While the concept of low-flow anesthesia, or LFA, is not new, having been first described in 1952 by Foldes et al, there has been some question as to what constitutes LFA versus minimal-flow anesthesia1. However, because LFA offers a multitude of benefits to the hospital, the patient and the environment, adopting the use of an LFA technique is an important consideration in the practice of anesthesia.
In this article, we’ll discuss not only the definition for LFA, but also the differences between low flow and minimal flow, as well as the benefits that may be achieved through LFA.
Open vs. closed anesthetic circuits
The basic requirement for conducting LFA is the use of a rebreathing system. With this system, the unused gases and anesthetic agent contained in the patient’s exhaled breath are reused in the next inhaled patient breath.
By enabling exhaled anesthetics to be rebreathed rather than wasted through the scavenging system, circle anesthesia circuits are intended to lower the amount of inhaled anesthetic waste. The amount of rebreathing is controlled by the fresh gas flow, and when the exhaled gas is rebreathed, a carbon dioxide absorbent is required. Closed circuit anesthesia occurs when the fresh gas flow matches a patient’s oxygen consumption, essentially when all the exhaled gas is delivered back to the patient. In principle, the patient's oxygen consumption equals the minimal safe, fresh gas flow, which is oxygen only2.
The minimum safe, fresh gas flow must consider extra losses (gas sampling and leaks) that most anesthetic delivery systems experience in addition to oxygen consumption. Open-circuit anesthesia develops when no exhaled gas is rebreathed and all of it exits the circuit via the scavenging system. An open circuit, as a rule, exists when the fresh gas flow exceeds minute ventilation (VE). More exhaled gas is rebreathed, and more carbon dioxide absorbent is consumed as fresh gas flow is decreased from an open circuit to a closed-circuit condition2.
The anesthesia circle system is considered open if the fresh gas is fed into the rebreathing system in the amount that exceeds 50% - 100% of minute volume3.
What is low-flow versus minimal-flow anesthesia?
The classification of gas flow rate was set by Baker in 1994 and is still considered valid. Based on his classification, fresh gas flow rate is divided into few categories: metabolic-flow (<250 ml/min), minimal-flow (250-500 ml/min), low-flow (500-1000 ml/min), medium-flow (1-2 L/min), high-flow (2-4 L/mi) and very high-flow (>4 L/min)6.
Therefore, the main difference between low- versus minimal-flow anesthesia is that in LFA, the fresh gas flow is reduced to 1.0 L/min, while in minimal-flow anesthesia, the fresh gas flow is reduced between 0.25 L/min and 0.5 L/min.
This is important because very little anesthetic gas is required to keep a patient asleep during surgery, and any extra is released into the atmosphere. The flow rate of the carrier gas, or FGF, has a significant role in determining the amount of anesthetic gas utilized during a surgical procedure. FGF is controlled by anesthesia practitioners using values between less than 1 L/min (low) and more than 10 L/min (very high). At the lower flow gas flow rates level (FGF), the patient rebreathes some of the exhaled anesthetic gas. So therefore, a significantly lower amount of inhalational anesthetic is vented into the atmosphere5.
While LFA is characterized as gas flows under 1 L/min, the term "LFA" is also typically used to refer to inhalation anesthetic procedures with a semi-closed rebreathing system and a rebreathing rate of at least 50%4.
Phases of low-flow anesthesia
According to standard procedure, premedication, preoxygenation and induction of anesthesia are carried out. A laryngeal mask or endotracheal intubation is followed by the patient's connection to a rebreathing system. The LFA approach usually goes through three phases: initial high-flow, low-flow, and the recovery phase4,6.
A high fresh gas flow of approximately 6.0 L/min of 100% oxygen is characteristic of the initial phase, which lasts 10 to 20 minutes. Denitrogenation involves high-flow 100% O2 ventilation to remove the nitrogen in the blood. Denitrogenation removes the nitrogen from the lungs, which makes room for more O2. The oxygen reserves and functional residual capacity both rise due to this phenomenon which helps to extend the safe apnea time3,4,6.
Following the induction phase, fresh gas flow is decreased to the appropriate level (1 L/min or less) during the low-flow phase. Re-breathing rate rises noticeably when the flow rate is decreased. When the flow is reduced, the oxygen concentration should be raised to at least 40% in order to maintain the inspired oxygen concentration above 30% of volume. The amount of anesthetic vapor delivered to the respiratory system will decrease when rebreathing occurs and in parallel to the decrease in flow rate. Thus, constant monitoring of oxygen and anesthetic drug concentration is a crucial concern in LFA4,6,7,8.
Carbon dioxide (CO2) wash-out is crucial for low fresh gas flow anesthesia. Due to absorbent exhaustion, CO2 content in the ventilation system dramatically increases, since rebreathing volume in LFA is higher than in high-flow anesthesia. As a result, CO2 absorbent should be monitored until it is exhausted by looking for color changes and increases in inspired CO2 (FiCO2) and then be replaced at a minimum of once a week. Double or big single canisters should be employed in anesthetic machines without CO2 monitoring devices. 4,7.
During emergence, the vaporizer and nitrous oxide, if used should be turned off sooner toward the end of the surgical procedure since the low flow does not immediately wash out the volatile anesthetic agent6,7,8. To efficiently remove any remaining anesthetic in the patient or circuit, the patient may be switched to high-flow oxygen at the end of the surgery. This enables much faster wash out of residual anesthetic gas in the circuit of the anesthetic machine and the lungs6,9.