What is Low Flow Anaesthesia?
This can be defined as any inhalation procedure wherein a circle system using an absorbent with a fresh gas flow of <patients alveolar minute volume, less than 1-1.5 l/min, 3 l/min or less, 0.5-2 l/m, less than 4 l/m, 500-1000 ml/min, 0.5-1 l/min or any FG flow which is less than the alveolar ventilation. The usage of the contemporary anesthetic equipment we can enact the FG flow when it is lessened to the least of 2L/min or even lesser. Using modern anesthetic machines, this can be achieved when FG flow (FGF) is reduced to at least 2 L/min or less.
Hidden facts on the usage of Low Flow Anesthesia in the past
The history behind the usage of Low Flow Anesthesia?
Inhalational anesthesia and closed system anesthesia are almost the same age. Almost/closed anesthesia systems have been in use since 1850. John Snow (1813-53) has found that inhalation anesthetics were exhaled mostly unchanged in the expired air of anesthetized patients. He based on his studies concluded saying that the narcotic effects of the volatile anesthetics can be marked as prolonged by reinhaling the unused vapors. The earlier anesthetic agent was chloroform, administrated via a closed system, where KOH was utilized as a CO2 absorber. However, that kind of CO2 absorption did not gain acceptance. Later, a quick and effective method of CO2 absorption was developed when the first soda-lime absorber was introduced in 1917.
Later in 1924 Ralph Waters introduced the rebreathing technique with carbon dioxide absorption into routine clinical practice and inaugurated the to-and-fro absorption system. At the same time, Carl Gauss has introduced his clinical experience studies of the use of a circular absorption system with arcylene, i.e. acetylene, as an inhalation anesthetic. Then in the mid-’50s, when HAL was brought forth, the use of LFA and closed system anesthesia diminished significantly. This was largely due to the inherent problem of the first-generation HAL vaporizers, which was the unreliable delivery of vapor at low FGF.
The introduction of ISO in the early 1980s gave way to a renewed interest in LFA and closed-circuit anesthesia. It was further enhanced by the fact that anesthetic agents are atmospheric pollutants, especially N2O, HAL, ENF, and to some extent ISO. The introduction of new low solubility agents, like DES and SEVO, has initiated a renaissance in the use of LFA, in order to contain costs associated with adapting FGF to patient demand.
Kleemann in 1990, had put forth the procedures of preserving the functional and the anatomical integrity of the epithelial cells of the respiratory tract using the LFA for the improvisation of heat and humidity of the rebreathing anesthetic gases. Baum and Aitken Head in 1995, renovated the LFA by imposing on important noted benefits from both environmental and economic aspects. The volatile substances like sevoflurane and desflurane became the most commercially acceptable during the usage with Low flow Anesthetic techniques and during the clinical practice Xenon as an anesthetic gas was more relevant.
The use of newer volatile agents such as sevoflurane and desflurane becomes more economically acceptable when used with low-flow anesthetic techniques. This is more applicable if Xenon were to be used as an anesthetic gas in clinical practice.
Definitions
Closed System Anesthesia is a form of LFA in which the FGF = uptake of anesthetic gases and oxygen by the patient and gas sampling. APL valve does not vent any gas. No gas is vented by the APL valve.
Low Flow Anesthesia is a form delivery, where FGF is below 1.5 l/min, but maintained slightly above the uptake of the patient. In addition, there is a low flow of excess gas that leaves the circuit through the excess gas valve.
Closed Circle Anesthesia is a form whereby FGF matches patient gas uptake and there is no excess gas leaving the circuit by way of the excess gas valve.
Minimal Flow Anesthesia is a form of anesthesia whereby FGF is 0.5 l/min.
Concept of Low Flow Anesthesia:
The idea of LFA is to restore the absorbed gases with as minimum FG as possible while making sure to remove CO2 before recirculating which results in minimizing the loss of anesthetic agents into the environment.
