Low flow Anesthesia system

Describes the low flow anesthesia technique in details

1. Low flow Anaesthesia system Dr P K Maharana KIMS, Bhubaneswar.

2. History of Low flow Anaesthesia As early as 1850 John Snow (1813-53 ) recognized that only a small fraction that inhalation anaesthetics gets metabolized and most of it exhaled unchanged in the expired air of the anaesthetized patients . He also concluded that the narcotic effect of the volatile anaesthetics could be markedly prolonged by re-inhaling these unused vapours.   About 75 years later, in 1924, the rebreathing technique with carbon dioxide absorption was introduced into routine clinical practice by Ralph Waters (1883-1979), the to-and-fro absorption system .  At the same time Carl Gauss (1875-1957) published his clinical experiences of the successful use of a circle absorption system for use with narcylene, i.e. acetylene, as an inhalation anaesthetic proved to be a milestone. Brian C. Sword introduced the circle (CO 2  absorption) system in the 1930s.  1933 onwards ; with introduction and increasing clinical use of cyclopropane it became nearly essential to close the overflow valve of the breathing system to avoid serious danger of explosions; and it forced to reuse the exhaled air completely

3. History of LFA( Conti- i ) In 1954 , the world noticed a land mark event the introduction halothane to clinical practice, a volatile anaesthetic characterized by high anaesthetic potency but a narrow therapeutic index . It’s use necessitated knowledge of the inhaled vapour concentration to ensure patient safety. It was easy to measure the concentration of halothane with high flow system with no or minimal rebreathing. Hence popularization of Halothane gave rise to use of high flow system. In spite of the fact that most anaesthetic machines were equipped with circle systems, but it became clinical obligation to use high fresh gas flows for patient’s safety because rebreathing became negligible . The introduction of Isoflurane in the early 1980s gave way to a renewed interest in LFA and use of closed-circuit anesthesia. This fact is further strengthen by the fact that anesthetic agents are atmospheric pollutants especially N 2 O, the volatile agents like HAL, ENF, and to some extent ISO, accused for ozone depletion & the green house effect .

4. History of LFA( Conti-ii) In 1990’s the low soluble volatile substances like sevoflurane and desflurane became the most popular clinically. Factors like cost and theatre pollution paved the way for the resurgence of low flow system and use of these newer drugs. Xenon as an anesthetic gas was more relevant. 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.

5. Definition Any technique that utilizes a fresh gas flow (FGF) that is less than the alveolar ventilation can be classified as “ Low flow anaesthesia. ” However, there is no clear definition as to what exactly the flow should be to level it as low flow? Baum et al : had defined it as a technique wherein at least 50% of the expired gases had been returned to the lungs after carbon dioxide absorption in the next inspiration. A technique for oxygen-nitrous oxide anaesthesia with a flow of 1liter per minute was described by Foldes in 1952. In 1974, Virtue described technique using a fresh gas flow of 500ml per minute, which is named as minimal flow anaesthesia . Baker in his editorial described fresh gas flow (FGF) in anesthetic practice as under; Metabolic flow  250ml/minute Minimal Flow  250-500ml/minute Low Flow  500-1000ml/minute Medium Flow  1000- 2000ml/minute For all practical purposes a FGF was less than about two litres per minute may be considered as LFA

6. Principle of working of LFA system Low flow anaesthesia involves utilizing a fresh gas flow which is higher than the metabolic flows but considerably lesser than the conventional flows. The larger than metabolic flows provides considerably greater margin of safety and helps in satisfactory maintenance of gas composition in the inspired mixture. Completely closed circuit anaesthesia is based upon the reasoning that anaesthesia can be safely be maintained if the gases which are taken up by the body alone are replaced into the circuit taking care to remove the expired carbon dioxide with sodalime. - Hence in LFA, the conduct of anaesthesia is greatly simplified and at the same time provides advantages for the economy of the fresh gas flows minimising theatre pollution.

7. Requirements for LFA 1 . A circle system with CO 2  absorption . 2. A flow meter for the adjustments of FGF below 1 l/min . 3. Gas-tight breathing system , recommended test leakage should be below 150 ml/min. 4. The breathing system must be having minimal internal volume a nd a minimum number of components and connections. 5. Continuous gas monitoring must be employed, essential in controlling the patient's alveolar gas concentrations. 6. The vaporizers delivering the high concentrations must be calibrated and must be accurate at low fresh gas flow .

