Breathing systems (2)
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The document defines and describes breathing systems used in anesthesia. It discusses the purpose of breathing systems to deliver anesthetic gases and oxygen safely. The key components and requirements of breathing systems are explained, including minimizing resistance to gas flow. Different types of breathing systems are classified and their mechanisms and advantages/disadvantages are summarized.
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Breathing systems (2)
- 1. Breathing Systems Dr. P. Narasimha Reddy Professor Dept. of Anaesthesiology 6 March 2011
- 13. Non-rebreathing system Unidirectional Flow 6 March 2011
- 27. AR Systems – Functional analysis Spontaneous Breathing 6 March 2011
- 28. Magill System – Controlled Ventilation First Breath Expiration Next Inspiration 6 March 2011
- 44. Bain is a Modified Mapleson D (APL) 6 March 2011
- 64. 6 March 2011
Editor's Notes
- 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.