breathing circuits (1) (1).pptx BY MOHAMED ANWER RIFKY

ANAESTHESIA BEATHING CIRCUITS AND SCVENGER SYSTEMS IN ANAESTHESIA

1. Breathing and Scavenging Systems THE OPEN SYSTEM In B, oxygen is delivered to an Edinburgh mask >>wide opening to room air>>negligible increase to the respiratory dead space. In C, oxygen is delivered by cannulae (patient's nostrils). A similar technique (gases being delivered by the side tube of a special gag). Advantage :no added resistance ,disadvantage : 1-little control over the concentration of oxygen and other gases (dilution is inevitable). 2-During peak inspiration (50 litre min- 1 ) >> the dilution with air is high. If the flow of gas exceeds his peak inspiratory flow>>known given concentrations of gases ( 'Venturi' masks achieve this) ),also to administer more accurately controlled concentrations of gases (a tightly fitting face mask or intubation) >> adequacy during peak insp. flow. This is the simplest system -The patient's airway remains open ( no tubing, valves or reservoir bag are used)- A >> anaesthetic gases fall over the child's face from a cupped hand (end of tube).A Schimmelbush mask was used in the past.   NON-REBREATHING VALVE SYSTEMS A typical non-rebreathing valve (' Ambu E valve’) , It contains moulded rubber (inspiratory and expiratory Valves) , a reservoir bag is present in the system and the gas supply to this must equal the tidal ventilation. The Quantiflex (an alternative non-rebreathing valve) machines is used in dental anaesthesia in which the inspiratory valve is positioned immediately after the reservoir bag. The Ambu E valve can also be used for controlled ventilation >>valve flap occludes the outlet. In a variety of the valves, the Ambu E2, used only in resuscitation apparatus, the expiratory flap valve is omitted >> nonrebreathing function is then no longer present. Valves>> cleaned and supervised >>valve in the inflation position >>build-up of dangerous airway pressure. As pressures needed to overcome the inspiratory and expiratory valve resistances can be 100-200 Pa (1-2 cmH20) during peak flow>> uncomfortable in conscious patients for long term. Non-rebreathing systems having special low resistance flap valves as the Entonox , are used for airmen, firemen and divers. A non-rebreathing valve system is used outside the hospital. and in anaesthesia >> Triservice apparatus . Inspiratory and expiratory valves (incorporated in the Laerdal valve) and as in the Ambu E, the inspiratory valve can also function as an inflating valve (the silicone rubber flaps of the valve occlude the expiratory ports)>> Air is drawn through a Triservice vaporizer (a modified OMV vaporize ) - The T-junction ( shown) allows oxygen supplementation. The advantages: system are of extreme lightness and portability (draw-over vaporizers). T-PIECE SYSTEMS The T-piece system ( Ayre's T-piece after Dr. Philip Ayre, 1937. ) ,with no valves in the simplest form . 1- Fresh gas being supplied to a T-piece at the limb which must be of suitable diameter (excessive resistance) >> 22 mm internal diameter (adult).Fresh gas flow of 2 to 2.5 times the tidal ventilation and the capacity of the open limb could be the same as the patient's tidal volume. 2- The gases from the patient's anatomical dead space are expired first >>released from the open end of the tube at A. During mid-expiration, the majority of the alveolar gases are expired at near peak expiratory flow, (only slightly diluted by the fresh gas flow) .At the finish of expiration the expiratory flow falls (C, contains more fresh gas). If there is an end expiratory pause, then a further volume of undiluted fresh gas is finally present as at D (figure). 3- During inspiration. Initially, fresh gas only is inspired until inspiratory flow becomes greater than the flow of fresh gas to the T-piece. At peak inspiration, the patient may in addition draw upon the gas at D in the open tube. An end-expiratory pause >>no rebreathing of alveolar air occurs. Figure ,illustrates the effect of the end-expiratory pause upon the fresh gas flow required to prevent any rebreathing of alveolar gas>>an end-expiratory pause of 1 second >>a flow of fresh gas of 8.5 litre min- 1 (under twice the tidal ventilation of 4.8 litre min- 1 ) ensures that the volume of fresh gas supplied always exceeds the volume of inspired gas from the open limb. 4-I f the open limb is omitted>>this leads to inspiration of air (unless the fresh gas flow is equal to the patient's peak inspiratory flow). 5- A bag with a valve to the open end of the T-piece >>possible to use lower fresh gas flows . 6- Suited to use in paediatric anaesthesia (an adequate end-expiratory pause) >>system is more effective). 7- A- Negligible apparatus dead space >>T-piece connection close to the patient's airway with B- no valves >> malfunction due to sticking at the valve. C- Requires high fresh gas flow ,with pollution from spilt gas.

