Low flow anaesthesia
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Low flow anaesthesia
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Low flow anaesthesia
- 2. Introduction John Snow- used caustic potash to absorb CO2 from expired gases. Waters introduced To & Fro System Sir Brian Sword – Circle system Term “Low Flow Anaesthesia” –by F. Foldes
- 3. Definition: Baum et al: Where at least 50% of the expired gases had been returned to the lungs after carbon dioxide absorption. Baker : Metabolic flow : about 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/min Very high flow >4l/min
- 4. The need for low flow anaesthesia. Completely closed circuit anaesthesia?? Advantage of low flow anaesthesia over closed circuit anaesthesia??
- 5. Technical requirements for safe conduct of low flow anaesthesia:
- 6. Monitors needed: Inspired oxygen concentration End tidal CO2 concentration End tidal anaesthetic agent concentration. Airway pressures and minute volume monitors Other requirements: Accurate flow meters for adjustment of FGF. Leaks should not exceed 100ml/mt at 20 cm H2O
- 7. The Practice Of Low Flow Anaesthesia: 1. Initiation of Low flow anaesthesia 2. Maintenance of Low flow anaesthesia 3. Termination of Low flow anaesthesia
- 8. Initiation phase: Aim:To achieve 1.3 MAC of anaesthetic Factors affecting build up of alveolar concentration
- 9. Time Constant Time for changes in the composition of the fresh gas to lead to corresponding changes in the composition of the gas in the anaesthetic system
- 10. Methods to achieve desired gas and agent concentration 1) Use of high flows for a short time : loading FGF of 4-6 L/mt for around 10 mts with 40% O2 and 60% N2O Vapouriser settings: 1- 1.5% Isoflurane 2-2.5% Sevoflurane 4-6% Desflurane.
- 11. After 10 mts Expired MAC of 0.8 of anaesthetic agent. 0.8MAC of anaesthetic agent + 0.6 MAC N2O 1.3 to 1.4 MAC(ED 95 to prevent reflex movement to skin incision)
- 12. After 10 mts Expired MAC of 0.8 of anaesthetic agent. + 0.6 MAC N2O 1.3 to 1.4 MAC(ED 95 to prevent reflex movement to skin incision
- 13. . Advantages : Rapidity in achievement of desired concentration Unexpected rise in agent concentration prevented Can use the commonly available vaporisers Can achieve better denitrogenation. Avoiding gas volume deficiency
- 14. Disadvantages : Compromise on the economy Need for scavenging systems
- 15. 2) Prefilled Circuit: 3) Use of large dose of anaesthetic agent: FGF with metabolic flow of oxygen and N2Oof 3-5 l/mt in the beginning FiO2 kept around 30-40% by reducing N2O. Disadvantage: Hypoxia if some errors occur
- 16. Can the duration of the initial phase be shortened? After 10 mts uptake is about 550 ml. If FGF reduced to 500 ml by 10mts gas volume deficiency occurs.
- 17. Shortening of initial phase by: Accelerating the denitrogenation and the wash-in phase. Using inhalational anaesthetic agent with low blood solubility and low individual uptake. Accelerating anaesthetic depth. Stepwise reducing the fresh gas flow rate: 2l/mt after 5mts ,then to 1l/mt after 10 mts & 0.5l/mt after 15 mts
- 18. Aims during maintenance phase of low flow anaesthesia: Maintain steady alveolar concentration of anaesthetic agent. Minimal uptake of anaesthetic agent by body. Prevent administration of hypoxic mixtures.
- 19. Low flow phase/Maintanence phase When fresh gas flow reduced to 1 L/mt increase inspired O2 conc to 50% to avoid hypoxia Vapouriser setting: Sevoflurane -3% Isoflurane - 2% With this expiratory conc of 0.7 – 0.8 MAC of anaesthetic agent is maintained To maintain inspiratory oxygen concentration during low flow anaesthesia increase fresh gas O2 conc by 50%
- 20. To guarantee safe inspiratory oxygen concentration: Increase O2 conc. with low flow Monitor inspiratory O2 concentration If inspiratory O2 falls below 30% increase O2 conc by 10 % of total flow & decrease N2O by 10%
- 21. Gothenburg Technique Loading phase with 1.5 l/mt O2 & 3.5l/mt N2O for 6mts. Maintanence phase : O2 @ 4ml/kg & N2O to maintain O2 >40%. Oxygen analyser is mandatory.
- 22. Other features to look for during maintenance phase Leaks if any Gas monitors : sample at 200ml/mt care to return the sample back to circuit to maximise economy of FGF utilisation.
- 23. Emergence phase: Close vapouriser prior to end of surgery due to long time constant. Here vapourisers closed after 2hrs of
- 24. Advantages of low flow anaesthesia without nitrous oxide: Performance of LFA simple and easy Initial phase short, increased excess gas volume, reduced risk for gas volume deficiency No contraindications for oxygen / air mixture Pressure within any air confinement remains constant Carrier gas ecologically safe Long lasting abdominal surgery
- 25. Minimal Flow Anaesthesia without nitrous oxide? Premedication and induction as routine. Additive iv opioids for analgesia Initial high flow phase: 4 L/min flow with 1 L/min O2, 3 L/min air . Vaporisers : 1.0-1.1 xMAC in end tidal gas Isoflurane to 2.5 vol%, Sevoflurane to 3.5 vol% Desflurane to 6.0 vol%.
