Breathing systems (2)

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  • REQUIREMENTS OF A BREATHING SYSTEM: The components when assembled should satisfy certain requirements, some essential and others desirable. Essential: The breathing system must a) deliver the gases from the machine to the alveoli in the same concentration as set and in the shortest possible time; b) effectively eliminate carbon-dioxide; c) have minimal apparatus dead space; and d) have low resistance. Desirable: The desirable requirements are a) economy of fresh gas; b) conservation of heat; c) adequate humidification of inspired gas; d) Light weight; e) convenience during use; f) efficiency during spontaneous as well as controlled ventilation (Efficiency is determined in terms of CO2 elimination and fresh gas utilization); g) adaptability for adults, children and mechanical ventilators; h) provision to reduce theatre pollution.
  • CLASSIFICATION OF BREATHING SYSTEMS: One will realise the reason for the failure of the attempts at classification in the 50's to 60's, if this definition and requirements are taken into account. There are numerous classifications of breathing systems according to the whims and fancy of the person classifying. Many of them are irrelevant as they do not define a breathing system. Different authors classified the same system under different headings, adding to confusion1. McMohan in 1951 classified them as open, semiclosed and closed taking the level of rebreathing into account. It as follows: Open no rebreathing Semiclosed partial rebreathing Closed total rebreathing Dripps et al have classified them as Insufflation, Open, Semiopen, Semiclosed and Closed taking into account the presence or absence of Reservoir, Rebreathing, CO2 absorption and Directional valves1. The ambiguity of the terminology used as open, semi open, semi closed and closed allowed inclusion of apparatus that are not breathing systems at all into the classification. To overcome this problem Conway2 suggested that a functional classification be used and classified according to the method used for CO2 elimination as: 1. Breathing systems with CO2 absorber and 2. Breathing systems without CO2 absorber. Miller.D.M.3 in 1988 widened the scope of this classification so as to include the enclosed afferent reservoir system. A new breathing system called 'The Maxima'4 has been designed by Miller in 1995 and to include it in the classification5, the enclosed afferent reservoir systems have been grouped under 'displacement afferent reservoir' systems. This classification also has a personal bias as the Humphrey ADE system is not included in the classification, even though he preferred to compare his system with that of Humphrey's6. The classification suggested in table.1. is a partial modification of Miller's3 classification.
  • Bi-Directional Flow: Systems with bi-directional flow are extensively used. These systems depend on the FGF for effective elimination of CO2. Understanding these systems is most important as their functioning can be manipulated by changing parameters like Fresh gas flow, alveolar ventilation, apparatus dead space, etc. We will analyze these in detail.
  • Fresh Gas Supply; Fresh gas flow (FGF) forms one of the essential requirements of a breathing system. If there is no FGF into the system, the patient will get suffocated. If the FGF is low, most systems do not eliminate carbon-dioxide effectively, and if there is an excess flow there is wastage of gas. So, it becomes imperative to specify optimum FGF for a breathing system for efficient functioning. If the system has to deliver a set concentration in the shortest possible time to the alveoli, the FGF should be delivered as near the patient's airway as possible.
  • Elimination Of Carbon-Dioxide: The following may be taken as an example for better understanding of CO2 elimination by the bi-directional flow systems. Normal production of carbon-dioxide in a 70 kg adult is 200 ml per minute and it is eliminated through the lungs. Normal end-tidal concentration of carbon-dioxide is 5%. Hence, for eliminating 200 ml of carbon-dioxide as a 5% gas mixture, the alveolar ventilation has to be: 200 x 100 = 4,000 ml. 5 This 4000 ml or 4 litres is the normal alveolar ventilation. Any breathing system connected to an adult's airway should provide a minimum of 4 litres per minute of carbon-dioxide free gas to the alveoli for eliminating carbon-dioxide. If the alveolar ventilation becomes less than 4 litres per minute, it would lead to hypercarbia. If the alveoli are ventilated with 5 litres/minute of a gas containing 1% carbon-dioxide, or 8 litres/minute of a gas containing 2.5% carbon-dioxide, it could still eliminate 200 ml of carbon-dioxide per minute from the alveoli. It may be construed as 4 litres of CO2 free gas and 1 litre of gas with 5% CO2 in the first instant and as 4 litres of CO2 free gas and 4 litres of gas with 5% CO2 in the second instant. In effect, 4 litres of alveolar ventilation with CO2 free gas is provided in both cases.
  • If the alveolar ventilation becomes less than 4 litres per minute, it would lead to hypercarbia. If the alveoli are ventilated with 5 litres/minute of a gas containing 1% carbon-dioxide, or 8 litres/minute of a gas containing 2.5% carbon-dioxide, it could still eliminate 200 ml of carbon-dioxide per minute from the alveoli. It may be construed as 4 litres of CO2 free gas and 1 litre of gas with 5% CO2 in the first instant and as 4 litres of CO2 free gas and 4 litres of gas with 5% CO2 in the second instant. In effect, 4 litres of alveolar ventilation with CO2 free gas is provided in both cases.
