Episode #110 How Low Can You Go? Keeping Patients Safe During Low-Flow Anesthesia Care

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. We are continuing our journey through the June 2022 APSF Newsletter. There are so many great topics to cover. What has been your favorite article from the most recent issue?

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

Today, we are turning up the dial on keeping our patients safe while we turn down the dial and discuss 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. To follow along with us, head over to APSF.org and click on the Newsletter heading. First one down is the current issue. Then scroll down until you see our featured article today. I will include a link in the show notes as well.

Do you use low-flow anesthesia routinely? Is this a common practice at your institution. There are quite a few benefits of low-flow anesthesia that include decreased waste of inhaled anesthetics, decreased cost, and less greenhouse gas effects. The reduction in greenhouse gas effects is something that we talked about with Meyer and MacBride during our 3-part interview show on sustainable healthcare. This was episodes 102, 103, and 104 and we hope that you check them out. Other benefits of low-flow anesthesia for the patient include a reduction in heat and humidity loss from the lungs. These benefits are important, but before turning the dial way down, it is important to understand the risks and safety concerns. The APSF is working on an APSF-ASA medical technology training initiative related to low flow anesthesia so keep an eye out for it in the future. 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. I will include the citation in the show notes as well.

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. Figure 2 in the article 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.”

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.

Are you team #lowflowanesthesia while continuously monitoring your patients to keep them safe during anesthesia care? Let us know on twitter by tagging us @APSForg.  Plus, if you are interested in learning more about reducing waste and pollution while using inhaled anesthetics, we hope that you will check out the APSF-ASA course on low-flow anesthesia that will be available on the APSF website this fall.

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

© 2022, The Anesthesia Patient Safety Foundation