2. COAXIAL T-PIECE SYSTEM A coaxial T-piece system is a more compact (the Bain (USA) or Penlon coaxial (UK) systems). As in other T-piece techniques, gas flows (spontaneous breathing )= 2.5 times Vt . Special care ( central tube carrying fresh gas is correctly attached)>>fresh gases may not be delivered . MAGILL SYSTEM The Magill system ( Sir Ivan Magill in 1920)- Made of a reservoir bag, delivery tube and expiratory valve >>spontaneous breathing in the Fig.(the tube at the end of expiration )>>Gases from the anatomical dead space travel along the tube at A, while fresh gas inflates the reservoir bag, distending the bag >>pressure ( 50 Pa or 0.5 cmH20 )>>open the expiratory valve>>alveolae gas are then expired -The system can conserve the dead space gas >> 70% of the patient's tidal ventilation (spontaneous breathing ).-The system can also be used to ventilate the patient>>valve being tightened >>intermittent positive pressure (reservoir bag) , for short-term use -Modern expiratory valves are larger than the former ' Heidbrink ' type>> bulk. If positioned remote from the patient , the valve need not to be changed with each fresh tubing. COAXIAL MAGILL SYSTEM The coaxial Magill system also uses a remote expiratory valve , known as the Lack system- Fresh gas flows from the reservoir bag through the wide tube which contains a narrower tube leading to the exp. valve-Unlike the coaxial T-piece the patient breathes through both tubes >> wider coaxial tubes are required to prevent increased resistance to breathing-Ensure that the internal tube is correctly fitted -As in the Magill system, the coaxial Magill is less suitable for prolonged controlled ventilation. CLOSED SYSTEMS In the fully closed system (closed circuit or circle system)-The patient's gases recirculate through soda lime (absorbs carbon dioxide), while oxygen is added ( metabolic uptake) -Arrangement and the positions of the components may vary-The soda lime granules >>critical size, large enough (excess resistance to breathing )and small enough (absorb carbon dioxide efficiently >> '4-8 mesh', >> granules will pass through a sieve having four strands per inch, but not through one with eight strands per inch. The anaesthetic may be supplied from a drawover vaporizer in the circle ( VIC), or by an out of circle vaporizer ( VOC)>> retains its accuracy at low flows-The variability of the patient's uptake of oxygen and anaesthetic agents>>gas analysers ( cost of analysers ), are recommended .Other disadvantages>>difficulty of sterilization (complexity of the closed system) , and the need for a special form of a ventilator- Has the advantage: 1- Oxygen uptake >>monitored, and the uptake of volatile anaesthetic agents to be demonstrated>>its economy though the cost of the soda lime . 2- Heat loss from the patient is slightly reduced (inspired gas is fully saturated). 3- Reduction of pollution >>scavenging -Excess gas spilt from the expiratory valve with 2 lites /min.>> closed system with leak for better control of the concentrations of gases=that conc.of those of flow supply>>lacks some of the claimed advantages -If nitrous oxide were to be used >>closed system with leak in the initial period >> nitrogen to be replaced by nitrous oxide. ALTERNATIVE CLASSIFICATION OF SYSTEMS In the UK>>the Magill system, which are neither closed nor open >> referred to as 'semi-closed', but in the USA the term 'semi open is used instead; the term 'semi-closed' being used for the closed system with leak So,terms semi-open and semiclosed are best avoided. Mapleson: 1- T-piece system (unmodified) =E 2- T-piece system with open ended bag =F 3- T-piece system with bag and valve =D 4- Magill system =A .The systems B and C in this classification are now seldom used ,and the open, non-rebreathing valve, and closed systems are not included in this categorization.

