Episode #167 Low Flow Anesthesia Revisited

September 12, 2023

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Welcome to the next installment of the Anesthesia Patient Safety podcast hosted by Alli Bechtel.  This podcast will be an exciting journey towards improved anesthesia patient safety.

Have you taken the APSF Technology Education Initiative Low Flow Anesthesia Course yet? The APSF Low-Flow Anesthesia Course is available on the American Society of Anesthesiologists website and includes 8 topics that will provide anesthesia professionals with knowledge to safely and effectively reduce anesthetic waste and pollution by using low-flows during anesthesia care.

We are turning the volume up and the flows down today as we return to Episode #110, “How Low Can You Go? Keeping Patients Safe During Low-Flow Anesthesia Care” for a review on how to keep patients safe during low-flow anesthesia. Our featured article today is “Patient Safety and Low-Flow Anesthesia” by Jeffrey Feldman and Samsun Lampotang from the June 2022 APSF Newsletter.

An important resource for implementing low flow anesthesia as part of your practice is the 2012 article by Jeffrey Feldman published in Anesthesia and Analgesia, “Managing fresh gas flow to reduce environmental contamination.” In the article, Feldman reviews steps for reducing fresh gas flows during induction, maintenance, and emergence while monitoring oxygen and anesthetic gas concentrations closely in order to reduce waste with the use of a circle anesthesia system and keep patients safe. Here is the citation:

  • Feldman JM. Managing fresh gas flow to reduce environmental contamination. Anesth Analg. 2012 May;114(5):1093-101. doi: 10.1213/ANE.0b013e31824eee0d. Epub 2012 Mar 13. PMID: 22415533.

Stay tuned for more information about the APSF’s new initiative on medical technology education. For more information, check out this article from the June 2022 APSF Newsletter.


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© 2023, The Anesthesia Patient Safety Foundation

Hello and welcome back to the Anesthesia Patient Safety Podcast.  My name is Alli Bechtel, and I am your host. Thank you for joining us for another show. One of our most popular episodes of all time is Episode #110, “How Low Can You Go? Keeping Patients Safe During Low-Flow Anesthesia Care? We are going to revisit that show today and talk about the new APSF Technology Education Initiative: Low Flow Anesthesia.

Before we dive into the episode today, we’d like to recognize Fresenius Kabi, a major corporate supporter of APSF. Fresenius Kabi has generously provided unrestricted support to further our vision that “no one shall be harmed by anesthesia care”. Thank you, Fresenius Kabi – we wouldn’t be able to do all that we do without you!”

Have you taken the Low Flow Anesthesia Course yet? This Low-Flow Anesthesia Course is available on the American Society of Anesthesiologists website and includes 8 topics that will provide anesthesia professionals with knowledge to safely and effectively reduce anesthetic waste and pollution by using low-flows during anesthesia care. Here is a description of the course overview:

“Using guided, simulation-based learning, basic concepts of low-flow anesthesia are explored along with strategies for reducing fresh gas flow during the induction, maintenance and emergence phases of inhalation anesthesia. Each topic is designed for about a 15-minute interaction but the motivated learner is encouraged to utilize the simulation platform to explore different strategies and reinforce the learning. The topics are designed to be followed in sequence but do not need to be done at the same time. The course is free of charge to all anesthesia professionals but a guest login is required for non-ASA members. Continuing education credits are available to physicians, nurses and anesthesia assistants. For physicians enrolled in the MOCA process, the CME credits are patient safety eligible.”

This is a great opportunity for anesthesia professionals around the world to learn more about low flow anesthesia, build this into your practice, and gain continuing education credits. I will include a link to the course information in the show notes as well.

To get even more excited about low-flow anesthesia, we are going to revisit our show from August 2022 with the featured article, “Patient Safety and Low-Flow Anesthesia” by Jeffrey Feldman and Samsun Lampotang from the June 2022 APSF Newsletter. To follow along with us, head over to APSF.org and click on the Newsletter heading. Fifth one down is Newsletter archives. From here, scroll down until you get to June 2022. Then, scroll down until you get to our featured article today. I will include a link in the show notes as well.

Let’s take a listen now:

[Episode #110]

“And now, let’s get into the APSF article. The author’s open with a description of the circle system use in anesthesia care which was designed to reduce waste since the exhaled anesthetic gases remain in the system and return to the patient in the inspired gases. There is a reduction in anesthetic waste when the fresh gas flows remain low enough to that the exhaled gases are able to return to the patient. Keep in mind that circle systems depend on carbon dioxide absorption with the use of CO2 absorbents. Check out Figure 1 in the article for a representation of the circle system.

