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low flow anesthesia

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  2. 2.  Closed circuit anaesthesia with carbon dioxide absorption was used as early as 1850 by John Snow. Because of the high costs and limited availability of anaesthetic agents, early investigators frequently used closed circuit techniques.  Brian Sword first described the circle breathing system and delivery of closed circuit anaesthesia in 1930 . Lack of efficient vaporizers along with poor understanding of the pharmacokinetics of the newly developed halothane led to the resurgence of high flow techniques. DR.Aditi
  3. 3. Contd..  Low flow, partial rebreathing anaesthetic techniques were first described by Foldes and colleagues in 1952 which involved fresh gas flows of 1 l/min.  Virtue, in 1974, reduced gas flows even further in his minimal flow anaesthesia technique 500 ml/min. DR.Aditi
  4. 4. Low flow anaesthesia has various definitions.  Any technique that utilizes a fresh gas flow (FGF) that is less than the alveolar ventilation can be classified as Low flow anaesthesia.  It can also be defined as a technique wherein at least 50% of the expired gases had been returned to the lungs after carbon dioxide absorption. This would be satisfied when the FGF is less than about two litres per minute. DR.Aditi
  5. 5. According to Dorsch :  Low-flow anesthesia has been variously defined as an inhalation technique in which a circle system with absorbent is used with a fresh gas inflow of  Less than the patient's alveolar minute volume,  less than 4 L/minute  3 L/minute or less  0.5 to 2 L/minute  less than 1 or 1.5 L/minute or  0.5 to 1 L/minute or 500 mL/minute DR.Aditi
  6. 6. Baker classified the FGF used in anaesthetic practice into : 1. Medium flow : 1 - 2 l/min. 2. Low flow : 500- 1000 ml/min. 3. Minimal flow : 250-500 ml/min. 4. Metabolic flow : about 250 ml /min For most practical considerations, utilisation of a fresh gas flow less than 2 litres/min may be considered as low flow anaesthesia. DR.Aditi
  7. 7.  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 soda lime.  No gas escapes out of the circuit and would provide for maximal efficiency for the utilisation of the fresh gas flow.  The very nature of this system requires that the exact amount of anaesthetic agent taken up by the body be known, since that exact amount has to be added into the circuit. DR.Aditi
  8. 8. Contd…  Any error in this could lead to potentially dangerous level of anaesthetic agent be present in the inspired mixture with its attended complications. So we have to be cautious while using low flow anaesthesia. DR.Aditi
  9. 9. 1. Circle rebreathing system with CO2 absorption 2. Accurate flow meters for adjustments of FGF below 1L/min 3. Gas tight breathing system. Recommended test leakage should be below 150 mL/min at 30 cm H2O test pressure. DR.Aditi
  10. 10. CONTD.. 5. The breathing system should have minimal internal volume and a minimum number of components and connections. 6. Continuous gas monitoring MUST be employed. From the clinical standpoint the measurement of expiratory gas concentrations close to the Y-piece is of crucial importance. That information is essential in controlling the patient’s alveolar gas concentrations, whereas Fi reflect the adequacy of gas concentrations into the breathing circuit. DR.Aditi
  11. 11. Contd..  7.VAPOURIZERS :  Calibrated vaporizers - Vaporizers capable of delivering high concentrations and that are accurate at low fresh gas flows are required.  Liquid injection - Anesthetic liquid can be injected directly into the expiratory limb  In circle vaporizer - In-system vaporizers have been used successfully with both spontaneous and controlled ventilation DR.Aditi
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  13. 13. Monitoring  Inspired O2 concentration should be monitored at all times if N2O is used in more than 65% concentration, as one of the adjuvant gas.  EtCO2 monitoring seems to be necessary to ensure proper functioning of the absorber.  If monitoring of end tidal anaesthetic concentration is available, the administration of low flow anaesthesia becomes very easy. DR.Aditi
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  15. 15. HOW TO ADJUST FGF AT DIFFERENT PHASES OF LFA Premedication, pre-oxygenation and induction of sleep are performed according to the usual practice. Concerning adjustment of FGF anesthesia can be divided into 3 phases: 1. Initial HIGH flow 2. Low flow 3. Recovery DR.Aditi
  16. 16. 1.Initial HIGH flow phase  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.  The factors that can influence the build up of alveolar concentration should all be considered while trying to reach the desired alveolar concentration. (MAC)  DR.Aditi
