Anesthesia Breathing Systems Juan E Gonzalez, CRNA, MS Assistant Clinical Professor Anesthesiology...

Anesthesia Breathing Systems

Juan E Gonzalez, CRNA, MS

Assistant Clinical Professor

Anesthesiology Nursing Program

Florida International University


PurposeTo deliver anesthetic gases and oxygenOffer a means to deliver anesthesia without significant

increase in airway resistanceTo offer a convenient and safe method of delivering

inhaled anesthetic agentsTo annoy you with yet one more thing to memorize


Basic PrinciplesAll anesthesia breathing systems have 2 fundamental

purposesDelivery of O2/Anesthetic gasesElimination of CO2

All breathing circuits create some degree of resistance to flow

Anesthesia Breathing Systems Resistance to flow can be minimized by:

Reducing the circuit’s length Increasing the diameter (who’s law is that??)

Hagen-Poiseuille P = (L)(v)(V) r4

P is pressure gradient. L is length. v is viscosity. V is flow rate RESISTANCE IS INDIRECTLY PROPORTIONAL TO FLOW RATE WITH LAMINAR FLOW

Flow = P1-P1/R where P1 is pressure at one end of a tube and P2 is pressure at the other end of the tube

FOR TURBULENT FLOW, GAS DENSITY IS MORE IMPORTANT THAN VISCOSITY RESISTANCE IS PROPORTIONAL TO THE “SQUARE” OF FLOW RATE (TURBULENT FLOW) IN CLINICAL PRACTICE, FLOW IS USUALLY A MIXTURE OF LAMINAR & TURBULENT

FLOW

Avoiding the use of sharp bends (turbulent flow) Eliminating unnecessary valves Maintaining laminar flow

Another look at Poiseuille’s Law Laminar flow: orderly movement of gas inside a “hose” (gas

in the center of the tube moves faster than gas closer to walls)

Turbulent flow: resistance is increased (seen with sudden narrowing or branching of tube)

Laminar flow becomes Turbulent when Reynold’s number is >2000

Poiseuille’s Law follows Laminar flow R = 8 n l (R: resistance, n: viscosity, l: length, r: radius)

r4

Example: doubling the radius of the tube will decrease the resistance 16 times (2)4=16

Anesthesia Breathing Systems 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 of A gas reservoir bag

provides gas for the moments during inspiration where flow in the trachea is greater than fresh gas flow (FGF)

Rebreathing of exhaled gases Means to chemically neutralize CO2 Unidirectional valves


ClassificationsOpenSemiopenSemiclosedClosed

Function of any breathing circuit

Deliver oxygen and anesthetic gases Eliminate CO2 (either by washout with adequate

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


ClassificationsOpen

NO reservoirNO rebreathingNo neutralization of CO2No unidirectional valvesExamples include

Nasal Cannula Open drop ether


ClassificationsOpen

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

NO scavenging

Anesthesia Breathing Systems Classifications

Semiopen Gas reservoir bag present NO rebreathing No neutralization of CO2 No unidirectional valves Fresh gas flow needed exceeds minute ventilation (two to three times

minute ventilation to prevent rebreathing). Minimum FGF 5L/min Examples include

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


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 valves (insp, exp, APL)Fresh 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 (supply of O2, N2O and VAA just matches pt’s requirements) If pt spontaneously ventilating, APL valve should totally closed (no scavenging since no waste total rebreathing)

Anesthesia Breathing Systems Nonrebreathing circuits

Mapleson Classification – 1954 Mapleson D still commonly used

Modified Mapleson D is also called Bain. Arrangement of components (entry point of fresh gas, reservoir gas, APL valve) is similar in both. The main difference is that the Bain has the fresh gas hose inside the expiratory corrugated limb (tube within a tube). Unrecognized kinking of inner inspiratory hose will convert the expiratory outer hose into dead space.

Mapleson F is better known as Jackson-Rees modification of Ayre’s T-piece

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


(APL)

APL

Insp

Exp

Non-rebreathing Circuits

All non-rebreathing (NRB) circuits lack unidirectional valves (insp & exp) 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 pop-off (APL) valve.

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 out more thoroughly).

All NRB circuits are convenient, lightweight, easily scavenged (if using appropriate FGF).