  • Apparatus Dead Space: It is the volume of the breathing system from the patient-end to the point up to which, to and fro movement of expired gas takes place. The dynamic dead space will depend on the FGF and the alveolar ventilation. The dead space is minimal with optimal FGF. If the FGF is reduced below the optimal level, the dead space increases and the whole system will act as dead space if there is no FGF. Increasing the FGF above the optimum level will only lead to wastage of FG
  • The afferent limb is that part of the breathing system which delivers the fresh gas from the machine to the patient. If the reservoir is placed in this limb as in Mapleson A, B, C and Lack's systems, they are called afferent reservoir systems (ARS).
  • The afferent limb is that part of the breathing system which delivers the fresh gas from the machine to the patient. If the reservoir is placed in this limb as in Mapleson A, B, C and Lack's systems, they are called afferent reservoir systems (ARS). The efferent limb is that part of the breathing system which carries expired gas from the patient and vents it to the atmosphere through the expiratory valve/port. If the reservoir is placed in this limb as in Mapleson D, E, F and Bain systems, they are called efferent reservoir systems (ERS). Enclosed afferent reservoir system has been described by Miller and Miler.
  • The Mapleson D, E, F and Bain systems have a 6 mm tube as the afferent limb that supplies the FG from the machine. The efferent limb is a wide-bore corrugated tube to which the reservoir bag is attached and the expiratory valve is positioned near the bag. In Mapleson E system, the corrugated tube itself acts as the reservoir (Fig.8). In Bain system, the afferent and efferent limbs are coaxially placed (Fig.9).
  • All these ER systems are modifications of Ayre's T-piece. This consists of a light metal tube 1 cm in diameter, 5 cm in length with a side arm (Fig.10). Used as such, it functions as a non-rebreathing system. Fresh gas enters the system through the side arm and the expired gas is vented into the atmosphere and there is no rebreathing. The dead space is minimal as it is only up to the point of FG entry and elimination of CO2 is achieved by breathing into the atmosphere. FGF equal to peak inspiratory flow rate of the patient has to be used to prevent air dilution.
  • In an attempt to reduce FGF requirements, ER systems are constructed with reservoirs in the efferent limb. The functioning of all these systems are similar. These systems work efficiently and economically for controlled ventilation as long as the FG entry and the expiratory valve are separated by a volume equivalent to atleast one tidal volume of the patient. They are not economical during spontaneous breathing.
  • Factors that influence the composition of gas mixture in the corrugated tube with which the patient gets ventilated are the same as for spontaneous respiration namely FGF, respiratory rate, tidal volume and pattern of ventilation. The only difference is that these parameters can be totally controlled by the anaesthesiologist and do not depend on the patient. Using a low respiratory rate with a long expiratory pause and a high tidal volume, most of the FG could be utilized for alveolar ventilation without wastage.
  • Analyzing the performance of these systems during controlled ventilation, two relationships have become evident. 1) When FGF is very high the PaCO2 becomes ventilation dependent (as during spontaneous respiration). 2) When the minute volume exceeds the FGF substantially, the PaCO2 is dependent on the FGF17. Combining these influences a graph can be constructed as shown in Fig.13. An infinite number of combinations of FGF and minute ventilation can be chosen to achieve a desired PaCO2. One can use a high FGF and a normal minute volume of 70 ml/kg to achieve a normal PaCO2 of 40 mm Hg. This is uneconomical and leads to low humidity and heat loss. Alternately, a FGF equivalent to the predicted minute volume i.e., 70 ml/kg can be chosen and the patient ventilated with at least twice the predicted minute volume i.e. 140 ml/kg. Here a deliberate controlled rebreathing is allowed in order to maintain normal PaCO2 along with high humidity, less heat loss and greater economy of fresh gas. Combinations between these two extremes can also be used. It is important to remember that using a low FGF with normal minute ventilation, can lead to hypercarbia; a moderate FGF and hyperventilation, can lead to hypocarbia.
  • a sodalime canister, (2) Two unidirectional valves, (3) Fresh gas entry, (4) Y-piece to connect to the patient, (5) Reservoir bag (6) a relief valve and (7) low resistance interconnecting tubing. (1) There should be two unidirectional valves on either side of the reservoir bag, (2) Relief valve should be positioned in the expiratory limb only, (3) The FGF should enter the system proximal to the inspiratory unidirectional valve.
  • A) Economy: The FGF could be reduced to as low as 250 - 500 ml of oxygen. The consumption of Halothane/Isoflurane has been found to be around 3.5 ml/hour19. b) Humidification: In the completely closed system, once the equilibrium has been established, the inspired gas will be fully saturated with water vapour20. C) Reduction of heat loss: In addition to conserving water the totally closed system will also conserve heat. The CO2 absorption is an exothermic reaction and the system may actively assist in maintaining body temperature. D) Reduction in atmospheric pollution: Once the expiratory valve has been closed, no anaesthetic escapes, except for the small percutaneous loss from the patient.