- 26. Flow reduced to 0.5 l/mt after 10 mts. 0.3 L/min O2 + 0.2 L/min air. Vaporisers settings: Isoflurane to 5.0 vol%, Sevoflurane to 5.0 vol% Desflurane to 8.0 vol%. Expiratory anaesthetic concentration in the range of 1.0-1.1 MAC can be maintained
- 27. Anaesthetic machine prerequisites Calibraton of flowmeters to low flow range. Vapouriser –Temperature,pressure & flow compensated Leakage –not >100ml/mt.
- 28. Characteristics of Low Flow Anaesthesia Increased rebreathing volume. Less excess gas. Difference of gas composition – Fresh gas versus gas in the circuit. Long time constants
- 29. Concerns about Safety in Low Flow Anaesthesia Hypoxia Gas volume deficiency Misdosage of volatiles Reduced controllability Exhaustion of the absorbent
- 30. Advantages of low flow anaesthesia: 1) Economic. Comparison of cost for 2 hour inhalational anaesthesia with sevoflurane
- 31. 2) Decrease environmental pollution : N2O green house effect Halothane,Enflurane & Isoflurane – contain chlorine - cause ozone depletion Desflurane & Sevoflurane – do not contribute to green gouse effect. 3) Improved anaesthetic gas climate – due to conservation of heat and humidity
- 32. Efficiency of inhalational anaesthesia: Efficiency = Amount of agent taken by patient (Vu ) Amount of agent delivered into breathing system (Vd) With high FGF Vd increases & efficiency decreases
- 33. Disadvantages of low flow techniques: Capital investment for absorber breathing systems. Dependence on gas monitoring. Increased consumption of absorbent at low flows. Accumulation of unwanted gases in breathing system.
- 34. Limitations and contraindications of low flow anaesthesia: Low flow anaesthesia not suitable for: Short term anaesthesia with a face mask. Procedures with imperfectly gas-tight airways. Equipment with a high gas leakage. Inadequate monitoring Uncompensated diabetic states Intoxicated with alcohol CO poisoning
- 35. Drawbacks….. Accumulation of unwanted gases in breathing system Substances exhaled by patient – Alcohol, Acetone, CO, Methane Contaminants of medical gases – CO ,N2O ,N2 ,Argon Products of reaction with absorbents- Compound A Danger of hypoxia and hypercapnia. Inability to quickly change inspired gas mixture.
- 36. Future development: Liquid injection vapourisers – for more appropriate dosing of volatile agents. Non chemical systems for CO2 absorption
Editor's Notes
- Time for changes in the composition of the fresh gas to lead to corresponding changes in the composition of the gas in the anaesthetic system. At the end of 1 x T: 67 % change in concentration At the end of 3 x T : 95% change in concentration
- Duration of the initial phase : usually 3 time constant -time for desired anaesthetic gas composition to be established within the entire gas containing system, an adequate depth of anaesthesia must be reached, and denitrogenation must be completed
- After 10 mts gas uptake is about 570mL/min of gas. At that time the fresh gas flowrate can be reduced to 1.0 L/min without any problems. If the flow rate, however, would be reduced to 0.5 L/min already at that time, then more gas would be extracted from the breathing system than being supplied to it. The resulting gas volume deficiency in the system would lead to changes in ventilation pattern. Output of the vaporisers is limited. Even when an isoflurane or an enflurane vaporiser, for instance, is fully opened, at a fresh gas flow rate of 0.5 L/min not more than 25mL/min agent vapour can be supplied to the breathing system. When individual uptake is high this small amount of anaesthetic agent will not meet the needs to maintain an adequate depth of anaesthesia, particularly if a relatively high anaesthetic concentration is desired.
- Accelerating the denitrogenation and the wash-in phase with high fresh gas flow of 8-12L/min. Using inhalational anaesthetic agent with low blood solubility and low individual uptake. Accelerating anaesthetic depth through initially setting a high fresh gas concentration of the anaesthetic agent. Stepwise reducing the fresh gas flow rate. reducing the flow to 2 L/min after the initial 5minutes, to 1 L/min after 10 minutes and, finally, to 0.5 L/min after 15minutes
- Small amounts of the anaesthetic gases to match the uptake and providing oxygen for the basal metabolism should suffice. 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. For most practical purposes, in the absence of oxygen analyser the following technique is safe to use. 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 %
- The oxygen concentration of the fresh gas must be adequately increased when the flow rate is reduced(Low Flow Anaesthesia: 40-50 vol.% O2, Minimal Flow Anaesthesia: 50-60 vol.% O2). • The inspiratory oxygen concentration must be monitored continuously, and the lower alarm limit set to the nominal inspiratory value. If the inspiratory O2-concentration falls below 30 vol %, the O2-flow must be increased by l0% of the total gas flow, and the N2O-flow be reduced by the same value. Thus, during Low Flow Anaesthesia the O2-flow must be increased by 100mL/min and by 50mL/min during Minimal Flow Anaesthesia each time that the alarm limit is reached. The nitrous oxide flow must be reduced accordingly
- oxygen analyser is mandatory. since the nitrous oxide added is directly based on its readings and hence any errors would be dangerous.
