“Why would my patient wake up just because I reduced my oxygen flow rate?” At first, this question perplexes many of us who try low flow anesthesia. Later we realize that if our patients don’t get lighter when we reduce our oxygen flow rate, we have likely been over-anesthetizing them in the past. In this four-part series we’ll be taking a closer look at the advantages and disadvantages of low flow anesthesia, and dive deeply into how it all works.
“Quick and Dirty” Calculations
It’s no secret that I am a fan of low flow anesthesia. When your patient is on a circle system, there are some distinct advantages to reducing the oxygen flow rate: patients have a tendency to stay warmer during anesthesia, it helps retain moisture, it’s an efficient use of anesthetic gas, it costs about 90% less than using a non-rebreathing circuit, and it produces about 90% less pollution. Low flow anesthesia yields significant advantages to your patients, your practice, and your environment.
I am careful here to mention low flow anesthesia in the context of a circle system. Low flow techniques cannot be used with a non-rebreathing circuit. You’ll find more information about circuits in my blog post Rebreathing or Non-Rebreathing. That post will help you decide when to use a circle system. It’s more often than you may think. And to clarify further, here are some “quick and dirty” calculations for oxygen flow rates that are generally accepted as “low flow”.
- 35 pounds and under? Set oxygen flow rate at 500ml / min
- Over 35 pounds? Calculate oxygen flow at 30ml / kg / min
- [Hint] You don’t get to 1 Liter flow until you get to a 70 pound Labrador
There’s more information about oxygen flow rates in my blog post Go With The Flow – How to decide the oxygen flow rate for small animal anesthesia.
OK. Let’s do this!
Now that you’ve decided to challenge a long standing more-is-better oxygen philosophy, and you’re ready to try low flow anesthesia, this is a good time to ask about the downsides. In other words (as we’ve all asked all too often), “What could go wrong?”
When you change something as fundamental as the oxygen flow rate, you have to expect that other things may change as well. Here’s a short list of challenges you may see.
- You may scare your DVM.
- Your patient may inspire CO2.
- Your patient may wake up.
Anything out of the ordinary related to anesthesia is going to capture your doctor’s attention. So talk with them before you make changes to the way you normally do things. Explain the advantages and disadvantages of low flow oxygen delivery, and review your understanding of what to expect. Your doctor may also have experience to share with you. It’s a great way to put everyone on the same page.
If you monitor CO2 during anesthesia with a capnograph, you’ll be able to easily detect inspired carbon dioxide by watching the baseline of the waveform during inspiration. Notice how the wave form depicted below does not return to “0” at inspiration, but rather continues to migrate upward. At each inspiration phase the wave form should return to zero. If it doesn’t, the capnograph is detecting carbon dioxide in the inspired air.
There are a few reasons this may happen but for the sake of this conversation, number one on the list is insufficient oxygen flow. And it’s easily resolved: turn up the oxygen flow rate. Since you may be trying low flow anesthesia for the first time, you may want to scrap the whole idea right now and go back to an oxygen flow rate you are more comfortable with. Resist that urge for a little longer. Turn the oxygen flow rate up just a little, and wait for a few breaths. Repeat until the wave form comes back to base line on inspiration. It won’t take long.
If a capnograph is still on your wish list, all is not lost. Hypercapnia (elevated levels of carbon dioxide) has some pretty identifiable signs. If you are trying low flow anesthesia and your patient is experiencing rapid, shallow breathing (tachypnea) and/or red mucous membranes, hypercapnia is a pretty good guess as to the cause. Fortunately, the remedy is the same as if you had a capnograph: slowly turn up the oxygen flow rate until it resolves.
Going back to the first on the short list of challenges you may see (scaring your DVM), keep your doctor in the loop through all of this so everybody stays on the same page.
The third of the three challenges – Your Patient May Wake Up – is due simply to an insufficient amount of anesthetic gas. Over the next three posts in this series we will dive deeply into answering the question, “Why would my patient wake up just because I reduced my oxygen flow rate?” We’ll approach the answer from three directions:
- We’ll look at the anesthetic gas itself
- We’ll look at how a vaporizer works
- We’ll look at the flow of oxygen through a vaporizer
Next up, understanding how to calculate an appropriate dose of a gas – a look at anesthetic gas as a drug.
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.