Anesthesia Breathing Systems Mapleson

Advantages Used during transport of children Minimal dead space, low resistance to breathing Scavenging (variable ability, depending on FGF used)

Disadvantages Scavenging (variable ability, depending on FGF used) High flows required (cools children, more costly) Lack of humidification/heat (except Bain) Possibility of high airway pressures and barotrauma Unrecognized kink of inner hose in Bain Pollution and higher cost Difficult to assemble


FGF

FGF

FGF

FGF

FGF

FGF

Mapleson CircuitsMaskBreathing Bag

Press-relief valve (APL)

Press-relief valve

Press-relief valve

Breathing Bag

Press-relief valve

Mapleson Components

Breathing TubesCorrugated tubes connect components of Mapleson to ptLarge diameter (22mm) creates low-resistance pathway

for gases & potential reservoir for gasesVolume of breathing circuit = or > TV to minimize FGF

requirements Fresh Gas Inlet (position will determine type of

Mapleson performance and classification)

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

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

APL fully open during spontaneous ventilation APL partial closure while squeezing breathing bag (assisted

ventilation)

Breathing Bag Reservoir Bag of gases Method 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 (Poor choice)

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

http://www.capnography.com/Circuits/maplesona.htm

Mapleson D

*FGF forces alveolar gas away from pt toward APL valve

*Efficient during ControlleD Ventilation

Mapleson

Mapleson D Mapleson C Mapleson F (Jackson-

Rees)


Bain system (http://www.capnography.com/Circuits/bainsystem.htm)

Coaxial (tube within a tube) version of Mapleson DFresh gas enters through narrow inner tubeExhaled gas exits through corrugated outer tubeFGF required to prevent rebreathing:

200-300ml/kg/min with spontaneous breathing (2 times VE)70ml/kg/min with controlled ventilation

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’s 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 breathes in 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’s 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 (gas follows the path of least resistance)


Bain Advantages

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

Improved humidification with partial rebreathingEase of scavenging waste gasesOverflow/pressure valve (APL valve)Disposable/sterile

Anesthesia Breathing Systems Bain

DisadvantagesUnrecognized disconnectionKinking of inner fresh gas flow tubingRequires high flowsNot easily converted to portable when commercially used

anesthesia machine adapter Bain circuit usedLook at the Bain and identify what makes it modified

from the standard Mapleson D

Bain is a Modified Mapleson D

(APL)

Pethick’s Test for the Bain Circuit A unique hazard of the use of the Bain circuit is occult

disconnection or kinking of the inner hose (fresh gas delivery hose). To perform the Pethick’s test, use the following steps: Occlude the patient's end of the circuit (at the elbow). Close the APL valve. Fill the circuit, using the oxygen flush valve (like pressurizing

the circuit when you are doing a leak test) Release the occlusion at the elbow and flush. A Venturi effect

flattens the reservoir bag if the inner tube is patent.

Circle System

CAN: Canister of CO2 absorber

RB: Reservoir bag

Optimization of Circle Design

Unidirectional ValvesPlaced in close proximity to pt to prevent backflow into

inspiratory limb if circuit leak develops. Fresh Gas Inlet

Placed b/w absorber & inspiratory valve. If placed downstream from insp valve, it would allow FG to bypass pt during exhalation and be wasted. If FG were placed b/w expiration valve and absorber, FG would be diluted by recirculating gas

Optimization of Circle Design

APL valvePlaced 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 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 heatLow FGF’s saves money

Anesthesia Breathing Systems Circle systems

Unidirectional valvesPrevent inhalation of exhaled gases until they have passed

through the CO2 absorber (enforced pattern of flow)Incompetent valve will allow rebreathing of CO2Hypercarbia and failure of ETCO2 wave to return to baseline

Pop off (APL) ValveAllows pressure control of inspiratory controlled ventilationAllows for manual and assisted ventilation with mask, LMA,

or ETT (anesthetist will regulate APL valve to keep breathing bag not too deflated or inflated)


Circle systemAllows for mechanical ventilation of the lungs using

the attached ventilatorAllows for adjustment of ventilatory pressureAllows for semiopen, semiclosed, and closed systems

based solely on FGF Is easily scavenged to avoid pollution of OR

environment

Anesthesia Breathing Systems Advantages of rebreathing

Cost reduction (use less agent and O2) Increased tracheal warmth and humidityDecreased exposure of OR personnel to waste gasesDecreased 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 Systems 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 Use of a mask is associated with greater dead space than an ETT

Anesthesia Breathing Systems Carbon 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

TV should not exceed air space between absorbent granules (1/2 absorbent capacity)

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 (change canister!!)

Anesthesia Breathing Systems Carbon dioxide neutralization

Maximum absorbent capacity 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 (this is bad since the absorber is designed to absorb CO2 and not to further degradeVAA

Disadvantages of Circle System Greater size, less portability Increased complexity

Higher risk of disconnection or malfunction Increased resistance (of valves during spontaneous

ventilation)Dissuading use in Pediatrics (unless a circle pedi

system used) Difficult prediction of inspired gas concentration

during low fresh gas flow

Anesthesia Breathing Systems 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)

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

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)

The End 1. Mapleson WW. The

elimination of rebreathing in various semiclosed anaesthetic systems. British journal of Anaesthesia; 1954;26: 323-32.

2. Ward CS. In: Anaesthetic equipment. Physical principles and maintenance; W.B.Saunders, London; 2nd ed. 1985.

Anesthesia Breathing Systems Juan E Gonzalez, CRNA, MS Assistant Clinical Professor Anesthesiology...

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