  • E) Control of anaesthesia: It is possible to compute the time course of uptake of anaesthetic in a patient of known size and add the appropriate quantity of the anaesthetic to the circuit at a rate decreasing in a manner calculated to maintain a constant alveolar concentration21. In practice an alveolar concentration of about 1.3 x MAC is found to be suitable. The technique has several potential disadvantages. i) A greater knowledge of uptake and distribution is required to master closed circuit anaesthesia. ii) Inability to alter any concentration quickly. iii) Real danger of hypercapnia may result from, a) an inactive absorber, B) incompetent unidirectional valves and c) incorrect use of absorber bypass.
  • Breathing systems (2)

    1. 1. Breathing Systems Dr. P. Narasimha Reddy Professor Dept. of Anaesthesiology 6 March 2011
    2. 2. Definition <ul><li>A breathing system is defined as an assembly of components which connects the patient’s airway to the anaesthetic machine creating an artificial atmosphere, from and into which the patient breathes. </li></ul>6 March 2011
    3. 3. <ul><li>Purpose </li></ul><ul><ul><li>To deliver anesthetic gases and oxygen </li></ul></ul><ul><ul><li>Offer a means to deliver anesthesia without significant increase in airway resistance </li></ul></ul><ul><ul><li>To offer a convenient and safe method of delivering inhaled anesthetic agents </li></ul></ul><ul><ul><li>To annoy you with yet one more thing to memorize </li></ul></ul>6 March 2011
    4. 4. Components <ul><li>A fresh gas entry port / delivery tube </li></ul><ul><li>A port to connect it to the patient's airway; </li></ul><ul><li>A reservoir for gas, in the form of a bag </li></ul><ul><li>An expiratory port / valve </li></ul><ul><li>A carbon dioxide absorber if total rebreathing is to be allowed </li></ul><ul><li>Corrugated tubes for connecting these components. </li></ul><ul><li>Flow directing valves may or may not be used. </li></ul>6 March 2011
    5. 5. Requirements of a Breathing System <ul><li>Essential: </li></ul><ul><li>The breathing system must </li></ul><ul><li>D eliver the gases from the machine to the alveoli in the same concentration as set and in the shortest possible time; </li></ul><ul><li>E ffectively eliminate carbon-dioxide; </li></ul><ul><li>H ave minimal apparatus dead space; and </li></ul><ul><li>H ave low resistance. </li></ul>6 March 2011
    6. 6. Requirements of a Breathing System <ul><li>Desirable: </li></ul><ul><li>economy of fresh gas; </li></ul><ul><li>conservation of heat; </li></ul><ul><li>adequate humidification of inspired gas; </li></ul><ul><li>Light weight; </li></ul><ul><li>convenience during use; </li></ul><ul><li>efficiency during spontaneous as well as controlled ventilation </li></ul><ul><li>adaptability for adults, children and mechanical ventilators; </li></ul><ul><li>provision to reduce theatre pollution </li></ul>6 March 2011
    7. 7. Anesthesia Breathing Systems <ul><li>Resistance to flow can be minimized by: </li></ul><ul><ul><li>Reducing the circuit’s length </li></ul></ul><ul><ul><li>Increasing the diameter (who’s law is that??) </li></ul></ul><ul><ul><ul><li>Hagen-Poiseuille P = (L)(v)(V) </li></ul></ul></ul><ul><ul><ul><ul><li> r 4 </li></ul></ul></ul></ul><ul><ul><ul><ul><li>P is pressure gradient. L is length. v is viscosity. V is flow rate </li></ul></ul></ul></ul><ul><ul><ul><ul><li>RESISTANCE IS INDIRECTLY PROPORTIONAL TO FLOW RATE WITH LAMINAR FLOW </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Flow = P1-P1/R where P1 is pressure at one end of a tube and P2 is pressure at the other end of the tube </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><li>FOR TURBULENT FLOW, GAS DENSITY IS MORE IMPORTANT THAN VISCOSITY </li></ul></ul></ul></ul><ul><ul><ul><ul><li>RESISTANCE IS PROPORTIONAL TO THE “SQUARE” OF FLOW RATE (TURBULENT FLOW) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>IN CLINICAL PRACTICE, FLOW IS USUALLY A MIXTURE OF LAMINAR & TURBULENT FLOW </li></ul></ul></ul></ul><ul><ul><li>Avoiding the use of sharp bends (turbulent flow) </li></ul></ul><ul><ul><li>Eliminating unnecessary valves </li></ul></ul><ul><ul><li>Maintaining laminar flow </li></ul></ul>6 March 2011