So, what exactly is low-flow anesthesia? Is it when the total fresh gas flows are at 1L/min? The authors remind us that low-flow anesthesia depends on the clinical situation since at times 1L/min may be too high to prevent excess waste reduction or may be too low leading to delivery of an inadequate concentration of oxygen or anesthetic to the patient. Here is the author’s definition of low-flow anesthesia. Get your pencils ready!

Low-flow anesthesia involves “Reducing fresh gas flow below minute ventilation to the lowest level consistent with equipment capabilities and provider comfort while ensuring safe and effective care for the patient.”

In order to provide safe low-flow anesthesia it is important to ensure adequate oxygen delivery with an inspired oxygen concentration that is able to provide and maintain the desired concentration of oxygen in the blood. Another important component is the delivery of an adequate anesthetic concentration to provide an appropriate level of hypnosis while maintaining physiologic stability during surgical procedures. This level changes depending on the patient and surgical procedure. When you turn down the dial of the fresh gas flows, there is increased rebreathing which results in a difference in the concentrations of oxygen and anesthetic gas delivered in the fresh gas and the concentrations that are inspired by the patient. In addition, during low-flow anesthesia changes in the gas and anesthetic agent concentrations take more time compared to when the fresh gas flows are higher. This is an important point especially when you need to make changes to the oxygen and anesthetic concentrations that are delivered to the patient. There is definitely an art to provide low-flow anesthesia care. And what about the carbon dioxide concentrations? There is no change in carbon dioxide concentrations as a result of changes in the fresh gas flow as this concentration depends on the minute ventilation.

Next, we are going to discuss how to make sure that you are delivering an adequate concentration of oxygen during low-flow anesthesia. Have you ever turned down the fresh gas flows without making changes to the inspired concentration of oxygen? What happened? There is a risk for hypoxemia and inadvertent low inspired concentration of oxygen with low flow anesthesia. Anesthesia professionals need to be aware of this risk. Remember, the concentration of expired oxygen is lower than the inspired oxygen concentration as a result of the patient’s oxygen consumption. With increased percentage of rebreathing gas, there is a bigger effect of the exhaled gas concentration of oxygen on the inspired concentration of oxygen. Any leaks in the system and the patient’s oxygen consumption will affect the inspired oxygen concentration. Thus, it is critical to continuously monitor inspired oxygen concentration during low-flow anesthesia and adjust the delivered oxygen concentration to maintain an adequate inspired oxygen concentration. Remember, changes in FiO2 occur slowly with low flows, so it is important to set a low oxygen concentration alarm above the minimum safe level to have time to make the appropriate changes in the delivered oxygen concentration and the fresh gas flow.

We are going to tackle management of inspired anesthetic agent concentration next. This is a little more complicated. While the patient’s oxygen consumption during the procedure remains pretty constant, this is not the case for the uptake of anesthetic agents during the procedure. Instead, there is an exponential decrease in uptake, and we will need to account for this during low-flow anesthesia. The expired concentration of anesthetic agent will always be less than the inspired concentration due to uptake until emergence. This difference is the greatest at the beginning of the procedure when the uptake of anesthetic agent is high. This is why higher fresh gas flows may be used during induction to maintain the desired anesthetic concentration. During maintenance of anesthesia, the uptake has decreased and there is less of an impact of the expired concentration of anesthetic agent on the inspired concentration. Once again, monitoring is required to provide safe anesthesia care with continuous monitoring of the inspired and expired anesthetic agent concentrations. Uptake of anesthetic agents can be figured out by looking at the difference between the inspired and expired anesthetic agent concentration. A good time to decrease fresh gas flows is when you see the difference narrow representing decreased uptake and then it will be easier to maintain an adequate anesthetic concentration in the circuit with the lower flows. The delivered agent concentration depends on the vaporizer setting. The inspired agent concentration is the concentration of agent that is inspired by the patient. During low flow anesthesia, it may be necessary to set the vaporizer setting higher than the desired minimum alveolar concentration to ensure delivery of adequate anesthetic agent to the patient. The practice of low-flow anesthesia requires the anesthesia professional to monitor the anesthetic agent closely and make changes when necessary to the vaporizer setting and fresh gas flows.