The FGF classification was stated by Baker. The classifications of FGF usage in the anesthetic practice are:
Baker has classified the FGF used in anesthetic practice into:
- Medium Flow : 1.2 l/min
- Low Flow : 500-1000 ml/min
- Minimal Flow : 250-500 ml/min
- Metabolic Flow : about 250 ml/min
Requirements
A circular rebreathing system with CO2 absorption.There should be an accurate flow meter for the adjustments of FGF below 1 l/min.
Accurate flow meters for adjustments of FGF below 1 l/min.
The suggested test leakage must be 150 ml/min at cm H2O test pressure in the gas-tight breathing system. The process of inhaling and exhaling should be minimal internal volume and a minimum number of connections and components.
Gas-tight breathing system, recommended test leakage should be below 150 ml/min at cm H2O test pressure.
The breathing system must be having minimal internal volume and a minimum number of components and connections.
Regular gas checks must be check listed. In view of the clinical aspect, the evaluation of the expiratory gas concentrations close to the Y-piece is of pivotal condition. The data is crucial in regulating the alveolar gas concentrations of the patients.
Continuous gas monitoring must be employed. From a clinical standpoint, the measurement of expiratory gas concentrations close to the Y-piece is of crucial importance. This information is essential in controlling the patient’s alveolar gas concentrations.
The vaporizers delivering the high concentrations must be calibrated and must be accurate at low fresh gas flow.
Calibrated vaporizers that can deliver high concentrations and that are accurate at low fresh gas flow are required.
The liquid injection can be injected directly into the expiratory limb
Monitoring for the safe performance of LFA:
Inspiratory oxygen concentration
Airway pressure and/or minute volume
Anesthetic agent concentration in the circuit
Expiratory CO2 concentration
Adjustments of FGF at different phases of LFA:
Premedication, pre-oxygenation, and induction of sleep are conducted as per the usual practice.
Concerning the adjustment, it can be classified as
- Initial High Flow: Parameters that can influence the building up of alveolar concentration.
- Sufficient denitrogenation
- Rapid wash in the desired gas composition into the breathing system
- Constituting the required anesthetic concentration
Establishing of the desired anesthetic concentration
- Avoiding the gas volume deficiency
- Low Flow
- Recovery
Injection Techniques:
The exact dose can be calculated:
Primary dose (ml vapor) = Desired concentration×(FRC + Circuit Volume) + (Cardiac output × BG Coefficient)}
An alternative method for administering large amounts of agents is by directly injecting the agent into the circuit.
The procedure is age-old and is also the method tested which is reliable. 1ml of liquid halothane yields 226 ml of vapor on vaporization and 1ml of liquid isoflurane at 20°C gives the output of 196 ml of vapor and thus the demand of 2 ml agent is infused in tiny portions into the circuit. The volatile gets coupled in the circle at high temperature resulting in the spontaneous vaporization of the agent. The recurrent injections, in general, are manually made in 0.2-0.5ml aliquots. The dosage must not exceed 1 ml foe each time and the exceeding doses of 2 m bolus result in inviting the disaster.
This is an old, time tested method and is extremely reliable. Each ml of the liquid halothane, on vaporization yields 226 ml of vapor and each ml of liquid isoflurane yields 196 ml of vapor at 20°C. Hence the requirement of about 2 ml of the agent is injected in small increments into the circuit. The high volatile coupled with the high temperature in the circle results in the instantaneous vaporization of the agent. The intermittent injections are often made in 0.2-0.5ml aliquots manually. Doses should never exceed 1ml at a time. Doses exceeding 2ml bolus invites disaster.
Low Flow Phase:
The desired low flow can be maintained after the high flow phase of 5-15 min and the target gas concentrations have been achieved with the reduction in FGF.
Lesser the FGF the more is the disparity in vaporizer setting and inspired concentration of the anesthetic agent in the breathing circuit. The time taken to reach the required concentration in the inspiratory is delayed with the lower FGF.
The lower the FGF the greater is the difference between the vaporizer setting and the inspired concentration of the anesthetic agent in the breathing circuit. With low FGF, the time to reach the desired concentration in the inspiratory gas is prolonged.
Thus, the control of oxygen and the anesthetic agent is crucial and paramount in LFA.
