through baffles and up through the wick to rejoin the main flow. Once it merges with the main flow of oxygen, the combined gases flow out of the vaporizer to the breathing circuit. The percentage of the flow of oxygen that was diverted from the mainstream and into the sump of the vaporizer has now been saturated with anesthetic gas. When it recombines with the main flow, that diverted percentage becomes the percent of anesthetic gas you set with the vaporizer dial. In other words, if you set the vaporizer dial at 2%, then 2% of the main oxygen flow is diverted down into the vaporizer, through the wick, into the sump where it’s saturated with anesthetic gas, and back up to rejoin the main oxygen flow. As it mixes into the main flow, it changes the concentration of the gas out-flowing from the vaporizer. The mixed gas leaving the vaporizer is now 98% oxygen and 2% anesthetic gas.

It’s interesting to learn the general principles of how a precision vaporizer works, but the question we’re trying to answer is why a patient might wake up simply because you turn down your oxygen flow rate. The most important message to take from this section is that all of this concerns a percentage of the total oxygen flow. In the final part of this four-part series we’ll look at the role that oxygen flow rate plays on anesthetic delivery, and how 2% of one oxygen flow rate does not equal the anesthetic delivery of 2% of another oxygen flow rate.

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Ken Crump AAS, AHT is a writer and animal anesthetist, and writes Making Anesthesia Easier for DarvallVet, a division of Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary anesthesia and oncology across the United States and in Canada.  Ken retired from the Veterinary Teaching Hospital at Colorado State University in 2008.

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