Now, that we have discussed considerations for managing the inspired oxygen concentration and inspired anesthetic concentration, let’s turn our attention to the management of fresh gas flows during low-flow anesthesia.  Changing fresh gas flows has a significant impact on the rate of change of oxygen and anesthetic agent concentrations in the circuit. We are going to get technical for a minute so grab your pencils and notepads. The time constant for the rate of change equals the internal volume of the anesthesia machine and the breathing circuit in liters divided by the fresh gas flows in liters per minute. Now, let’s fill in some numbers for these variables. If the internal volume is 5 liters (which it may be or even more) with fresh gas flows of 1L/min, then the time constant equals 5 minutes and it can take about 4 time constants (or 20 minutes) to get close to equilibrium. As you turn down the fresh gas flows, the concentrations of oxygen and anesthetic will change slower to reach a new equilibrium and any changes to the gas mixture or vaporizer setting may be seen only after several minutes. Continuous monitoring of oxygen and agent concentrations in the circuit with appropriately set alarms is important to make sure that these slow changes are noticed when necessary. Plus, when you need changes to happen quickly, you can increase the fresh gas flows. If we go back to our equation with 5 liters internal volume and fresh gas flows of 5L/min, the time constant is reduced to only 1 minute.

We have been talking all about low-flow anesthesia. If you remember from our podcast shows on sustainable healthcare, avoiding or minimizing the use of nitrous oxide and desflurane is important for reducing greenhouse gases. So, can we do low-flow anesthesia with Sevoflurane and is there a minimum safe fresh gas flow? The package insert for Sevoflurane states that fresh gas flow should be not less than 1 L/min for up to 2 MAC-hours and at least 2L/min for longer procedures. Whoa, wait a minute?! How can we do low-flow anesthesia given this recommendation? There are many practicing anesthesia professionals who may be hesitant to turn down the flows in order to adhere to the FDA recommendation. Using low-flow anesthesia and sevoflurane would be considered an off-label use. We are going to hear from a lawyer Brian Thomas who is the vice-president for Risk Management, Preferred Physicians Medical when we review his article from the June 2022 APSF Newsletter, “Off-label Low-Flow Sevoflurane: Regulatory Red Herring or Liability Landmine?” So stay tuned!

Today, we will continue with our featured article as the authors review the science and the safety for using lower flows. The lower limits for fresh gas flows when using sevoflurane were set due to the concern for the accumulation of Compound A and the potential for renal toxicity. Compound A may be formed from the combination of sevoflurane with some absorbent formulations, but renal toxicity has never occurred in humans.  If we look a little closer at the science behind the formation of Compound A, we can see that the formation of Compound A depends on the interaction of sevoflurane with specific absorbents, those with strong bases including potassium hydroxide and sodium hydroxide. Absorbents that do not contain potassium hydroxide and have less than 2% sodium hydroxide do not interact with Sevoflurane to form compound A. It is not even produced so there would be no reason to maintain fresh gas flows above a certain level. Check out the safety data sheet for the absorbents that are used at your institution. What is the chemical composition? There are several effective carbon dioxide absorbents that have less than 2% sodium hydroxide available. Check out Figure 2 in the article. It reveals the medical safety data sheet for Dragersorb Free which has a sodium hydroxide concentration of 0.5-2% and would not be able to form Compound A following interaction with Sevoflurane. As long as you are carefully monitor oxygen concentration and anesthetic agent concentration, you do not need to maintain fresh gas flows above a set lower limit of 1-2L./min when using sevoflurane.

As we wrap up for today, I’m going to read the author’s conclusion now: “The practice of safe and effective low-flow anesthesia is an art that requires the practitioner to understand the capabilities and limitations of the circle system, set fresh gas flow and vaporizer concentrations to estimate patient needs, and continuously monitor the concentrations that result in the circuit.”

We hope you enjoyed this Revisited Episode. If you have any questions or comments from today’s show, please email us at [email protected]. Please keep in mind that the information in this show is provided for informational purposes only and does not constitute medical or legal advice. We hope that you will visit APSF.org for detailed information and check out the show notes for links to all the topics we discussed today.

We hope you will check out Episode #111 of the Anesthesia Patient Safety Podcast when we headed into the court room to discuss FDA regulations, off-label use, informed consent and liability for anesthesia professionals. If you are hesitant to turn down the flows when using Sevoflurane due to the FDA recommendations, then you do not want to miss this show.

For even more resources dedicated to low-flow anesthesia, head over to APSF.org and click on the Patient Safety Resources. The sixth one down is the APSF technology education initiative, then click on low-flow anesthesia and scroll down until you get to the additional resources section. There are some great articles as well as manufacturer information to check out while you are waiting for the next podcast episode to drop.

Until next time, stay vigilant so that no one shall be harmed by anesthesia care.

© 2023, The Anesthesia Patient Safety Foundation