    8. 8. Another look at Poiseuille’s Law <ul><li>Laminar flow: orderly movement of gas inside a “hose” (gas in the center of the tube moves faster than gas closer to walls) </li></ul><ul><li>Turbulent flow: resistance is increased (seen with sudden narrowing or branching of tube) </li></ul><ul><li>Laminar flow becomes Turbulent when Reynold’s number is >2000 </li></ul><ul><li>Poiseuille’s Law follows Laminar flow </li></ul><ul><ul><li>R = 8 n l (R: resistance, n: viscosity, l: length, r: radius) </li></ul></ul><ul><ul><ul><li>r 4 </li></ul></ul></ul><ul><ul><ul><li>Example: doubling the radius of the tube will decrease the resistance 16 times (2) 4 =16 </li></ul></ul></ul>6 March 2011
    9. 9. Classification Of Breathing Systems <ul><li>McMohan in 1951 </li></ul><ul><li>Open no rebreathing </li></ul><ul><li>Semiclosed partial rebreathing </li></ul><ul><li>Closed total rebreathing </li></ul><ul><li>Dripps et al </li></ul><ul><li>Insufflation, Open, Semiopen, Semiclosed and Closed </li></ul><ul><li>taking into account the presence or absence of Reservoir, Rebreathing, CO2 absorption and Directional valves </li></ul>6 March 2011
    10. 10. <ul><li>Conway </li></ul><ul><li>Breathing systems with CO2 absorber </li></ul><ul><li>Breathing systems without CO2 absorber. </li></ul><ul><li>Sub-classified by Miller in 1988 </li></ul>Classification Of Breathing Systems 6 March 2011
    11. 11. <ul><li>Unidirectional flow : </li></ul><ul><li>Non rebreathing systems. </li></ul><ul><li>Circle systems. </li></ul>Breathing Systems Without CO 2 Absorption <ul><li>Bi-directional flow: </li></ul><ul><li>Afferent reservoir systems. </li></ul><ul><ul><li>Mapleson A </li></ul></ul><ul><ul><li>Mapleson B </li></ul></ul><ul><ul><li>Mapleson C </li></ul></ul><ul><ul><li>Lack’s system. </li></ul></ul><ul><li>Enclosed afferent reservoir systems </li></ul><ul><ul><li>Miller’s (1988) </li></ul></ul><ul><li>Efferent reservoir systems </li></ul><ul><ul><li>Mapleson D </li></ul></ul><ul><ul><li>Mapleson E </li></ul></ul><ul><ul><li>Mapleson F </li></ul></ul><ul><ul><li>Bain’s system </li></ul></ul><ul><li>Combined systems </li></ul><ul><ul><li>Humphrey ADE </li></ul></ul>6 March 2011
    12. 12. Breathing Systems With CO 2 Absorption. <ul><li>Unidirectional flow </li></ul><ul><ul><li>Circle system with absorber. </li></ul></ul><ul><li>Bi-directional flow </li></ul><ul><ul><li>To and Fro system. </li></ul></ul>6 March 2011
    13. 13. Non-rebreathing system Unidirectional Flow 6 March 2011
    14. 14. Disadvantages of Nonrebreathing system <ul><li>Fresh gas flow has to be constantly adjusted </li></ul><ul><li>There is no humidification of inspired gas. </li></ul><ul><li>There is no conservation of heat. </li></ul><ul><li>They are not convenient as the bulk of the valve has to be positioned near the patient. </li></ul><ul><li>The valves can malfunction due to condensation of moisture and lead to complications. </li></ul>6 March 2011
    15. 15. Bi-Directional Flow <ul><li>These systems depend on the FGF for effective elimination of CO2. </li></ul><ul><li>functioning can be manipulated by changing parameters like Fresh gas flow, alveolar ventilation, apparatus dead space </li></ul>6 March 2011
    16. 16. Fresh Gas Supply <ul><li>If there is no FGF into the system, the patient will get suffocated. </li></ul><ul><li>If the FGF is low, most systems do not eliminate carbon-dioxide effectively, and </li></ul><ul><li>I f there is an excess flow there is wastage of gas. </li></ul><ul><li>I t is imperative to specify optimum FGF for a breathing system for efficient functioning. </li></ul><ul><li>FGF should be delivered as near the patient's airway as possible. </li></ul>6 March 2011
    17. 17. Elimination Of Carbon-Dioxide <ul><li>Normal production of carbon-dioxide in a 70 kg adult is 200 ml per minute </li></ul><ul><li>Normal end-tidal concentration of carbon-dioxide is 5%. </li></ul><ul><li>F or eliminating 200 ml of carbon-dioxide as a 5% gas mixture, the alveolar ventilation has to be </li></ul><ul><li>200 x 100 / 5 = 4,000 ml. </li></ul>6 March 2011
    18. 18. <ul><li>alveolar ventilation </li></ul><ul><li>< 4 L/min Hypercarbia </li></ul><ul><li>> 4 L/min Hypocarbia </li></ul><ul><li>alveolar ventilation </li></ul><ul><li>5 L/min with 1 % CO2 </li></ul><ul><li>8 L/min with 2.5 % CO2 </li></ul>Elimination Of Carbon-Dioxide Normocarbia 6 March 2011
    19. 19. Apparatus Dead Space <ul><li>It is the volume of the breathing system from the patient-end to the point up to which, to and fro movement of expired gas takes place </li></ul>6 March 2011
    20. 20. <ul><li>The dynamic dead space will depend on the FGF and the alveolar ventilation </li></ul><ul><li>The dead space is minimal with optimal FGF. </li></ul><ul><li>If the FGF is reduced below the optimal level, the dead space increases and </li></ul><ul><li>T he whole system will act as dead space if there is no FGF </li></ul>Apparatus Dead Space 6 March 2011
