Remembering the Classics: The Art of Low Flow Anesthesia

Jeffrey M. Feldman MD, MS, Justn Lockman MD MSEd, Myron Yaster MD

OK, I’ve (MY) got to admit it… I have always loved low-flow and closed-circuit anesthesia. It kept me focused and in the moment. In my own hands, this meant just enough oxygen to cover oxygen consumption (around 200-300 mL/min/m2 or 3-5 mL/kg/minute) and just enough vapor to maintain end-tidal sevoflurane (or in days of yore, halothane) at MAC.  I would let patients breathe spontaneously (with or without pressure support), letting the end-tidal CO2 rise to the low 50s.  This is an ideal way to teach residents and fellows the mechanics of the circle system, the CO2 absorber, oxygen consumption, CO2 elimination, uptake and distribution of anesthetic gasses, and the physiology of the control of ventilation.  The technique requires incredible vigilance and attention to detail to keep it safe.  Indeed, the only real problem with it was no one would ever relieve me or my resident/fellow for breaks – or if they did they would immediately turn up the flows to “normal”! 

Today there is another, perhaps even greater, reason to use low-flow anesthesia: our volatile anesthetics are TERRIBLE for the environment, and we should not carelessly waste them.  To help keep the technique alive and to teach a new generation about how to help save the world (or at least the environment), we asked Jeff Feldman of Children’s Hospital of Philadelphia to review his classic paper on “Managing fresh gas flow to reduce environmental contamination” as a “remembering the classics” for the PAAD readership.  Also perhaps of interest to PAAD readers, the SPA meeting in Tampa this week will feature a table hosted by the Sustainability Special Interest Group – please stop by and learn about what else you can be doing to help the environment while you work! Myron Yaster MD and Justin Lockman MD MSEd

Original article

Jeffrey M Feldman. Managing fresh gas flow to reduce environmental contamination. Anesth Analg. 2012 May;114(5):1093-101. PMID: 2241553

In 2012, I (JMF) wrote an article entitled: “Managing Fresh Gas Flow to Reduce Environmental Contamination.”1  Ten years later, the topic of low-flow anesthesia continues to be prevalent in the anesthesia literature, if not increasingly visible given the conversation on climate change and well-established data on the green house impact of anesthetic gases.2,3  The title of my article was chosen in part to emphasize the primary role of the bedside clinician for controlling anesthetic waste by managing fresh gas flow.  An Ovid search revealed 28 citations for the article, but there is no readily available evidence to understand if the content had any meaningful impact on practice, or pollution for that matter.  While the recommendations in the article remain valid and relevant for modern practice, despite being a full-time pediatric anesthesiologist, (mea culpa) the content of that article did not address some of the aspects of low flow anesthesia unique to pediatric anesthesia.  The PAAD format and the targeted pediatric anesthesia audience provides an opportunity to revisit the topic for our community.

As pediatric anesthesiologists, how can we reduce inhaled anesthetic waste and pollution by refining the art of managing fresh gas flow?  Here are specific suggestions:

1.      Limit Fresh Gas Flow to No More than Minute Ventilation during Inhalation Induction:  The goal of inhalation induction is to establish an effective concentration of Sevoflurane in the circuit while providing as positive an experience as possible for the patient.  Sevoflurane pollution cannot be avoided since “high” fresh gas flow is needed to achieve the desired rate of change of anesthetic concentration in a circle system.  But not everyone is aware that once fresh gas flow reaches minute ventilation, further increases do not increase the inspired concentration more rapidly – this practice just dumps more Sevoflurane into the atmosphere via the scavenging system.  Since minute ventilation is based upon patient size, smaller patients require lower fresh gas flows to achieve inhalation induction than larger patients.  The Society for Pediatric Anesthesia has developed a weight-based guideline for maximum fresh gas flow during inhalation induction that is useful for refining the art of inhalation induction while attempting to reduce waste (see Table copied below).4  There are additional strategies to consider for reducing waste during inhalation induction like reducing FGF once the circuit is saturated with anesthetic, turning off FGF during airway management and deciding when to transition to maintenance anesthesia, but these require more consideration and we can write more about them another time.

