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


Basic PrinciplesAll anesthesia breathing systems have

2 fundamental purposes Delivery of O2/Anesthetic gases Elimination of CO2 (either by washout with

adequate fresh gas flow (FGF) or by soda lime absorption)


Resistance to flow can be minimized by: Reducing the circuit’s length Increasing the diameter Avoiding the use of sharp bends Eliminating unnecessary valves Maintaining laminar flow


Classifications (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 CO2Unidirectional valves


ClassificationsOpenSemiopenSemiclosedClosed


ClassificationsOpen

No reservoirNo rebreathingNo neutralization of CO2No unidirectional valves


ClassificationsOpen

Nasal cannulaOpen drop etherThink of it as anything where there is

NO rebreathing and NO scavenging

+Anesthesia Breathing SystemsClassifications

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

Mapleson A, B, C, D Bain Jackson-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)

+How do NRB’s work?During expiration, fresh gas flow (FGF) pushes

exhaled gas down the expiratory limb, where it collects in the reservoir (breathing) bag and opens the expiratory valve (pop-off or APL).

The next inspiration draws on the gas in the expiratory limb. The expiratory limb will have less carbon dioxide (less rebreathing) if FGF inflow is high, tidal volume (VT) is low, and the duration of the expiratory pause is long (a long expiratory pause is desirable as exhaled gas will be flushed more thoroughly).

All NRB circuits are convenient, lightweight, easily scavenged.


Mapleson Used during transport of children Minimal dead space, low resistance to breathing Disadvantages

Scavenging (variable ability) High flows Lack of humidification/heat Possibility of high airway pressures and barotrauma


FGF

FGF

FGF

FGF

FGF

FGF

Mapleson Circuits

MaskBreathing Bag

Press-relief valve

Press-relief valve

Press-relief valve

Breathing Bag

Press-relief valve

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22Mapleson Systems

+Mapleson Components Breathing Tubes

Corrugated tubes connect components of Mapleson to pt

Large diameter (22mm) creates low-resistance pathway for gases & potential reservoir for gases

Volume = or > TV to minimize FGF requirements

Fresh Gas Inlet (position will determine type of Mapleson performance)

+Mapleson Components Pressure-Relief Valve (Pop-Off Valve, APL)

If gas inflow > pt’s uptake & circuit uptake = press buildup (out via scavenger)

APL fully open during spontaneous ventilation

APL partial closure while squeezing breathing bag

Breathing Bag Reservoir Bag of gasesMethod of generating positive pressure

ventilation

Mapleson A

Mapleson A Since No gas is vented during expiration, high

unpredictable FGF (> 3 times minute ventilation) needed to prevent rebreathing during mechanical ventilation

Most efficient design during spontAneous ventilation since a FGF = minute ventilation will be enough to prevent rebreathing)

Mapleson A (Magill) System

The Mapleson A or Magill system is good for spontaneous breathing patients, so the fresh gas flow can be lower. However as the APL valve is close to the patient, it is regarded by many as difficult to use.

1950’s

Mapleson A (Lack) System

The Mapleson A or Lack system is a modification of the Magill where the valve is moved to the machine end of the system using another length of tubing. This adds volume to the system and makes it rather heavy at the patient end.

1976

+Mapleson D

FGF forces alveolar gas away from pt toward APL valveIt requires very high fresh gas flows to prevent rebreathing of CO2Efficient during ControlleD Ventilation

FGIAPLvalve

+Mapleson F (Jackson Rees Modification)

The Mapleson F or Jackson Rees modification of the Ayres T Piece is a basic system for use with very small patients. It is a big disadvantage that you cannot remove waste gases safely.

Because this has a bag with an open tail, it is technically a Jackson-Rees Modification system

Ayres – 1937JR - 1950

Mapleson C Bagging System

The Mapleson C is more than an anesthesia system. It can be found all over the hospital for use as an emergency bagging system for resuscitation or manual ventilation using oxygen, as well as being a standard induction system in some countries.


Bain system Coaxial version of Mapleson D Fresh gas enters through narrow

inner tube Exhaled gas exits through

corrugated outer tube FGF required to prevent

rebreathing: 200-300ml/kg/min with

spontaneous breathing (2 times VE)

70ml/kg/min with controlled ventilation

1972

+Bain at work (spontaneous)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.

During expiration, there is a continuous FGF into the system at the pt 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.

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.

+Bain at work (spontaneous)

During the next inspiration, the pt breaths 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.

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.

Bain at work (controlled)

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.

During expiration, the expired gas continuously gets mixed with the fresh gas that is flowing into the system at the patient end.

During the expiratory pause the FG continues to enter the system and pushes the mixed gas towards the reservoir.

+Bain at work (controlled)

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).

As the pressure in the system increases, the APL valve opens and the contents of the reservoir bag are discharged into the scavenger.


Bain Advantages

Warming of fresh gas inflow by surrounding exhaled gases (countercurrent exchange)

Improved humidification with partial rebreathing

Ease of scavenging waste gases Overflow/pressure valve (APL valve) Disposable/sterile


BainDisadvantages

Unrecognized disconnection Kinking of inner fresh gas flow tubing Requires high flows Not easily converted to portable when

commercially used anesthesia machine adapter Bain circuit used

+Bain is a Modified Mapleson D


SemiclosedA type of “circle system”Always has a gas reservoir bagAllows for PARTIAL rebreathing of exhaled

gasesAlways provides for chemical neutralization

of CO2Always contains 3 unidirectional valvesFresh gas flow is less than minute

ventilationExamples: The machine we use everyday!


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)


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


Circle systemsMost commonly usedAdult and child appropriate sizesCan be semiopen, semiclosed, or

closed dependent solely on fresh gas flow (FGF)

Uses chemical neutralization of CO2Conservation of moisture and body

heat


Circle systemAllows 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


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

+Anesthesia Breathing SystemsDead 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

+Anesthesia Breathing SystemsCarbon dioxide neutralization

Influenced 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


Carbon 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 byproducts The drier the soda lime, the more likely it will absorb &

degrade volatile anesthetics

+Disadvantages of Circle System

Greater size, less portability Increased complexity

Higher risk of disconnection or malfunction

Increased resistance Dissuading use in Pediatrics

Difficulty of predicting inspired gas concentration during low fresh gas flow

+Anesthesia Breathing SystemsAirway 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)

+Bacterial Contamination

Slight risk of microorganism retention in Circle system that could (theoretically) lead to respiratory infections in subsequent pts

Bacterial filters are incorporated into EXPIRATORY LIMB of the circuit

Anesthesia Breathing Systems

Mode Reservoir Rebreathing ExampleOpen 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:

anesthesia breathing systems

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