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When using a circle anesthesia system, any anesthetic gas that enters the scavenging system is ultimately vented outside the hospital. Whenever fresh gas flow exceeds the patient‘s metabolic oxygen requirement, excess anesthetic gases enter the scavenging system and ultimately pollute the atmosphere. By minimizing total fresh gas flow, the environmental impact of anesthetic vapors can be reduced. Although the environmental impact of a single case may appear inconsequential, the collective impact of low fresh gas flow management over millions of anesthetics is significant.
The following discussion is intended to describe the rationale and methods for minimizing fresh gas flows and empower clinicians to adopt environmentally responsible practices for delivering inhalational anesthesia. Many resources are available on the principles of low-flow and closed-circuit anesthesia. One review offers more detail on the strategies described here.89.
In addition, the Anesthesia Patient Safety Foundation and the ASA offer a course on low-flow anesthesia at no cost that also provides safety CME credits. Information can be found at apsf.org/tei/lfa and the course is available at Low Flow Anesthesia (education.asahq.org).
In an anesthesia circuit, fresh gas flow rates determine the degree of rebreathing of exhaled gases, which impacts anesthetic wastage and the extent of environmental contamination.
In practice, there are opportunities during induction, maintenance, and emergence to minimize fresh gas flow and thereby anesthesia-related greenhouse gas emissions. The optimal fresh gas flow setting during each anesthetic phase is a balance between reducing flow to avoid waste while ensuring sufficient flow to manage the desired concentrations of oxygen and inhaled anesthetics. Proper knowledge and utilization of anesthesia equipment is important to safely implement low flow strategies.
Clinical consideration of the time to change blood and brain concentrations of the anesthetic in fragile or unstable patients must be part of the assessment of when and how to use low flow management. Reducing fresh gas flow should never take priority over maintaining safe and effective concentrations of oxygen and anesthetic in the patient.
The maintenance phase begins when the uptake of anesthetic from the lungs has slowed and the brain concentration is adequate. During this phase, rapid changes in the established gas concentrations are not typically required. The maintenance phase, therefore, is the simplest opportunity to safely minimize fresh gas flow to reduce the environmental impact of inhaled anesthetics.
Continuous measurement of inspired oxygen and expired vapor concentrations is essential to safely reduce fresh gas flow. At low fresh gas flows, there is a risk of inadequate oxygen and anesthetic delivery, the latter being especially true early in the anesthetic when there is significant uptake of anesthetic from the lungs. Inspired oxygen monitoring is critical to prevent hypoxemia from excessively low fresh gas flows and inadequate oxygen delivery.
The minimum safe fresh gas flow supplies enough oxygen to satisfy the patient’s oxygen consumption plus enough additional gas flow to replace gases lost due to leaks in the circuit and/or via a side-stream gas analyzer. Oxygen consumption during anesthesia varies between patients and even phases of the anesthetic. Consuming 3-5 mL/kg/minute of oxygen, an adult (70 kg) will use between 210 and 350 mL of oxygen per minute. Assuming there are no leaks from the circuit, a fresh gas flow of oxygen of between 210 and 350 mL/minute is all that is required. Any greater fresh gas flow will spill the excess gas into the scavenging system and ultimately the environment. If oxygen consumption is underestimated and the fresh gas flow of oxygen is set too low, the inspired oxygen concentration will progressively diminish. More likely, if oxygen flow is set to 350 mL/min, the inspired oxygen concentration will slowly rise, and total oxygen flow can be reduced to maintain a stable oxygen concentration.
Unless the patient has a large oxygen consumption (e.g., trauma, sepsis, pregnancy), it should be possible during the maintenance phase of anesthesia to set the fresh gas flow to a maximum of well below 1 L/minute. There is still some environmental contamination with this technique, since the total fresh gas flow exceeds what is consumed, but it is easier to manage than a true “closed circuit” technique. For smaller patients with even lower oxygen consumption requirements, the fresh gas flow during maintenance can be reduced even further with the same caveat of monitoring inspired oxygen concentration.
Leaks (or potential leaks) from the circuit also need to be considered to determine the minimum safe fresh gas flow. After passing routine preoperative pressure testing, true circuit leaks should be minimal (less than 150ml/min for most machines). Anesthesia machines with automated leak testing typically report the measured leak which can be used to determine fresh gas flow. If the leak in the circuit has not been quantified but the circuit has passed a pressure test, another 100-150 mL/min should be added to accommodate any leaks from the circuit. If using a side-stream gas analyzer that does not return sampled gas to the breathing circuit, add 200 mL/min to the required fresh gas glow calculation.
Induction is the most challenging time to minimize fresh gas flow since a rapid increase in anesthetic concentration is desired. However, several practices during induction can be employed to reduce anesthetic waste and excess greenhouse gas emissions.
A fresh gas flow greater than minute ventilation creates waste with no clinical benefit. In general, once the fresh gas flow equals minute ventilation, there is little to no rebreathing of exhaled gas and the patient inspires only fresh gas. Increasing fresh gas flow further does not significantly speed the increase in anesthetic concentration but does force more anesthetic to the scavenging system and ultimately the atmosphere. In other words, increasing fresh gas flow beyond minute ventilation creates pollution without clinical benefit. The only caveat is that fresh gas flow greater than minute ventilation may be needed to aid mask ventilation if there is a large leak around the mask. However, for most patients, good mask skills are sufficient to limit the maximum fresh gas flow to minute ventilation. For adult patients the total fresh gas flow should not exceed 5-6 L/min. The Society for Pediatric Anesthesia has published guidance for weight-based maximum fresh gas flows during inhalation induction that are useful when caring for smaller patients.90,91
Priming can be used to load the circuit with anesthetic more efficiently than just using high fresh gas flow. However, the method of priming is important if excessive waste and pollution is to be avoided. Most modern anesthesia machines have a self-test procedure that involves occluding the end of the breathing circuit. One common approach to priming is to turn on high fresh gas flows with the circuit occluded and open the APL valve. This approach will prime the internal parts of the breathing system but not the circuit itself since the occluded circuit holds both inspiratory and expiratory valves closed. Furthermore, if flows are left running beyond the time required to effectively prime, waste is generated without clinical benefit. An alternate approach is to empty the reservoir bag manually just before turning on the vaporizer. This approach will reduce the time required to establish anesthetic in the circuit as the reservoir bag volume does not dilute the incoming anesthetic.
