anesthesia breathing systems

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Anesthesia Breathing SystemsProf. Karen Haddock, CRNA

+ Anesthesia Breathing Systems Purpose

To deliver anesthetic gases and oxygen Offer a means to deliver anesthesia without significant increase in airway resistance

To offer a convenient and safe method of delivering inhaled anesthetic agents

+ Anesthesia Breathing Systems Basic Principles

All anesthesia breathing systems have 2 fundamental purposesDelivery of O2/Anesthetic gasesElimination of CO2 (either by washout with adequate fresh gas flow (FGF) or by soda lime absorption)

+ Anesthesia Breathing SystemsResistance to flow can be minimized by:Reducing the circuit’s lengthIncreasing the diameter Avoiding the use of sharp bendsEliminating unnecessary valvesMaintaining laminar flow

+ Anesthesia Breathing SystemsClassifications (controversial)Traditional attempts to classify circuits combine functional aspects (eg, extent of rebreathing) with physical characteristics (eg, presence of valves)

Based on the presence or absence ofA gas reservoir bagRebreathing of exhaled gasesMeans to chemically neutralize CO2

Unidirectional valves

+ Anesthesia Breathing SystemsClassifications

OpenSemiopenSemiclosedClosed

+ Anesthesia Breathing Systems


OpenNo reservoirNo rebreathingNo neutralization of CO2No unidirectional valves


OpenNasal cannulaOpen drop etherThink of it as anything where there is NO rebreathing and NO scavenging


SemiopenGas reservoir bad presentNO rebreathingNo neutralization of CO2No unidirectional valvesFresh gas flow exceeds minute ventilation

Examples includeMapleson A, B, C, DBainJackson-Rees

+ Anesthesia Breathing SystemsNon-rebreathing circuits

Mapleson – 1954 Mapleson D still commonly used Mapleson F is better known as Jackson-Rees Modified Mapleson D is also called Bain

Used almost exclusively in children Very low resistance to breathing The degree of rebreathing is influenced by method of ventilation

Adjustable overflow valve Delivery of FGF should be at least 2x the minute volume

+ Non-rebreathing Circuits

All non-rebreathing (NRB) circuits lack unidirectional valves and soda lime CO2 absorption

Amount of rebreathing is highly dependent on fresh gas flow (FGF)

Work of breathing is low (no unidirectional valves or soda lime granules to create resistance)


Closed Always has a gas reservoir bag Allows for TOTAL rebreathing of exhaled gases Always provides for chemical neutralization of CO2

Always contains unidirectional valves We don’t use these….Suffice to say you can do this with the machines we have now if you keep your fresh gas flow to metabolic requirements around 150ml/min

+ Circle System

+ Optimization of Circle Design Unidirectional Valves

Close to pt to prevent backflow into inspiratory limb if circuit leak develops.

Fresh Gas Inlet Placed between absorber & inspiratory valve. If placed downstream from insp valve, it would allow FG to bypass pt during exhalation and be wasted. FG placed between expiration valve and absorber would be diluted by recirculating gas

+ Optimization of Circle Design APL valve

Placed immediately before absorber to conserve absorption capacity and to minimize venting of FG

Breathing Bag Placed in expiratory limb to decrease resistance to exhalation. Bag compression during controlled ventilation will vent alveolar gas thru APL valve, conserving absorbent

+ Circle system can be:

closed (fresh gas inflow exactly equal to patient uptake, complete rebreathing after carbon dioxide absorbed, and pop-off closed)

semi-closed (some rebreathing occurs, FGF and pop-off settings at intermediate values), or

semi-open (no rebreathing, high fresh gas flow)

+ Anesthesia Breathing Systems Circle systems

Unidirectional valves Prevent inhalation of exhaled gases until they have passed through the CO2 absorber (enforced pattern of flow)

Incompetent valve will allow rebreathing of CO2

Hypercarbia and failure of ETCO2 wave to return to baseline

Pop off (APL) Valve Allows pressure control of inspiratory controlled ventilation

Allows for manual and assisted ventilation with mask, LMA, or ETT

+ Anesthesia Breathing SystemsCircle systems

Most commonly usedAdult and child appropriate sizes

Can be semiopen, semiclosed, or closed dependent solely on fresh gas flow (FGF)

Uses chemical neutralization of CO2

Conservation of moisture and body heat

+ Anesthesia Breathing SystemsCircle system

Allows for mechanical ventilation of the lungs using the attached ventilator

Allows for adjustment of ventilatory pressure

Is easily scavenged to avoid pollution of OR environment

Low FGF’s saves money

+ Anesthesia Breathing Systems Advantages of rebreathing

Cost reduction (use less agent and O2) Increased tracheal warmth and humidity Decreased exposure of OR personnel to waste gases

Decreased pollution of the environment REMEMBER that the degree of rebreathing in an anesthesia circuit is increased as the fresh gas flow (FGF) supplied to the circuit is decreased


Dead space Increases with the use of any anesthesia system Unlike Mapleson circuits, the length of the breathing tube of a circle system DOES NOT directly affect dead space

Like Mapleson’s, length DOES affect circuit compliance (affecting amount of TV lost to the circuit during mech vent)

Increasing dead space increases rebreathing of CO2 To avoid hypercarbia in the face of an acute increase in dead space, a patient must increase minute ventilation

Dead space ends where the inspiratory and expiratory gas streams converge


Carbon dioxide neutralizationInfluenced by

Size of granules Presence or absence of channeling in the canister (areas of loosely packed granules, minimized by baffle system)

Tidal volume in comparison to void space of the canister

Ph sensitive dye Ethyl violet indicator turns purple when soda lime exhausted (change when 50-70% has changed color)

Regeneration: Exhausted granules may revert to original color if rested, no significant recovery of absorptive capacity occurs

+ Anesthesia Breathing SystemsCarbon dioxide neutralization

Maximum absorbent capacity 23-26L of CO2/100g granules

Granules designated by Mesh size (4-8 mesh)A compromise between higher absorptive surface area of small granules & the lower resistance to gas flow of larger granules

Toxic byproductsThe drier the soda lime, the more likely it will absorb & degrade volatile anesthetics

+ Disadvantages of Circle SystemGreater size, less portabilityIncreased complexity

Higher risk of disconnection or malfunction

Increased resistanceDissuading use in Pediatrics

Difficulty of predicting inspired gas concentration during low fresh gas flow


Airway Humidity Concerns Anesthesia machine FGF dry and cold

Medical gas delivery systems supply dehumidified gases at room temp.

Exhaled gas is saturated with H2O at body temp High flows (5 L/min) low humidity Low flows (<0.5 L/min) allow greater H2O saturation Absorbent granules: significant source of heat/moisture (soda lime 14-19% water content)

Normal upper airway humidification bypassed under General Anesthesia

Passive heat and humidity (“Artificial Nose”) Active heat and humidity (electrically heated humidifier)

Anesthesia Breathing Systems

Mode Reservoir Rebreathing

Example

Open No No Open drop

Semi-open Yes No Nonrebreathing circuit or Circle at high FGF (>VE)

Semi-closed

Yes Yes, partial

Circle at low FGF (<VE)

Closed Yes Yes, complete

Circle (if APL valve closed)

+Reference

Morgan, G. E., Mikhail, M. S., Murray, M. J., & Larson, P. C. , (2013). Clinical anesthesiology (4rd ed.). New York: The McGraw-Hill Companies, Inc..

Stoelting, R. K., Miller, R. D. ,(2007). Basic of Anesthesia (5th ed.). New York: Churchill Livingstone Elsevier, Inc. (2011). MemoryMaster. Des Moines:

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