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  18. 18. • Use of high flows for a short time • Prefilled circuit • Use of large doses of anaesthetic agents. Injection techniques :The exact dose to be used is calculated thus: Priming dose (ml vapour) = Desired concentration x {( FRC + Circuit volume) +( Cardiac output x BG Coeff.)} DR.Aditi
  19. 19.  An alternative method for administering the large amounts of the agents is by directly injecting the agent into the circuit.  This is an old, time-tested method and is extremely reliable. Each ml of the liquid halothane, on vaporisation 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. The high volatility coupled with the high temperature in the circle results in instantaneous vaporisation of the agent. DR.Aditi
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  21. 21.  The intermittent injections are often made in 0.2- 0.5 ml aliquots manually.  Doses should never exceed 1ml at a time. Doses exceeding 2 ml bolus invite disaster.  Intermittent injections can often be easily substituted with a continuous infusion with the added advantage of doing away with the peaks and troughs associated with intermittent injections. DR.Aditi
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  23. 23.  This priming dose is the dose required to bring the circuit volume + FRC to the desired concentration and is injected over the first few minutes of the closed circuit anaesthesia.  Besides this, an amount of agent necessary to compensate for the uptake of the body must also be added and this is calculated depending on the uptake model being used. DR.Aditi
  24. 24. 2. Low flow phase After the high flow phase of 5-15 min, or when the target gas concentrations has been achieved FGF can be reduced at the desired low flow level.  The lower the FGF the greater the difference between the vaporizer setting and inspired concentration of the anesthetic agent in the breathing circuit will be. With low FGF, time to reach the desired concentration in the inspiratory gas will be prolonged. Hence, monitoring of oxygen and anesthetic agent concentration is essential and necessary in LFA. DR.Aditi
  25. 25.  If the flow provided is too small for the patient’s needs the bellow will gradually go down, down down... DR.Aditi
  26. 26. THE MAINTENANCE OF LOW FLOW ANAESTHESIA  This phase is characterised by 1. Need for a steady state anaesthesia often meaning a steady alveolar concentration of respiratory gases. 2. Minimal uptake of the anaesthetic agents by the body. 3. Need to prevent hypoxic gas mixtures. DR.Aditi
  27. 27.  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. Management of the oxygen and nitrous oxide flow during the maintenance phase DR.Aditi
  28. 28. Constant circuit volume :  Constant reservoir bag size : if the bag decreases in size. The fresh gas flow rate is increased. If the bag increases in size, the flow is decreased.  Ventilator with Ascending bellows: constant volume can be achieved by adjusting the fresh gas flow so that bellows is below the top of its housing at the end of exhalation.  Ventilator with Descending bellows: the fresh gas flow so that bellows just reaches the bottom of its housing at the end of exhalation. DR.Aditi
  29. 29. Important Equations :  VO2 = 10 × BW KG ^ ¾ BRODY EQUATION  V N20 = 1000 × t ^ ½ SEVERINGHAUS EQUATION  V AN = desired concentration × λB/G × Q × t − 1/2 LOWE EQUATION DR.Aditi
  30. 30.  Time constant :  The time constant is a measure for the time it takes, that alterations of the fresh gas composition will lead to corresponding alterations of the gas composition within the breathing system. According to a formula given by Conway the time constant (T) can be calculated by the division of the system's volume (VS) by the difference between the amount of anaesthetic agent delivered into the system with the fresh gas (VD) and the individual gas uptake (VU):  T = VS / (VD - VU) DR.Aditi
  31. 31.  The Gothenburg Technique :  Initially high flows, oxygen at 1.5 l/min and nitrous oxide at 3.5 l/min had to be used for a period of six minutes after the induction of anaesthesia and this constitutes the loading phase.  This is followed by the maintenance phase in which the oxygen flow is reduced to about 4ml/kg  And nitrous oxide flow adjusted to maintain a constant oxygen concentration in the circuit.  The usual desired oxygen concentration is about 40%. DR.Aditi
  32. 32. Management of the potent anaesthetic agents during maintenance phase 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. DR.Aditi
  33. 33. They had approximated isoflurane 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, they had suggested that the above said 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. DR.Aditi
  34. 34. Category High flow 5L/Min Inter Mediate 2.5 l/min Low flow 1 l/min Minimal Flow 0.5 l/min Closed circuit 0.3 – 0.5 l/min Percentage rebreathing 0 % 52% 86% 97% 100% Circuit concentratio n with vaporizer set to 1% (at 1 h) 1 % 0.94% 0.68% 0.54% N2O 0.52% NO N2O 0.39 % Vaporizer setting for in inspired concentratio n of 1% ( at 1 h) 1% 1.1% 1.4% 2.0% N2O 2.4% NO N20 3.2% DR.Aditi