8. Circle System The minimum requirement for conduct of low flow anaesthesia is the effective absorption of CO2 from the expired gas, so that the CO2 free expired gas can be reutilized for alveolar ventilation. In the past, two systems were in use; “To and Fro” system introduced by Waters and the “ circle system” introduced by Brian Sword. But because of its bulkiness near the patient and other disadvantages the “ To and Fro ” has gone out of favour and the “circle system” using large sodalime canisters is in use.

9. Describing a Circle System. The circle system should have the basic configuration with two unidirectional valves on either side of the sodalime canister , fresh gas entry, reservoir bag, pop off valve, and corrugated tubes and „Y‟ piece to connect to the patient. The relative position of fresh gas entry, pop off valve, and reservoir bag are immaterial as long as they are positioned between the expiratory and the inspiratory unidirectional valves that functions properly and CO2 absorption is efficient at all times.

10. THE PRACTICE OF LOW FLOW ANAESTHESIA: The practice of low flow anaesthesia can be dealt with under the following three headings: 1. Initiation of Low flow anaesthesia 2. Maintenance of Low flow anaesthesia 3. Termination of Low flow anaesthesia.

11. INITIATION OF LOW FLOW ANAESTHESIA Primary aim at the start of low flow anaesthesia is to achieve an alveolar concentration of the anaesthetic agent that is adequate for producing surgical anaesthesia (approximately 1.3 MAC). Factors influencing to buildup of alveolar concentration should be considered . These factors can broadly be classified into three groups : 1) Factors governing the inhaled tension of the anaesthetic agent, 2) Factors responsible for rise in alveolar tension, 3) Factors responsible for reducing the alveolar tension, uptake from the lungs.

12. Factors governing the inhaled tension of the anaesthetic. Three most important factors responsible for inhaled tension of anesthetics are : 1. BREATHING CIRCUIT VOLUME 2. RUBBER GAS SOLUBILITY 3. SET INSPIRED CONCENTRATION

13. Breathing Circuit Volume Circle system is often bulky and volume roughly equal to 6-7 liters, FRC of the patient, which is roughly 3 litres, together constitutes a reserve volume of 10 liters; to which the anaesthetic gases and vapours have to be added. With the addition of FGF, the rate of change of composition of the reserve volume is exponential, time required for the changes to occur is governed by the time constant, which is equal to this reserve volume divided by the fresh gas flow. Three time constants are needed for a 95% change in the gas concentration to occur . Hence, if a FGF of 1L/min is used, then 30 minutes will be required for the circuit concentration to reflect the gas concentration of the FGF. If the FGF is still lower, then correspondingly longer time will be required.

14. Rubber Gas Solubility of the agent The anaesthetic agent could be lost from the breathing system due to solubility of the agent in rubber, and permeability through the corrugated tubes. The concentration will take a longer time if solubility is more. Though the amount of loss will be minimal, it should be considered at the start if the aimed anaesthetic concentration is low.

15. SET INSPIRED CONCENTRATION The rate of rise of alveolar partial pressure of the anaesthetic agent must bear a direct relationship to the inspired concentration . Higher the inspired concentration, the more rapid is the rise in alveolar concentration. The Inspired concentration of the agent depends on several factors. Denitrogenation of the circuit volume. Alveolar ventilation. Concentration effect.

16. Denitrogenation of FRC & Breathing Circuit Volume The functional residual capacity of the lung and the body as a whole contain nitrogen which will try to equilibrate with the circuit volume and alter the gas concentration if satisfactory denitrogenation is not achieved at the start of anaesthesia. Hence, as a prelude to the initiation of closed or low flow anaesthesia, thorough denitrogenation must be achieved with either a non-rebreathing circuit or the closed circuit with a large flow of oxygen and a tight fitting facemask .

17. Alveolar ventilation: The second factor governing the delivery of anaesthetic agent to the lung is the level of alveolar ventilation. The greater the alveolar ventilation, the more rapid is the rise of alveolar concentration towards the inspired concentration. This effect is limited only by the lung volume, the larger the functional residual capacity, the slower the wash. in of the new anaesthetic gas.

18. Factors responsible for uptake from the lungs thus reducing the alveolar tension 1. CARDIAC OUTPUT . 2. BLOOD GAS SOLUBILITY . 3. ALV – VENOUS GRADIENT.

19. Cardiac output Because blood carries anaesthetic away from the lungs, the greater the cardiac output, the greater the uptake, and consequently the slower the rate of rise of alveolar tension . At low inspired concentration, the alveolar concentration results from a balance between the ventilatory input and circulatory uptake. If the later removes half the anaesthetic introduced by ventilation, then the alveolar concentration is half that inspired.

