1. Anesthesia Breathing Systems
Juan E Gonzalez, CRNA, MS
Assistant Clinical Professor
Anesthesiology Nursing Program
Florida International University

2. 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
To annoy you with yet one more thing to memorize

3. Anesthesia Breathing Systems
Basic Principles
All anesthesia breathing systems have 2 fundamental purposes
Delivery of O2/Anesthetic gases
Elimination of CO2
All breathing circuits create some degree of resistance to flow

4. 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

5. 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

6. 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

7. Anesthesia Breathing Systems
Classifications
Open
Semiopen
Semiclosed
Closed

8. 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)

9. Anesthesia Breathing Systems

10. Anesthesia Breathing Systems
Classifications
Open
NO reservoir
NO rebreathing
No neutralization of CO2
No unidirectional valves
Examples include
Nasal Cannula
Open drop ether

11. Anesthesia Breathing Systems
Classifications
Open
Nasal cannula
Open drop ether
Think of it as anything where there is NO rebreathing and NO scavenging

12. 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

13. Anesthesia Breathing Systems
Classifications
Semiclosed
A type of “circle system”
Always has a gas reservoir bag
Allows for PARTIAL rebreathing of exhaled gases
Always provides for chemical neutralization of CO2
Always contains 3 unidirectional valves (insp, exp, APL)
Fresh gas flow is less than minute ventilation
Examples – The machine we use everyday!

14. Anesthesia Breathing Systems
Classifications
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)

15. 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

16. Anesthesia Breathing Systems
(APL)
APL
Insp
Exp

17. 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)

18. 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).

19. 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
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

20. Anesthesia Breathing Systems
Press-relief valve (APL)
Press-relief valve
FGF
FGF
Mask
Breathing Bag
Mapleson Circuits
FGF
FGF
Breathing Bag
Press-relief valve
FGF
FGF
Press-relief valve

21. 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 of breathing circuit = or > TV to minimize FGF requirements
Fresh Gas Inlet (position will determine type of Mapleson performance and classification)

22. 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

23. 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)

24. Mapleson D *FGF forces alveolar gas away from pt toward APL valve
*Efficient during ControlleD Ventilation

25. Mapleson
Mapleson D Mapleson C Mapleson F (Jackson-Rees)

26. Anesthesia Breathing Systems
Bain system (
Coaxial (tube within a tube) version of Mapleson D
Fresh gas enters through narrow inner tube
Exhaled gas exits through corrugated outer tube
FGF required to prevent rebreathing:
ml/kg/min with spontaneous breathing (2 times VE)
70ml/kg/min with controlled ventilation

27. 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.

28. 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.

29. 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.  

30. 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)

31. Anesthesia Breathing Systems
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

32. Anesthesia Breathing Systems
Bain
Disadvantages
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
Look at the Bain and identify what makes it modified from the standard Mapleson D

33. Bain is a Modified Mapleson D

34. Anesthesia Breathing Systems

35. 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.

36. Circle System
CAN: Canister of CO2 absorber
RB: Reservoir bag

37. Optimization of Circle Design
Unidirectional Valves
Placed 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

38. 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

39. 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)

40. Anesthesia Breathing Systems
Circle systems
Most commonly used
Adult 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
Low FGF’s saves money

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

42. Anesthesia Breathing Systems
Circle system
Allows for mechanical ventilation of the lungs using the attached ventilator
Allows for adjustment of ventilatory pressure
Allows for semiopen, semiclosed, and closed systems based solely on FGF
Is easily scavenged to avoid pollution of OR environment

43. 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

44. Anesthesia Breathing Systems

45. 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

46. 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!!)

47. 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 byproducts
The 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

48. 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

49. 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)

50. 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

51. Mode
Reservoir
Rebreathing
Example
Open No
Open drop
Semi-open
Yes
Nonrebreathing circuit or
Circle at high FGF (>VE)
Semi-closed
Yes, partial
Circle at low FGF (<VE)
Closed
Yes, complete
Circle (if APL valve closed)

52. The End
1. Mapleson WW. The elimination of rebreathing in various semiclosed anaesthetic systems. British journal of Anaesthesia; 1954;26:  
2. Ward CS. In: Anaesthetic equipment. Physical principles and maintenance; W.B.Saunders, London; 2nd ed