    21. 21. Sub-Classification Of Bi-Directional Flow Systems <ul><li>Afferent reservoir system (ARS). </li></ul><ul><li>Enclosed afferent reservoir systems (EARS). </li></ul><ul><li>Efferent reservoir systems (ERS). </li></ul><ul><li>Combined systems. </li></ul>6 March 2011
    22. 22. Bi-Directional Flow Systems <ul><li>If the reservoir is placed in this limb as in Mapleson A, B, C and Lack's systems, they are called afferent reservoir systems (ARS). </li></ul>Enclosed afferent reservoir system has been described by Miller and Miler. If the reservoir is placed in this limb as in Mapleson D, E, F and Bain systems, they are called efferent reservoir systems (ERS). The efferent limb is that part of the breathing system which carries expired gas from the patient and vents it to the atmosphere through the expiratory valve/port. The afferent limb is that part of the breathing system which delivers the fresh gas from the machine to the patient. Afferent Limb Efferent Limb 6 March 2011
    23. 23. Afferent Reservoir Systems <ul><li>The Mapleson A, B and C systems have the reservoir in the afferent limb, and do not have an efferent limb </li></ul><ul><li>Lack system has an afferent limb reservoir and an efferent limb through which the expired gas traverses before being vented into the atmosphere . This limb is coaxially placed inside the afferent limb. </li></ul>6 March 2011
    24. 24. Afferent Reservoir Systems <ul><li>These AR systems work efficiently during spontaneous breathing provided the expiratory valve is separated from the reservoir bag and FGF by at least one tidal volume of the patient and apparatus dead space is minimal </li></ul>6 March 2011
    25. 25. <ul><li>If the FGF is close to the expiratory valve as in Mapleson B & C, the system is inefficient both during spontaneous and controlled ventilation. </li></ul>Afferent Reservoir Systems 6 March 2011
    26. 26. Mapleson ’s analysis <ul><li>Gases move enbloc. They maintain their identity as fresh gas, dead space gas and alveolar gas. There is no mixing of these gases. </li></ul><ul><li>The reservoir bag continues to fill up, without offering any resistance till it is full. </li></ul><ul><li>The expiratory valve opens as soon as the reservoir bag is full and the pressure inside the system goes above atmospheric pressure. </li></ul><ul><li>The valve remains open throughout the expiratory phase without offering any resistance to gas flow and closes at the start of the next inspiration. </li></ul>6 March 2011
    27. 27. AR Systems – Functional analysis Spontaneous Breathing 6 March 2011
    28. 28. Magill System – Controlled Ventilation First Breath Expiration Next Inspiration 6 March 2011
    29. 29. Lack's system <ul><li>This system functions like a Mapleson A system both during spontaneous and controlled ventilation </li></ul>6 March 2011
    30. 30. Anesthesia Breathing Systems <ul><li>Mapleson </li></ul><ul><ul><li>Advantages </li></ul></ul><ul><ul><ul><li>Used during transport of children </li></ul></ul></ul><ul><ul><ul><li>Minimal dead space, low resistance to breathing </li></ul></ul></ul><ul><ul><ul><li>Scavenging (variable ability, depending on FGF used) </li></ul></ul></ul><ul><ul><li>Disadvantages </li></ul></ul><ul><ul><ul><li>Scavenging (variable ability, depending on FGF used) </li></ul></ul></ul><ul><ul><ul><li>High flows required (cools children, more costly) </li></ul></ul></ul><ul><ul><ul><li>Lack of humidification/heat (except Bain) </li></ul></ul></ul><ul><ul><ul><li>Possibility of high airway pressures and barotrauma </li></ul></ul></ul><ul><ul><ul><li>Unrecognized kink of inner hose in Bain </li></ul></ul></ul><ul><ul><ul><li>Pollution and higher cost </li></ul></ul></ul><ul><ul><ul><li>Difficult to assemble </li></ul></ul></ul>6 March 2011
    31. 31. Enclosed Afferent Reservoir (Ear) Systems <ul><li>Coaxial version </li></ul><ul><li>Non coaxial version </li></ul>6 March 2011
    32. 32. Efferent Reservoir Systems <ul><li>6 mm tube as the afferent limb that supplies the FG from the machine. </li></ul><ul><li>The efferent limb is a wide-bore corrugated tube to which the reservoir bag is attached </li></ul><ul><li>T he expiratory valve is positioned near the bag </li></ul><ul><li>All these ER systems are modifications of Ayre's T-piece </li></ul>6 March 2011
