Low flow anaesthesia

Low flow anaesthesia

1. Low Flow Anaesthesia Dr. ZIKRULLAH

2. Introduction John Snow- used caustic potash to absorb CO 2 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 CO 2 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 H 2 O

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 Use of high flows for a short time : loading FGF of 4-6 L/mt for around 10 mts with 40% O 2 and 60% N 2 O 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 N 2 O 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 N 2 Oof 3-5 l/mt in the beginning FiO 2 kept around 30-40% by reducing N 2 O. 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 O 2 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 O 2 conc by 50 %

20. To guarantee safe inspiratory oxygen concentration: Increase O2 conc. with low flow Monitor inspiratory O2 concentration If inspiratory O 2 falls below 30%  increase O 2 conc by 10 % of total flow & decrease N 2 O by 10%

21. Gothenburg Technique Loading phase with 1.5 l/mt O 2 & 3.5l/mt N 2 O for 6mts. Maintanence phase : O 2 @ 4ml/kg & N 2 O to maintain O 2 >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 anaesthesia

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 O 2 , 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 O 2 + 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 : N 2 O  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 ,N 2 ,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 CO 2 absorption