Hence the monitoring of oxygen and the anesthetic agent concentration is essential and necessary in LFA.
Maintenance:
The stage of maintenance is marked by
This phase is characterized by
- The requirement of the constant state of anesthesia generally meant as a substantial alveolar concentration of respiratory gases
- The need for steady-state anesthesia often meant as a steady alveolar concentration of respiratory gases.
- The basic uptake of anesthetic agents through the body
- Minimal uptake of anesthetic agents by the body
- The requirement of preventing the different mixture of gases
- Need to prevent hypoxic gas mixtures.
Administering the flow of both nitrous oxide and the O2 in the process of maintenance. Management of oxygen and nitrous oxide flow during the maintenance phase
During the initial stage, the flow must be a high flow of 10 L/min for 3 min pursuing the next flow of 400 ml of O2 and 600 ml of N2O in the first 20min and thereafter flow of 500ml of O2 and 500ml of N2O. This state of maintenance states the 33-44% of O2 concentrations each time.
A high flow of 10 L/min at the beginning for a period of 3 min followed by flow of 400 ml of O2 and 600 ml of N2O for the initial 20 min and flow of 500 ml of O2 and 500 ml of N2O thereafter. This shows to maintain the oxygen concentration between 33 and 40% at all times.
Equations:
VO2 = 10 × BW KG ˄ ¾ Brody Equation
V N2O = 1000 × t ˄ ½ Severinghaus Equation
V AN = Desired Concentration × λ B/G × Q × t-1/2 Lowe Equation
Precautions:
If there is a desire in the change of any component of the inspired mixture, FGF is to be increased.
Before returning to the low flow, if the integrity of the circle is cut the high flow for several minutes must be used.
If the integrity of the circle is cut high flow should be used for several minutes before returning to low flow.
If a CA is used its recommended to use high flow for 1-2 min each hour to remove gases such as N and CO2.
Emergency Phase:
In general, 100% O2 must be required in facilitating the washout of the anesthetic agent in the patient and later removing the agent to the scavenging the system by following high FGF.
At the end of anesthesia high FGF, usually 100% O2 is required in the patient to facilitate the washout of the anesthetic agent and then removing the agent to the scavenging system.
A charcoal filter can also be placed in the expiratory limb, which causes a rapid decrease in the concentration of the volatile agent.
Advantages:
Economic benefits like reduced anesthetic gas consumption, significant savings of 60-70% volatile anesthetic agents.
Ecological benefits like the reduced flow of FCs and nitrous oxide which damages or the Ozone layer, reduced the greenhouse effect caused by nitrous oxide and volatile agents.
Physiologically preserves the heat and humidity of the inspired gases and conserving the temperature reducing the water loss. Improves mucociliary clearance, airway epithelial health, flow dynamics of the inhaled anesthetic gases.
Environmental benefits like lessening operating room pollution and since less exposure to anesthetic vapors in the process of filling.
Disadvantages:
Inability to quickly alter the inspired concentrations
More attention is required
Danger of hypercarbia
Uncertainty about inspired concentrations
Faster absorbent exhaustion
Accumulation of undesirable trace gases in the system like CO, acetone, methane, hydrogen, ethanol, argon, nitrogen, compound A, etc.
Concerns about Safety in LFA:
Hypoxia
Misdosage of the volatiles
Exhaustion of the absorbent
Gas volume deficiency
Reduced controllability
Summary
Many new anesthesia machines have been designed and developed in Europe in the past few years like the Physioflex and Drager Zeus machines.
The modern equipment’s has an implanted and innate algorithm for the calculations of the uptake and later adjustments of the desired volumes and concentrations in accordance with playing the most crucial role. Today there are ample varieties of gas analyzers for the monitoring processes of FiO2, ETCO2 and agent monitoring in modern anesthesia workstations which results in effortless and pragmatic conduct of LFA.
These have an inbuilt algorithm in calculating the uptake and adjusting the concentrations and volumes in accordance, which is very useful. Anaesthesiologists must take up LFA as their professional commitment for the present and future generations.