    33. 33. Ayre's T-piece <ul><li>Light metal tube 1 cm in diameter, 5 cm in length with a side arm </li></ul><ul><li>Used as such, it functions as a non-rebreathing system </li></ul><ul><li>FGF equal to peak inspiratory flow rate of the patient </li></ul>6 March 2011
    34. 34. Efferent Reservoir Systems <ul><li>In an attempt to reduce FGF requirements ER systems are constructed </li></ul><ul><li>The functioning of all these systems are similar </li></ul>These systems work efficiently and economically for controlled ventilation as long as the FG entry and the expiratory valve are separated by a volume equivalent to atleast one tidal volume of the patient 6 March 2011
    35. 35. ER systems - Functional Analysis <ul><li>Factors affecting CO2 elimination </li></ul><ul><li>FGF, </li></ul><ul><li>respiratory rate, </li></ul><ul><li>expiratory pause, </li></ul><ul><li>tidal volume and </li></ul><ul><li>CO2 production </li></ul>Spontaneous Breathing Factors other than FGF cannot be manipulated in a spontaneously breathing patient 6 March 2011
    36. 36. <ul><li>Factors affecting CO2 elimination </li></ul><ul><li>FGF, </li></ul><ul><li>respiratory rate, </li></ul><ul><li>tidal volume and </li></ul><ul><li>pattern of v entilation </li></ul>ER systems - Functional Analysis Controlled Ventilation T hese parameters can be totally controlled by the anaesthesiologist 6 March 2011
    37. 37. Anesthesia Breathing Systems <ul><li>Bain system (http://www.capnography.com/Circuits/bainsystem.htm) </li></ul><ul><ul><li>Coaxial (tube within a tube) version of Mapleson D </li></ul></ul><ul><ul><li>Fresh gas enters through narrow inner tube </li></ul></ul><ul><ul><li>Exhaled gas exits through corrugated outer tube </li></ul></ul><ul><ul><li>FGF required to prevent rebreathing: </li></ul></ul><ul><ul><ul><li>200-300ml/kg/min with spontaneous breathing (2 times V E ) </li></ul></ul></ul><ul><ul><ul><li>70ml/kg/min with controlled ventilation </li></ul></ul></ul>6 March 2011
    38. 38. Bain at work ( spontaneous ) <ul><li>Spontaneous : The breathing system should be filled with FG before connecting to pt. During inspiration, the FG from the machine, the reservoir bag and the corrugated tube flow to the pt. </li></ul><ul><li>During expiration, there is a continuous FGF into the system at the pt’s end. The expired gas gets continuously mixed with the FG as it flows back into the corrugated tube and the reservoir bag. Once the system is full, the excess gas is vented to the scavenger. </li></ul><ul><li>During the expiratory pause the FG continues to flow and fill the proximal portion of the corrugated tube while the mixed gas is vented through the valve. </li></ul>6 March 2011
    39. 39. Bain at work ( spontaneous ) <ul><li>During the next inspiration, the pt breathes in FG as well as the mixed gas from the corrugated tube. Many factors influence the composition of the inspired mixture (FGF, resp rate, expiratory pause, TV and CO2 production in the body). Factors other than FGF cannot be manipulated in a spontaneously breathing pt. </li></ul><ul><li>It has been mathematically calculated and clinically proved that the FGF should be at least 1.5 to 2 times the patient’s minute ventilation in order to minimize rebreathing to acceptable levels. </li></ul>6 March 2011
    40. 40. Bain at work ( controlled ) <ul><li>Controlled : To facilitate intermittent positive pressure ventilation, the APL has to be partly closed so that it opens only after sufficient pressure has developed in the system. When the system is filled with fresh gas, the patient gets ventilated with the FGF from the machine, the corrugated tube and the reservoir bag. </li></ul><ul><li>During expiration, the expired gas continuously gets mixed with the fresh gas that is flowing into the system at the patient’s end. </li></ul><ul><li>During the expiratory pause the FG continues to enter the system and pushes the mixed gas towards the reservoir.   </li></ul>6 March 2011
    41. 41. Bain at work ( controlled ) <ul><li>When the next inspiration is initiated, the patient gets ventilated with the gas in the corrugated tube (a mixture of FG, alveolar gas and dead space gas). </li></ul><ul><li>As the pressure in the system increases, the APL valve opens and the contents of the reservoir bag are discharged into the scavenger (gas follows the path of least resistance) </li></ul>6 March 2011