2.       Use less than 2 (or even less than 1 L/min for maintenance with Sevoflurane:  OK, I expect this to be a controversial recommendation.  FDA-required labeling for Sevoflurane still has a warning which states: “To minimize exposure to Compound A, sevoflurane exposure should not exceed 2 MAC·hours at flow rates of 1 to < 2 L/min. Fresh gas flow rates < 1 L/min are not recommended.”  While Compound A has been shown to be nephrotoxic in rats, there has never been any evidence of Compound A toxicity in humans.  Perhaps more importantly, readily available CO2 absorbent formulations that limit the concentration of strong base (NaOH) to less than 2% do not produce Compound A at all in the presence of Sevoflurane.5  These absorbents have become widely available since the last update of the package insert in 2003.  It is safe and reasonable to reduce FGF to less than 1 L/min for any duration when using Sevoflurane, especially when an absorbent is selected that limits the NaOH concentration.

3.      Reduce FGF during Maintenance based upon Patient Size:  For those adventurous practitioners interested in further refining the art of low-flow anesthesia to approach metabolic or closed-circuit conditions, patient size is an important consideration.  Oxygen consumption is the basis for estimating the minimum safe fresh gas flow.  A useful estimate of oxygen consumption is 3-5 mL/kg/min.  During maintenance, a minimum safe fresh gas flow can be estimated based upon oxygen consumption, sampled gas flow rate (if it is not returned to the circuit) and any leaks in the circuit. Setting the minimum estimated fresh gas flow is a start, but monitoring inspired oxygen and exhaled anesthetic concentrations is essential to the safe and effective practice of low-flow anesthesia.  See the original article from 2012 for more detailed guidance.

4.      Don’t use Desflurane:  Enough said.  The environmental impact of Desflurane, as measured by the metric Global Warming Potential (GWP) is much greater (as much as 10x) than any of the other inhaled anesthetics.  Indications for Desflurane in pediatric anesthesia are difficult to identify or justify.

5.      Turn off Vaporizer during Emergence and Manage Anesthetic with FGF:  This recommendation is not unique to pediatric anesthesia, but useful to minimizing waste during emergence.  By reducing fresh gas flow to well below minute ventilation, exhaled anesthetic is rebreathed and can be used to maintain the desired depth of anesthesia for quite some time.  When needed, increasing fresh gas flow can be used to control the progress of emergence without adding more inhaled anesthetic to the circuit.  The alternative strategy of reducing the vaporizer setting progressively at a high fresh gas flow rate provides excellent control over anesthetic concentration at the expense of adding to anesthetic waste, cost, and environmental pollution. 

The art of low-flow anesthesia lies in learning to manage fresh gas flow during every phase of the anesthetic to achieve the clinical goals while minimizing waste and pollution.  Like all aspects of pediatric anesthesia, variations in patient size provide an opportunity, indeed an imperative, to refine our practice in ways not available to the adult practitioner.  Induction and maintenance, in particular, are the phases of the anesthetic most amenable to considering patient size when setting fresh gas flow.  Monitoring inspired oxygen and exhaled anesthetic concentrations during low-flow anesthesia is essential to a safe and effective practice.  While not intended to be a detailed tutorial on low-flow anesthesia, this article will hopefully stimulate thinking about how to refine the art of managing fresh gas flow when caring for children, and how to do our part to save the world!

Do you have other suggestions for how our readers can minimize the environmental impact of the care we provide? If so, please add a comment below or send to Myron directly…myasterster@gmail.com and don’t forget to visit the Sustainability SIG table in Tampa this week!

References:

1.          Feldman JM. Managing fresh gas flow to reduce environmental contamination. Anesthesia and analgesia 2012;114(5):1093-101.

2.          Ryan SM, Nielsen CJ. Global warming potential of inhaled anesthetics: application to clinical use. Anesthesia and analgesia 2010;111(1):92-8.

3.          Varughese S, Ahmed R. Environmental and Occupational Considerations of Anesthesia: A Narrative Review and Update. Anesthesia and analgesia 2021;133(4):826-835.

4.          Glenski T, Narayanasamy S. Low flow anesthesia in pediatric patients. (https://pedsanesthesia.org/wp-content/uploads/2021/08/Low-Flow-Anesthesia-in-Pediatric-Patients.pdf). Accessed 03/20/2022

5.          Feldman JM, Hendrickx J, Kennedy RR. Carbon Dioxide Absorption During Inhalation Anesthesia: A Modern Practice. Anesthesia and analgesia 2021;132(4):993-1002.