20. Blood gas solubility If other things are equal, the greater the blood gas solubility coefficient, the greater the uptake of anaesthetic , and slower the rate of rise of alveolar concentration. “ Solubility” is the term used to describe how a gas or vapour is distributed between two media. At equilibrium , that is when the partial pressure of the anaesthetic in the two phases is equal, the concentration of the anaesthetic in the two phases might differ. This is calculated as a coefficient. When it is between blood and gas it is called blood gas solubility coefficient.

21. Alveolar to venous partial pressure gradient: During induction the tissues remove all the anaesthetic brought to them by the blood. Tissue uptake lowers the venous anaesthetic partial pressure far below that of the arterial blood. If tissue uptake is more then it results in a large alveolar to venous anaesthetic partial pressure difference, which promotes maximum anaesthetic uptake and hence lowers the alveolar partial pressure.

22. Concentration effects The concentration effect modifies this influence of uptake . When appreciable volumes are taken up rapidly, the lungs do not collapse; instead the sub atmospheric pressure created in the lung by the anaesthetic uptake causes passive inspiration of an additional volume of gas to replace that lost by uptake, thus increasing the alveolar concentration and offsetting the mathematical calculations. Similarly, if an insoluble gas (e.g., nitrogen) is present in the inspired mixture, as the blood takes up the anaesthetic gas, the concentration of the insoluble gas will go up in the alveoli, reducing the concentration of the anaesthetic agent. Concentration effect : The concentration effect helps in raising the alveolar tension towards the inspired tension, but hinders with it if an insoluble gas is present in the mixture.

23. Methods to achieve desired gas and agent concentration. High flow for short time Use of Prefilled Circuits with gases Use of large doses of anaesthetic agents. Injector technique.

24. Use of high flows for a short time This is by and far the commonest and the most effective technique of initiating closed circuit. By using high flows for a short time, the time constant is reduced thereby bringing the circuit concentration to the desired concentration rapidly. Often, a fresh gas flow of 10L of the desired gas concentration and 2 MAC agent concentration is used so that by the end of three minutes (three time constants) the circuit would be brought to the desired concentration. Advantages: --- Rapidity with which the desired concentration is achieved, the ability to prevent unexpected raise in the agent concentration and the ability to plenum vaporizers to achieve the desired concentration. This also has the added advantage of achieving better denitrogenation, so vital to the conduct of the low flow anaesthesia. The chief disadvantage: - Not economical & need scavenging systems to prevent theatre pollution Mapleson3 using a spreadsheet model of a circle breathing system has calculated that, by using a FGF equal to minute ventilation and setting the anaesthetic agent partial pressure to 3 MAC, the end expired partial pressure of halothane will reach 1 MAC in 4 minutes and that of isoflurane in 1.5 minutes.

25. Use of Prefilled circuit The second method is utilizing a different circuit like Magill’s for pre-oxygenation. Simultaneously, the circle is fitted with a test lung and the entire circuit is filled with the gas mixture of the desired concentration. Following intubation, the patient is connected to the circuit thereby ensuring rapid achievement of the desired concentration in the circuit.

26. Use of large doses of anaesthetic agents. The third method consists of adding large amounts of anaesthetic agent into the circuit so that the circuit volume + FRC rapidly achieves the desired concentration as well as compensates for the initial large anaesthetic gas uptake. To execute this, Pre-oxygenation after intubation, fresh gas flow is started with metabolic flows of oxygen and a large amount of nitrous oxide often in the range of 3-5 litres per minute. Oxygen concentration in the circuit gradually falls, is continuously monitor Oxygen and the nitrous oxide concentration, N2O flow is reduced once the desired oxygen concentration (33 - 40%) is achieved . Disadvantage of this method : is hypoxia if the oxygen monitor were to malfunction. Hence this method is seldom used for N2O. In the contemporary anaesthesia machines it is not possible to administer lesser than 25% of oxygen and the described technique cannot be executed.

27. Injection techniques. An alternative method for administering the large amounts of the agents is by directly injecting the agent into the circuit. The high volatility coupled with the high temperature in the circle results in instantaneous vaporization of the agent. The injection is made through a self sealing rubber diaphragm covering one limb of a metal t piece or a sampling port, inserted into either the inspiratory or the expiratory limb. This is an old, time-tested method and is extremely reliable. Each ml of the liquid halothane, on vaporization yields 226 ml of vapour and each ml of liquid isoflurane yields 196 ml of vapour at 20oC. Hence, the requirement of about 2ml of the agent is injected in small increments into the circuit.