    42. 42. Anesthesia Breathing Systems <ul><li>Bain </li></ul><ul><ul><li>Advantages </li></ul></ul><ul><ul><ul><li>Warming of fresh gas inflow by surrounding exhaled gases (countercurrent exchange) </li></ul></ul></ul><ul><ul><ul><li>Improved humidification with partial rebreathing </li></ul></ul></ul><ul><ul><ul><li>Ease of scavenging waste gases </li></ul></ul></ul><ul><ul><ul><li>Overflow/pressure valve (APL valve) </li></ul></ul></ul><ul><ul><ul><li>Disposable/sterile </li></ul></ul></ul>6 March 2011
    43. 43. Anesthesia Breathing Systems <ul><li>Bain </li></ul><ul><ul><li>Disadvantages </li></ul></ul><ul><ul><ul><li>Unrecognized disconnection </li></ul></ul></ul><ul><ul><ul><li>Kinking of inner fresh gas flow tubing </li></ul></ul></ul><ul><ul><ul><li>Requires high flows </li></ul></ul></ul><ul><ul><ul><li>Not easily converted to portable when commercially used anesthesia machine adapter Bain circuit used </li></ul></ul></ul><ul><ul><li>Look at the Bain and identify what makes it modified from the standard Mapleson D </li></ul></ul>6 March 2011
    44. 44. Bain is a Modified Mapleson D (APL) 6 March 2011
    45. 45. Pethick’s Test for the Bain Circuit <ul><li>A unique hazard of the use of the Bain circuit is occult disconnection or kinking of the inner hose (fresh gas delivery hose). To perform the Pethick’s test, use the following steps: </li></ul><ul><ul><li>Occlude the patient's end of the circuit (at the elbow). </li></ul></ul><ul><ul><li>Close the APL valve. </li></ul></ul><ul><ul><li>Fill the circuit, using the oxygen flush valve (like pressurizing the circuit when you are doing a leak test) </li></ul></ul><ul><ul><li>Release the occlusion at the elbow and flush. A Venturi effect flattens the reservoir bag if the inner tube is patent. </li></ul></ul>6 March 2011
    46. 46. PaCO2 Isopliths <ul><li>When FGF is very high the PaCO2 becomes ventilation dependent (as during spontaneous respiration). </li></ul><ul><li>When the minute volume exceeds the FGF substantially, the PaCO2 is dependent on the FGF </li></ul>40 mmHg 35 mmHg 6 March 2011
    47. 47. Combined System <ul><li>Humphry ADE:with two reservoirs one in afferent and onein efferent limb </li></ul><ul><li>System can be changed from ARS to ERS by changing the position of the lever. </li></ul><ul><li>Can be used for adults and children . </li></ul>6 March 2011
    48. 48. Breathing Systems with CO2 Absorption <ul><li>sodalime canister, </li></ul><ul><li>Two unidirectional valves, </li></ul><ul><li>Fresh gas entry, </li></ul><ul><li>Y-piece to connect to the patient, </li></ul><ul><li>Reservoir bag </li></ul><ul><li>a relief valve and </li></ul><ul><li>low resistance interconnecting tubing. </li></ul>Circle System Functional Analysis 6 March 2011
    49. 49. 3 Essential Factors <ul><li>There should be two unidirectional valves on either side of the reservoir bag and the canister , </li></ul><ul><li>Relief valve should be positioned in the expiratory limb only, </li></ul><ul><li>The FGF should enter the system proximal to the inspiratory unidirectional valve. </li></ul>6 March 2011
    50. 50. Optimization of Circle Design <ul><li>Unidirectional Valves </li></ul><ul><ul><li>Placed in close proximity to pt to prevent backflow into inspiratory limb if circuit leak develops. </li></ul></ul><ul><li>Fresh Gas Inlet </li></ul><ul><ul><li>Placed b/w absorber & inspiratory valve. If placed downstream from insp valve, it would allow FG to bypass pt during exhalation and be wasted. If FG were placed b/w expiration valve and absorber, FG would be diluted by recirculating gas </li></ul></ul>6 March 2011
    51. 51. Optimization of Circle Design <ul><li>APL valve </li></ul><ul><ul><li>Placed immediately before absorber to conserve absorption capacity and to minimize venting of FG </li></ul></ul><ul><li>Breathing Bag </li></ul><ul><ul><li>Placed in expiratory limb to decrease resistance to exhalation. Bag compression during controlled ventilation will vent alveolar gas thru APL valve, conserving absorbent </li></ul></ul>6 March 2011
    52. 52. Circle system can be: <ul><li>closed (fresh gas inflow exactly equal to patient uptake, complete rebreathing after carbon dioxide absorbed, and pop-off closed) </li></ul><ul><li>semi-closed (some rebreathing occurs, FGF and pop-off settings at intermediate values), or </li></ul><ul><li>semi-open (no rebreathing, high fresh gas flow) </li></ul>6 March 2011