3. RESISTANCE OF BREATHING SYSTEMS The open system gives no added resistance to breathing systems ,but others >>resistance >>1 - tubings and 2- valves used. If the tubing diameter is large ( laminar flow )>>additional pressure is overcomed .The critical flow for a smooth tube of 22 mm , carrying a nitrous oxide oxygen mixture could be about 22 litre min- 1 . Anaesthetic systems using corrugated tubing and peak flows may exceed 22 litre min- 1 >>(laminar and turbulent flow) ,but extra pressure is generated to drive the gases through the tubing if small (25 Pa =0.25 cmH20) during peak flow. So tubing of a larger diameter, e.g. 30 mm is used. In laminar flow resistance is inversely proportional to the fourth power of the radius >>diameter from 22 mm to 30 mm gives a 3.5 times reduction >>no risk of turbulent air flow . In simple T-piece systems without any expiratory valve or reservoir bag>>pressure required to be generated for the resistance is greater during expiration (expiratory flow is increased by the fresh gas ,whereas inspiratory flow in the limb is reduced by the fresh gas flow)>> higher fresh gas flow >back pressure during expiration (in most breathing systems).A typical valve may have an opening pressure of 50 Pa , during peak flows (30 litre min -1 )>> turbulence in the valve results in higher pressures of 100-500 Pa (1-5 cmH20). The reservoir bag used in a system can also affect the pressures which develop during spontaneous breathing. WORK USED IN BREATHING SYSTEMS The additional work required in breathing systems >>the area of a pressure-volume loop ( Figure) illustrates the type of loop for various systems). The pressure at the patient connector is plotted against the volume change for a tidal volume of 0.4 litre . The main source of resistance is in the valves , so 1- The simple T-piece system >>little added work. 2- The non-rebreathing ,circle system or the Magill system >>mean pressure swing could be 200 Pa (2 cmH20) for the volume change of 0.4 litre >>80 mJ (300 mJ for normal inspiration )>> 27% increase in the work >>uncomfortable for conscious individuals>>use valves with lower resistances. 3- In the anaesthetic non-rebreathing valve system the resistance of the inspiratory and expiratory valves are comparable (equal areas above and below the zero pressure axis). 4- A circle system would give a similar result. 5-I n the Magill system (no inspiratory valve) and the assistance to inspiration from the slightly distended reservoir bag and from the fresh gas flow overcomes the small flow resistance of the 22 mm diameter tube, but work is required during expiration, and the pressure-volume loop is almost entirely above the zero pressure axis. 6- The coaxial Magill system performs similarly, but the expiratory resistance in this case is that of the valve with its central 12.5 mm diameter supply tube. 7- The T-piece system with a bag and valve >>similar tracing but with a high fresh gas flow of 2 to 2.5 times tidal ventilation >>increased expiratory flow >>higher pressures generated in expiration through the valve .The high gas flow maintains a positive pressure during part of an inspiration patient uses less energy during inspiration. A similar tracing could be obtained with 8- the coaxial T-system . 9- In anaesthesia in a spontaneously breathing patient>>excessive increase in the work of breathing >>augment the depression of ventilation . 10- The added expiratory resistance >>rise in intrathoracic pressure >>reduce venous return . 11- Air tight fit face mask >>expiration with increases pressure >>leakage and so air pollution . SAFETY OF BREATHING SYSTEMS The components of breathing systems must be 1- correctly assembled and 2- Couplings with screw collars >>prevent unin - tentional disconnections with cone fitting to allow rapid change ,and a taper gauge can be used to test the dimensions of connectors 3- Plastic connections may distort during autoclaving . 4- tubes should not be too narrow and with no sharp angles >>turbulent flow. 5- Grossly excessive pressures can arise if the expiratory valve sticks or if it is omitted from the breathing system ,or build up if the scavenging tubing is obstructed. 6- Some disposable reservoir bags lack elasticity and give no such protection so that pressures in the system then rise to much higher values >>Laplace's law. This pressure is usually about 4 kPa ( 40 cmH20).The bag elasticity !!!. 7- safety valve on the anaesthetic machine. Typically this could be 35 kPa and such a valve only protects the mechanical components of the machine and gives no protection to the patient. 8- Magill system >>the expiratory valve, must be positioned close to the patient's airway>>no excess anatomical dead space and care of the central tube of a coaxial system (detached). T-piece with reservoir bag and valve . Magill system . Non-rebreathing valve or circle system . Simple T-piece system.

