CIRCLE SYSTEM in Breathing Systems (anesthesia)

CIRCLE SYSTEM additional components than mapleson circuits in attempt to avoid the disadvantages of the mapleson circuit COMPONENTS carbon dioxide absorbents carbon dioxode absorbers unidirectional valves PERFORMANCE CHARACTERISTICS of the Circle System fresh gas requirement dead space resistance humidity and heat conservation bacterial contamination OPTIMIZING CIRCLE SYSTEM unidirectional valves fresh gas inlet pressure release valve DISADVANTAGES of Circle system COMPONENTS OF CIRCLE SYSTEM Carbon Dioxide Absorbents: -eliminates C02 from exhaled gases to prevent hypercarbia and allows for rebreathing alveolar gas -rebreathing alveolar gas allows for conservation of heat and humidity -C02 combines with water to form carbonic acid which the carbonic acid is neutralized by C02 absorbents -C02 absorbents: soda lime and barium hydroxide lime neutralize carbonic acid into: calcium carbonate, water, and heat -Soda lime: more common absorbent and allows for absorbent of 23L of C02/100g of absorbent -color indicator: signals absorbant exhaustion: pH indicator senses increasing H+ concentration creating color conversion -absorbents should be replaced when 50 - 70% color change occurs -small granules: greater surface area but increased resistance to gas flow -large granules: lesser surface area but decreased resistance to gas flow -addition of silica to soda lime reduces the dust formation within the inhalational gases which can irritating the patients lungs -unlike soda lime, barium hydroxide lime is sufficeintly hard and does not produce dust, therefore addition of silica is not needed -absorbent granules have the potential of absorbing and later releasing significant amounts of volatile anesthetics -the absorbent granules' ability to absorb and later release volatile anesthetics may delay emergence and/or emergence -calcium hydroxide and calium chloride are newer absorbents which may result is less degredation of volatile anesthetics Carbon Dioxide Absorbers: -contain the absorbent granules -canister is either single or double compartments -double canisters allow for: greater complete C02 absorption, less frequent changes of absorbent, and lower gas flow resistance -patients tidal volume should not exceed the air space between the absorbent granules in order to ensure complete C02 absorption -air space between the absorbent granules is approximately 1/2 absorber's capacity -baffle system allows for less channeling of gases through areas of loosley packed granules Unidirectional Valves -function as check valves -rubber, plastic, or mica disc rest horizontal on an annular valve seat -forward flow: displaces disc upward allowing gas to move forward through the circuit -reverse flow: pushes disc downward against the annular valve seat preventing reflux -valve incompetence may occur from: warped disc, annular valve seat irregularities -expiratory valve is vulnerable to damage due to exposure of humidity from alveolar gases During inspiration -inspiratory valve opens during inhalation allowing patient to breathe in a mixture of fresh and exhaled alveolar gas devoid C02 -expiratory valve closes during inhalation preventing rebreathing of alveolar gas still containing C02 During expiration -expiratory valve opens allowing for the flow of alveolar glas to move away from the patient into the breathing circuit -inspiratory valve closes preventing expiratory gases to mix with fresh gases in the inspiratory limb of the breathing circuit malfunction of either the expiratory or inspiratory valve may allow for rebreathing of C02 and therefore hypercarbia PERFORMANCE CHARECTERISTICS of the CIRCLE SYSTEM fresh gas requirements: -low fresh gas flows (< or equal to 1L/min) the rebreathing of C02 is prevented or minimal due to C02 absorbers -low fresh gas flows have greater variation of oxygen and ansethetic gas concentrations between fresh gas and inspired gas -high fresh gas flows (> 5L.min) the rebreathing of C02 is minimal and may be no need for a C02 absorber -high fresh gas flow less time needed for fresh anesthetic concentrations to reflect a change in the inspired gas concentration -high fresh gas flow rates speed induction and recovery, and compensate for leaks in the breathing circuit (fresh gas flows equal to the anesthetic uptake from the patient constitute a closed breathing system) Dead space -ventilated airways which are not perfused therefore no gas exchange occurs -dead space in the circle system breathing circuits begins distal to the Y-piece where inspiratory and expiratory gases mix -length of the circle system breathing tube does not affect the dead space directly (unlike the mapleson breathing circuits) -length of the circle system breathing tube does affect the compliance and therefore the amount of tidal volume lost to the circuit Resistance -increased circle system resistance occurs from unidirectional valves and the C02 absorber -circle system resistance is increased especially with high respiratory rates and large tidal volume Humidity and heat conservation -heat and humidity in the inspired gas depends on the relative portion of the rebreathed alveolar gas to fresh gas high flow rates: are associated with low relative humidity low flow rates: are associated with high relative humidity bacterial contamination -smal risk of microorganisms remaining in the circle system components which could possibly lead to respiratory infections -bacterial filters are sometimes added into the inspiratory or expiratory limbs of the circle system breathing circuits OPTIMIZING THE CIRCLE SYSTEM DESIGN -arrangement of the circle system components can differentiate an optimized from nonoptimzed circle system design preferred arrangement of circle system components for optimal design are as follows: unidirectional valves should be: -close to the patient which prevents backflow into the inspiratory limb of the breathing circuit -not placed in the breathing Y-piece making it difficult to confirm proper positioning fresh gas inlet should be: -between the absorber and the inspiratory valve -downstream positon avoided since this would allow fresh gas to bypass the patient during exhalation and therefore wasted -postioned between expiratory valve and absorber avoided since fresh gas would be diluted by recirculating alveolar gas pressure-relief valve should be: -immediately before the C02 absorber in order to conserve absortion capacity and to minimize venting of fresh gas resevoir bag should be -located in the expiratory limb of the breathing circuit in order to decrease the resistance of exhalation Disadvantages of the Circle System -larger size compared with the mapleson system -less portability due to greater size -greater complexity -higher risk of disconnection or malfunction amongst the complexity of components -increased air flow resistance -difficulty predicting inspired gas concentrations while using low fresh gas flow rates