28. THE MAINTENANCE OF LOW FLOW ANAESTHESIA This is the most important phase as this is stretched over a long period of time and financial savings result directly from this; this phase is characterized by : 1 . Need a steady alveolar concentration of respiratory gases. 2. Minimal uptake of the anaesthetic agents by the body. 3. Need to prevent delivery of hypoxic gas mixtures. Since the uptake of the anaesthetic agent is small in this phase, adding small amounts of the anaesthetic gases to match the uptake and providing oxygen for the basal metabolism should suffice.

29. Management of the oxygen and nitrous oxide flow during the maintenance phase The need to discuss the flow rates of N2O and O2 arises specifically because of the possible danger of administration of a hypoxic mixture. Let us analyze the following example. 33% oxygen is set using a flow of 500 ml of O2 and 1000 ml of N2O. Oxygen is taken up from the lungs at a constant rate of about 4 ml/kg/min. N2O is a relatively insoluble gas and after the initial equilibration with the FRC and vessel rich group of tissues, the up take is considerably reduced. In this situation, there is a constant removal of O2 at a rate of 200 - 250 ml/min, where as the insoluble gas N2O uptake is minimal. Hence the gas that is partly vented and partly returning to the circuit will have more N2O and less of O2. Over a period of time, due to the mixing of fresh gas that has 66% N2O and the expired CO2 free gas that has N2O much higher than that, the percentage of N2O will go up and that of O2 will fall, sometimes dangerously to produce hypoxic mixtures. A high flow of 10 lit/min at the start, for a period of 3 minutes, is followed by a flow of 400 ml of O2 and 600 ml of N2O for the initial 20 minutes and a flow of 500 ml of O2 and 500 ml of N2O thereafter. This has been shown to maintain the oxygen concentration between 33 and 40 % at all times.

30. Management of the potent anaesthetic agents during maintenance phase This is easily accomplished by dialling in the calculated concentration on the plenum vaporizer for the flow being used. For example, suppose the anaesthetic uptake for a desired concentration of 0.5% halothane is 7.5ml/min. If a FGF of 500ml/min is being used, then the dial setting should be 1.5% for at this setting and for the used flow, the total vapour output would be 7.5ml/min. If a flow of 1000ml/min is being used, then the dial setting should be 0.8%. In practice the actual dial setting often over estimates the actual output since the plenum vaporizer under delivers the agent at low flows. Hence, the dial setting is fine-tuned depending on the endpoints being achieved.

31. Lowe's formula T approximated infusion (in liquid ml/hr) based on the Lowe's formula as follows: 0 - 5 min. 14 + 0.4X wt. ml/hr. 5 - 30 min. 0.2 X initial rate. 30-60 min. 0.12Xinitial rate. 60-120min. 0.08X initial rate. For halothane infusion, the above rates be multiplied by 0.8 and for enflurane multiplied by 1.6. These rates had been suggested to produce 1.3 MAC without the use of nitrous oxide. The infusion rates had to be halved if nitrous oxide is used.

32. Weir and Kennedy recommend infusion of halothane Weir and Kennedy recommend infusion of halothane (in liquid ml/hr) at the following rates for a 50 kg adult at different time intervals. 0-5 min 27 ml/hr 5-30 min 5.71 ml/hr 30-60 min 3.33ml/hr 60-120 min 2.36 ml/hr These infusion rates had been derived from the Lowe's theory of the uptake of anaesthetic agent.

33. Salient points to be considered during the maintenance phase The other salient points to be considered during the maintenance phase are the following: a) Leaks must be meticulously sought for and prevented , flows must be adjusted to compensate for the gas lost in the leaks. b) Most of the gas monitors sample gases at the rate of 200 ml/min, which may be sometimes as high as half the FGF. Hence, care must be taken to return the sample back to the circuit to maximize the economy of FGF utilisation.

34. Sevoflurane Controversies Compound A is produced by degradation of sevoflurane in the presence of soda lime or Baralyme, is proven to be a nephrotoxic in rats. As such, it is not a metabolite produced by biotransformation of sevoflurane in the body, but is rather a degradation product generated in the anaesthesia circuit. Changing the composition of the absorbent by eliminating the potassium hydroxide has reduced the formation of compound A to a large extent. Eliminating the NaOH also has made it safer. Amsorb ® plus is now available in India.