  35. 35. Low flow with Dräger machine :  With the connection of the patient to the anaesthesia machine, an initial phase of high fresh gas flow (4-6 L/min) follows. During this initial phase, adequate denitrogenation and distribution of anaesthetic gases in the desired concentration throughout the system is achieved and the desired level of anaesthesia is attained.  The duration of the initial phase is determined by the amount of flow reduction and the individual gas uptake (4-5 L/min, 6-8 min). DR.Aditi
  36. 36.  After this time, oxygen and nitrous oxide levels reach 30% O2 and 65% N2 O, respectively . If isoflurane is used, the vaporizer is set to 1.5%; enflurane and sevoflurane require 2.5% setting and desflurane requires a 4-6% setting.  After 6-8 min, an expiratory concentration corresponding to 0.8 MAC of the respective anaesthetic agent is attained. At an additive nitrous oxide concentration of 50-60%, this value approximately corresponds to AD95, or agent concentration at which 95% of patients tolerate a skin incision without noticeable reaction.  After 10 min, the total gas uptake of an adult patient is about 600 ml/min, so that at this time, the flow can be reduced to 1.0 L/min and low flow anaesthesia can be initiated. DR.Aditi
  37. 37.  An inspiratory oxygen concentration level of 30% can only be maintained if the fresh gas oxygen concentration is increased to 50% once the flow has been reduced.  Flow reduction also decreases the amount of anaesthetic agent, which is introduced into the fresh gas flow by the vaporizer.  If the initially selected anaesthetic agent concentration of 0.8 × MAC is to be maintained, the vaporizer must be adjusted for 3.0%.  For desflurane, no changes are necessary.  Before reducing fresh gas flow rates to even lower levels, i.e., 0.5 L/min, to perform minimal flow anaesthesia, the initial phase should be extended to 15 min and for very large patients to 20 min. DR.Aditi
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  39. 39.  Because the rebreathing fraction is increased compared with low flow anaesthesia, oxygen concentration should be increased to 60%; 50% represents an absolute minimum value.  An expiratory anaesthetic agent concentration of 0.8 × MAC should be maintained with appropriate dial setting.  In low flow anaesthesia, the composition of anaesthetic gases changes continuously during the course of anaesthesia, making intermittent adjustments to fresh gas composition necessary. DR.Aditi
  40. 40. Precautions :  If a rapid change in any component of inspired mixture is desired, FGF should be increased.  If integrity of circle is broken high flows should be used for several minutes before returning to low flow.  If closed anaesthesia system is used its recommended that high flow be used for 1-2 minutes every hour to eliminate gases such as nitrogen and carbon monoxide. DR.Aditi
  41. 41. 3. Emergence phase At the end of anesthesia high FGF, usually 100% O2, is necessary, to facilitate the washout of the anesthetic agent from the patient and to remove the agent to the scavenging system. A charcoal filter can be placed in expiratory limb. It will cause a rapid decrease in volatile agent concentration. DR.Aditi
  42. 42. There are only two recognised methods of termination of the closed circuit. They are as follows: 1 )Towards the end of the anaesthesia, the circuit is opened and a high flow of gas is used to flush out the anaesthetic agents which accelerates the washout of the anaesthetic agents. This has the obvious advantage of simplicity but would result in wastage of gases. 2 )The second method is the use of activated charcoal. Activated charcoal when heated to 220oC adsorbs the potent vapours almost completely. DR.Aditi
  43. 43. Advantage :  Economic Benefits  Reduce Operating Room Pollution  Environmental Benefits  Estimation Of Anesthetic Agent Uptake And Oxygen Consumption  Buffered Changes In Inspired Conc.  Heat And Humidity Conservation  Less Danger Of Barotrauma  Quality Of Patient Care DR.Aditi
  44. 44. Disadvantage : More Attention Required Inability to Quickly Alter Inspired Concentrations Danger of Hypercarbia Accumulation of Undesirable Gases in the System(CO, Methane, Acetone, Hydrogen, Ethanol, Compound A, Argon, Nitrogen) Uncertainty about Inspired Concentrations Faster Absorbent Exhaustion DR.Aditi
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  46. 46.  Many new anesthesia machines have been developed in Europe within the past few years. One of these is the Physioflex machine . Another is Drager Zeus.  These machines have an inbuilt algorithm to calculate the uptake and adjust concentration and volume accordingly. DR.Aditi
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