4. VENTILATORS T he Manley Pulmovent ventilator: 1- A fresh gas flow of, for example, 7 litre min- 1 inflates a reservoir bellows, at a pressure of about 10-12 kPa by springs. When the bellows are inflated to the preset volume , the cycling mechanism opens a valve controlled by a lever >>inflate the patient's lungs>>flow control valve restricts the flow ( 'volume cycled' ventilator). However, clear-cut classification of the cycling of ventilators is diffecult ),and a safety valve at the patient's side of the cycling valve limits pressure to a maximum of 7 kPa. ( valve limits the pressure) 3- During expiration the airway is open to atmosphere through the expired gas port while the reservoir bellows re-inflates. 4-I t is convenient to classify ventilators as 'constant pressure generators ', in which the gas is supplied at a constant pressure during inspiration, and 'constant flow generators ' in which gas is supplied at constant flow. 5- The tidal volume measured by a Wright respirometer is 0.5 litre , the ventilation rate is 14. Thus, the ventilator is sometimes referred to as a minute volume divider. 6- A near constant pressure is generated, it is applied via a flow control, (a conflict regarding the vent.classification ) 7- The breathing system is converted to a T-piece system (manual control or spontaneous breathing). 8- Some forms of Manley Pulmovent also incorporate an extra bellows to give a facility for negative pressure during expiration. 9- Two adjustable pressure sensors. One sensor is set to the maximum pressure desired ,the other sensor is set at a suitable pressure below the peak inspiratory pressure >>if it exceeds about 15 s >>disconnection alarm . SCAVENGING An increased incidence of spontaneous abortion in early reports (trace quantities of anaesthetic agents) >>suggestion not substantiated ,but reducing the level of pollution is still reasonable .Systems can be classified as active >>source of power as a pump draws away the scavenged gas or passive >>pressure generated by the patient during expiration>>exterior of the building .The simplest technique for active scavenging is an open technique (local scavenging), Other techniques of anaesthetic gas scavenging are more complex >>( transfer , receiving and disposal of the gases). OPEN SCAVENGING TECHNIQUE Suction is applied to the waste gases >>open to the atmosphere without reservoir bag >>generally less effective than the other systems>> a funnel is positioned near the expiratory valve and mask >>aspirate spilt anaesthetic agents ( anaesthesia for dental extractions )>>difficult airtight mask .Alternatively, the funnel can be inverted (form of a dish) collect gases released from the end of a T-piece breathing system.Disadvantages: 1- lack of control. 2- A high scavenging flow is needed and 3- the funnel must be very close to the point of release of the gases to be effective. COMPONENT SYSTEMS OF STANDARD TECHNIQUES The term 'transfer system' is used for the wide bore 30 mm diameter tubing >>transfers the gases collected from the exhaust port of the anaesthetic breathing system or ventilator to the 'receiving System (an open ended cylinder). Finally the 'disposal system' exhausts the gases to the exterior of the building. RECEIVING SYSTEM An open ended reservoir and can use a simple T-piece but the reservoir, usually a cylinder is used>>may incorporate channelling and the open end of the reservoir or 'air break’ for safety .The system is designed to prevent any risk of obstruction (excess negative or positive pressure ).Scavenging flow, typically 80 litre min- 1 , removes all expired gas with flow indicator ).In other receiving systems still in use a reservoir bag and safety valves are used >>valves preventing any risk of excess positive or negative pressure. The system becomes a passive one if it is used without the injector. DISPOSAL SYSTEM To dispose of the scavenged gases some hospitals use the centralized vacuum system but an independent system is recommended. A fan instead of a pump >.remove the scavenged gases ( not necessarily safer )>>risks of pulmonary oedema >>the pressure at the exhaust port must not exceed 50 Pa (0.5 cmH20) when tested with a flow of 30 litre min- 1 into the system. In a single anaesthetic machine>>injector powered by compressed air .Oxygen too has been used to drive such a system >> fire risk . Faults in the scavenging system have also been found >>the expiratory valve or the ventilator.

5. PASSIVE SCAVENGING SYSTEM THEATRE VENTILATION A wide bore tube conducts the expired gases outside the building. As excessive positive or subatmospheric pressures may be caused by wind movements ,and if the outlet is above roof level >>re-entry of the scavenged gases to the building (denser anaesthetics as nitrous oxide )>> back pressure .The collection system with bag and valves has been used without the injector (a passive scavenging system). An exit grill may be also used . Some spillage of anaesthetic vapours is inevitable ( 1- the end of anaesthesia , 2- at induction, or when using a 3- face mask>>Efficient theatre ventilation, e.g. 15 air changes per hour (volatile anaesthetics ). Reservoir >> improves efficiency