35. TERMINATION OF LOW FLOW ANAESTHESIA. Unlike the initiation or the maintenance of the closed circuit, termination is less controversial. There are only two recognized methods of termination of the closed circuit . A. Use of high flow of gases : the circuit is opened and a high flow of gas is used to flush out the anaesthetic agents . B . Use of activated charcoal. Activated charcoal when heated to 220oC adsorbs the potent vapours almost completely. Hence, a charcoal-containing canister with a bypass is placed in the circuit, towards the end of the anaesthesia, the gas is directed through the activated charcoal canister. This results in the activated charcoal adsorbing the anaesthetic agent resulting in rapid recovery and at the same time, reducing theatre pollution. Nitrous oxide, due to its low solubility is washed off towards the end by using 100% oxygen.

36. Emergence Phase In general, 100% O 2  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. A charcoal filter can also be placed in the expiratory limb, which causes a rapid decrease in the concentration of the volatile agent.

37. 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

38. Maintenance The stage of maintenance is marked by 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.

39. Flow Pattern during LFA 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 O 2  and 600 ml of N 2 O in the first 20min and thereafter flow of O 2 500ml and N 2 O 500ml,this state of maintenance states the 33-44% of O 2  concentrations each time.

40. Advantages of LFA system. 1.Enormous financial savings due to use of low fresh gas flows as well as the agent. 2. High humidity in the system leads to fewer post anaesthetic complications. 3.Maintenance of body temperature during prolonged procedures due to conservation of heat. 4. Reduction in the theatre pollution. 5. Environmental Pollution ( Ozone depletion/green house effect)

41. Disadvantages of LFA 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.

42. Undesired Gas effects A matter of concern remains the accumulation of trace gases resulting from the diminution of the wash out effects, may decrease the concentration of nitrous oxide and oxygen in the delivered mixture. That may, for instance, be the case if nitrogen accumulates because of insufficient denitrogenation , or the argon concentration may rise as a result of the use of an oxygen concentrator. Methane, exhaled physiologically by the patient, in high concentrations may compromise anaesthetic gas monitoring, e.g. measurement of halothane concentration. Accumulation of acetone may prolong the emergence from anaesthesia and provoke nausea or vomiting. However, only in the very rare cases of severely ketoacidotic patients does this become clinically relevant. Recently it was revealed, that the volatile anaesthetic agents desflurane, enflurane, isoflurane, and presumably also halothane, are liable to react with absolutely dry carbon dioxide absorbents to generate carbon monoxide . Subsequent recommendations have been made not to use fresh gas flows lower than 5 L min −1  to safely avoid accidental carbon monoxide intoxication resulting from trace gas accumulation. Halothane and sevoflurane , in their part, may react with carbon dioxide absorbents by generating haloalkenes, e.g. 1, bromo-1,chloro-2,2,difluoro-ethylene, (BCDFE), or fluoromethyl-2,2,difluoro-1,trifluoromethyl-vinyl-ether, (Compound A). Compound A is nephrotoxic.

43. Monitoring for the safe performance of LFA Inspiratory oxygen concentration. Airway pressure and minute volume. Anesthetic agent concentration in the circuit. Expiratory CO 2  concentration.

44. Concerns about Safety in LFA  Points of concern in providing safe anaesthesia are: Hypoxia. Misdosage of the volatiles. Exhaustion of the absorbent. Gas volume deficiency. Reduced controllability.

45. Anesthesia Machines & low flow delivery The lower is the flow, the greater the difference between the fresh gas and the gas composition within the breathing system. If the fresh gas flow is lower than 1 L min −1  even experienced anaesthetists may fail to estimate precisely the gas composition within the breathing system from the settings of the fresh gas controls. Furthermore, in a flow range lower than 1 L min −1  the performance of the fine needle valves and the flow meter tubes approaches the limits of accuracy. Older machines are not suitable for this. Newer Plenum vaporizers and newer machines can solve this problem The implementation of advanced computer technology, electronically controlling the fresh gas supply by closed loop feedback according to preset values, will overcome these technical problems. The Dräger company already offers commercially such a machine, featuring electronic control of the anaesthetic gas composition and volume circulating within the rebreathing system.

46. 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 FiO 2 , ETCO 2  and agent monitoring in modern anesthesia workstations which results in effortless and pragmatic conduct of LFA . Anaesthesiologists  must take up LFA as their professional commitment for the present and future generations.

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