    53. 53. Anesthesia Breathing Systems <ul><li>Circle systems </li></ul><ul><ul><li>Most commonly used </li></ul></ul><ul><ul><li>Adult and child appropriate sizes </li></ul></ul><ul><ul><li>Can be semiopen, semiclosed, or closed dependent solely on fresh gas flow (FGF) </li></ul></ul><ul><ul><li>Uses chemical neutralization of CO2 </li></ul></ul><ul><ul><li>Conservation of moisture and body heat </li></ul></ul><ul><ul><li>Low FGF’s saves money </li></ul></ul>6 March 2011
    54. 54. Functional analysis <ul><li>During inspiration fresh gas flow goes through inspiratory limb passing through U 1 . </li></ul><ul><li>No flow in expiratory limb. </li></ul><ul><li>During expiration no flow in the inspiratory limb, gases pass through U 2 goes to canister, CO 2 is absorbed then mixes with fresh gas flow in reservoir bag. </li></ul><ul><li>When bag is full expiratory valve opens. </li></ul>6 March 2011
    55. 55. Maintanence of closed systems <ul><li>Condensed water in the circuit must be removed. </li></ul><ul><li>Breathing hoses should be changed in between cases </li></ul><ul><li>Reusable tubes washed and hanged </li></ul><ul><li>Valves opened and cleaned with alcohol </li></ul><ul><li>Sodalime should be changed regularly </li></ul><ul><ul><li>Color change in majority of granules </li></ul></ul><ul><ul><li>If etco 2 shows base line elevation </li></ul></ul><ul><ul><li>When it was not used for longer time </li></ul></ul>6 March 2011
    56. 56. Advantages <ul><li>Economy: </li></ul><ul><ul><li>The FGF could be reduced to as low as 250 - 500 ml of oxygen. </li></ul></ul><ul><ul><li>The consumption of Halothane / Isoflurane has been found to be around 3.5 ml/hour </li></ul></ul><ul><li>Humidification </li></ul><ul><li>Reduction of heat loss </li></ul><ul><li>Reduction in atmospheric pollution </li></ul>6 March 2011
    57. 57. Disadvantages <ul><li>A greater knowledge of uptake and distribution is required to master closed circuit anaesthesia. </li></ul><ul><li>Inability to alter any concentration quickly. </li></ul><ul><li>Real danger of hypercapnia exists with </li></ul><ul><ul><li>an inactive absorber, </li></ul></ul><ul><ul><li>incompetent unidirectional valves and </li></ul></ul><ul><ul><li>incorrect use of absorber bypass. </li></ul></ul>6 March 2011
    58. 58. Disadvantages of Circle System <ul><li>Greater size, less portability </li></ul><ul><li>Increased complexity </li></ul><ul><ul><li>Higher risk of disconnection or malfunction </li></ul></ul><ul><li>Increased resistance (of valves during spontaneous ventilation) </li></ul><ul><ul><li>Dissuading use in Pediatrics (unless a circle pedi system used) </li></ul></ul><ul><li>Difficult prediction of inspired gas concentration during low fresh gas flow </li></ul>6 March 2011
    59. 59. Bacterial Contamination <ul><li>Slight risk of microorganism retention in Circle system that could (theoretically) lead to respiratory infections in subsequent pts </li></ul><ul><li>Bacterial filters are incorporated into EXPIRATORY LIMB of the circuit </li></ul>6 March 2011
    60. 60. Gases and vapour concentration in circle system <ul><li>Internal volume of apparatus with 2 kg absorbent </li></ul><ul><ul><li>Inter granular space 1 litre </li></ul></ul><ul><ul><li>Breathing hoses 1 litre </li></ul></ul><ul><ul><li>Pathways with in the absorber 1 litre </li></ul></ul><ul><ul><li>Patient FRC 1.25 litres </li></ul></ul><ul><ul><li>Total 4.25 litres </li></ul></ul><ul><li>Into this anesthetic gas is diluted. </li></ul>6 March 2011
    61. 61. Checking breathing systems – WHY???? <ul><li>Fatalities have occurred when the user </li></ul><ul><ul><li>User was unfamiliar with equipment </li></ul></ul><ul><ul><li>Included a non standard item </li></ul></ul><ul><ul><li>Not noticied improper assembly </li></ul></ul><ul><ul><li>Not noticed the development of a fault </li></ul></ul>6 March 2011
    62. 62. protocols <ul><li>The components of the system must conform to the international quality </li></ul><ul><li>Assembly should be proper </li></ul><ul><li>All tapered connections are push and twist technique </li></ul><ul><li>APL valve if closed air tight fitting must be there </li></ul><ul><li>If valve opened gases must go freely </li></ul><ul><li>Coaxial breathing systems integrity should be tested </li></ul>6 March 2011
    63. 63. Summary <ul><li>Defined a Breathing System </li></ul><ul><li>Classified the Breathing Systems </li></ul><ul><ul><li>With CO2 Absorber Unidirectional </li></ul></ul><ul><ul><li>Without CO2 Absorber Bidirectional </li></ul></ul><ul><li>Analyzed the functional analysis </li></ul>6 March 2011
    64. 64. 6 March 2011

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