- Leaks must be meticulously sought for and prevented since they would decrease the efficacy of the system. 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 maximise the economy of FGF utilisation. Some gas analysers like Ohmeda Rascal add air to the sample exhaust. This if returned to the circuit would result in dilution of the anaesthetic mixture and accumulation of nitrogen within the circuit and hence should be vented. This mandates utilisation of a flow adequate to compensate for this loss
- Due to the long time constants, the vaporiser can be closed distinctly prior to the definite end of the surgical procedure. If the low fresh gas flow is kept unchanged, the time required for the wash-out process increases with the extent of flow rate reduction . The decline of the concentration of the respective anaesthetic, given in vol%, seems to be faster if the less soluble agents like sevoflurane or desflurane used. During Minimal Flow Anaesthesia the vaporiser can be closed about 10minutes before the end of surgery -or about 20minutes before in prolonged anaesthesia. In the time that follows the patient is weaned on to spontaneous breathing via manually-assisted ventilation whilst maintaining low flow . Not earlier than about 5 to 10minutes before the definitive extubation the nitrous oxide supply is ceased and the oxygen flow increased to about 5 L/min to wash out the anaesthetic gases.
- Omitting the use of nitrous oxide consideralby simplifies the performance of low flow anaesthesia. Even at a fresh gas flow rate of 0.5 L/min the duration of the initial high flow phase can be kept at its minimum. It is only determined by the specific wash-in characteristic of the respective inhalation anaesthetic used. Neither denitrogenation or nitrous oxide wash-in, nor the initially high nitrous oxide uptake have to be considered as prolonging factors for his period any more. Thus, even in Minimal Flow Anaesthesia an initial high flow phase of only 10minutes sufficiently meets the needs (Fig. 15.1). The gas volume circulating within the breathing system increases as only oxygen and a negligibly small amount of anaesthetic agent vapour are still taken up by the patient. At a given fresh gas flow rate, the total gas uptake is less and the excess gas volume higher than with the use of nitrous oxide, resulting in reduced risk of gas volume deficiency
- Due to the fact that the gas uptake is less and the excess gas volume higher than with the use of nitrous oxide the flow can be reduced to 0.5 L/min early, already after 10minutes. Limiting factor for further shortening of the initial high flow phase remains the need to initially deliver a suitable high amount of inhalation anasthetic into the breathing system to establish a sufficient anaesthetic depth. Expiratory concentration of the volatile anasthetics should be increased by 0.2-0.25 times the MAC of the respective inhalation anaesthetic. Flow reduction to 0.5 L/min: with flow reduction the rebreathing fraction, containing less oxygen, increases demanding a considerable increase of the fresh gas oxygen content to about 68%. A mixture of 0.3 L/min O2 and 0.2 L/min air is needed to maintain an inspiratory oxygen concentration of about 40 vol%
- Flow reduction to 0.5 L/min: with flow reduction the rebreathing fraction, containing less oxygen, increases demanding a considerable increase of the fresh gas oxygen content to about 68%. A mixture of 0.3 L/min O2 and 0.2 L/min air is needed to maintain an inspiratory oxygen concentration of about 40 vol%. If the use of nitrous oxide is omitted the hypnotic state during emergence phase mainly is maintained by the inhalation anaesthetic. Therefore, to prevent early awakening of the patient, even if the low flow rate is maintained the delivery of anaesthetic agent may not be stopped earlier than 5-10minutes prior to the definite end of the surgical procedure.
- The leakage rate of the rebreathing systems must not exceed 100 mL/min at an internal system pressure of 20mbar
- Environmental Improvement of anaesthetic gas climate: Inspiratory cool dry gas impairs mucociliary function with subsequent microatelectasis,potential for infection and impaired gas exchange .Low fresh gas flow conserves heat and humidity & improve inspired gas humidification and temp thus improves anaesthetic gas climate. Sevoflurane or desflurane are both characterized - on the one hand - by low solubility and correspondingly low individual uptake, and - on the other hand - by comparatively low anesthetic potency andcorrespondingly high anaesthetic concentrations to beapplied. If such agents are used with high fresh gas flow, the efficiency becomes extremely small. From economical reasons, the use of such anaesthetics only can be justified with low flow anaesthesia
- Procedures with imperfectly gas-tight airways (i.e. bronchoscopies with a rigid bronchoscope); Use of technically unsatisfactory equipment with a high gas leakage; Inadequate monitoring (i.e. malfunction of the oxygen measuring device
- Substances exhaled by patient- alcohol , acetone, CO & methane.