One common practice during the anesthesia induction is to use a high fresh gas flow while mask ventilating the patient and turning off the vaporizer during intubation. Although the goal of this practice is to avoid contaminating the operating room with anesthetic vapor, in reality the continued high fresh gas flow through the circuit washes the accumulated anesthetic gas into the room. Alternatively, one can turn off the fresh gas flow during intubation and leave the vaporizer on. In the absence of fresh gas flow, none of the anesthetic vapor is washed into the room. This strategy facilitates setting a low fresh gas flow soon after intubation since the anesthetic vapor concentration in the circuit is preserved.76,89 When the anesthetic agent monitor indicates a clinically appropriate anesthetic dose and a small difference between inspired and expired agent concentrations, it is reasonable to use a minimum fresh gas flow setting and follow the maintenance strategy outlined previously.
The concentration of anesthetic vapor in the breathing circuit is determined by the vaporizer setting and the total fresh gas flow. The inspired concentration of anesthetic vapor may be less than the vaporizer setting depending upon the concentration of anesthetic agent in the exhaled gas and the amount of rebreathing. However, as fresh gas flow is reduced, the same inspired anesthetic concentration can be achieved by increasing the vaporizer setting to account for anesthetic uptake. For example, if one is accustomed to selecting 2% sevoflurane at a total fresh gas flow of 8 L/min during induction to rapidly wash gas into the circuit, the same amount of sevoflurane can be administered at a setting of 8% and 2 L/min. This approach will reduce gas waste by 75% and still maintain sufficient flow to maintain the volume in the circuit if the mask seal is tight or the patient is intubated.
An important caveat to this strategy is that over time, the uptake of anesthetic from the lungs will diminish and the inspired concentration will approach the vaporizer setting. If the vaporizer is set to deliver a high concentration of anesthetic agent and end-tidal concentrations are not carefully monitored, there is a risk of anesthetic overdose, although setting the inspired anesthetic concentration alarm can help to alert the clinician to the accumulation of anesthetic vapor in the inspired gas beyond a desired level.
A smooth, efficient emergence requires timing the elimination of anesthetic agent with the conclusion of surgery to facilitate a prompt transfer to the recovery room. Increasing fresh gas flow and gradually reducing the vaporizer setting is one approach that provides control over anesthetic concentration, but this strategy also wastes a significant amount of anesthetic through the scavenging system. Alternatively, turning off the vaporizer while maintaining low fresh gas flows also leads to a gradual elimination of anesthetic, but without unnecessary waste. Once the vaporizer is turned off, no additional anesthetic will contaminate the environment as the patient emerges. If adding nitrous oxide is avoided as well, there is no additional environmental impact from emergence. Fresh gas flow can then be used to manipulate the anesthetic concentration in the circuit. This approach to emergence takes practice and requires attention to an anesthetic agent monitor to follow agent concentrations, but it is easily learned.92
Modern anesthesia machines may include tools to help guide the safe reduction of fresh gas flow. These tools typically use minute ventilation and the difference between inspired and expired oxygen concentration to measure oxygen uptake. All are useful, although it is possible and encouraged to safely reduce flows even further by using the strategies outlined above. One important caveat is that these tools do not in general consider the desired anesthetic concentration and should not be used to reduce fresh gas flow outside of the maintenance phase.
Clinical decision support tools in electronic medical record platforms can also be implemented to identify extended periods of high fresh gas flow and prompt clinicians in real time to decrease fresh gas flows138. Finally, automated approaches to minimizing fresh gas flow are becoming available within newer generation anesthesia machines.
The selection of carbon dioxide absorbent is also an important consideration. Low fresh gas flow practices will increase consumption of carbon dioxide absorbent. Increased absorbent consumption also increases cost. While cost-containment is a valid concern of clinicians and institutions, the bottom line is that low fresh gas flow practices and the reduced consumption of sevoflurane and desflurane generate cost savings that will offset the increased consumption and cost of CO2 absorbent.93 In addition, absorbents differ in their efficiency (mass of CO2 absorbed per mass of absorbent), making specific product selection another stewardship opportunity.94 Importantly, carbon dioxide absorbent canisters should be changed based on the appearance of inspired carbon dioxide rather than a change in color indicator.95
Modern carbon dioxide absorbents do not utilize high concentrations of strong bases such as potassium hydroxide (KOH) or sodium hydroxide (NaOH). In the absence of these strong bases, low fresh gas flow anesthetic strategies do not lead to the accumulation of carbon monoxide or Compound A. All absorbents in common use today are predominantly calcium hydroxide. Absorbents that lack KOH and contain low concentrations (less than 2%) of NaOH are efficient for CO2 absorption and eliminate the risk of toxic compound production.77,80,95
NOTE: It may be possible to reduce the total fresh gas flow further if the circuit leak is less than 100 mL/min or oxygen consumption is less than the estimated value, but the inspired oxygen concentration must be monitored.
Curated by: the ASA Committee on Environmental Health
Date of last update: January 29, 2024
