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Flashcards in Anesthesia Breathing Circuits Deck (29)
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The breathing circuit system

-conduit through which gas flow to and from patient
1. delivers gases from machine to patient
2. eliminates CO2 (washout or soda lime absorption)
3. alters temperature and humidity of gases
4. converts continuous flow from the machine into intermittent flow to and from the patient
5. allows spontaneous, controlled, and assisted ventilation
6. allows for gas sampling, airway pressure, flow, and volume monitoring


Ideal breathing circuit:

-simple, delivers intended inspired gas mixture
-permits spontaneous and manual/controlled ventilation in all size groups
-efficient, requiring low fresh gas flow rate
-protects patient from barotrauma
-maintains moisture and heat
-sturdy and lightweight
-easy to remove gas waste
-easy to maintain



"impedance to flow"
pressure= flow X resistance
-a measure of the pressure drop between the inlet and the outlet as gas passes through a tube


Resistance: Laminar Flow

-smooth, orderly, parallel, fastest flow in middle (tip= parabolic)
pressure= (length x viscosity X flow rate)/ r ^4
biggest effect= r^4 *******


Resistant: Turbulent Flow

-non-parallel sides, eddies, same flow rate throughout
pressure= (length x flow rate ^2 x K)/ r^5
K= Konstant for gravity, friction, gas density and viscosity
-generalized turbulent flow when flow exceeds critical flow rate
-localized turbulent flow at bends and kinks



Distensibilty -- how stiff is it, how compliant is it?
-ratio of change in volume: change in pressure


Circle circuit vs Mapelson

Circle: carbon dioxide absorber, lots of rebreathing, conserves heat, unidirectional valves
Mapelson: no carbon dioxide absorber (depends on high flow rates)



-to inhale previously respired gases from which carbon dioxide may or may not have been removed


fresh gas flow

-amount of rebreathing varies INVERSELY with FGF rate
if FGF rate > minute ventilation= no rebreathing
IF scavenging or exhaust of exhaled gases at a point close to the respiratory tract

if FGF rate < minute ventilation= rebreathing to make up the required volume


What 2 factors affect rebreathing?

1. FGF
2. Mechanical dead space: volume in breathing system occupied by gases that are rebreathed without any change in composition. Can be minimized by separating the inspiratory and expiratory gas streams as close to the patient as possible


Circle system have high or minimal dead space?

-minimal dead space
-rebreathing is at the Y piece


Mapelson circuit: _______ dependent dead space

-Flow dependent dead space
-must have high FGF rates so that CO2 rebreathing does not occur


Effects of rebreathing

inhaled gas composition: breathing alveolar gas will cause a reduction in the inspired O2 content
-during induction rebreathing will reduce inspired anesthetic gas concentration and prolong induction
-during emergence alveolar concentration exceeds that of inspired gases so rebreathing will slow agent elimination
-breathing of CO2 will cause an increase in ETCO2 if its not absorbed well
-heat and moisture retention


Breathing Systems: General Components

-bushings (mounts) (modifies internal diameter)
-sleeves (modifies external diameter)
-connectors and adaptors
-reservoir bag
-breathing tubes
-adjustable pressure limiting valve (APL) pop off
-PEEP valves


APL valve

-the only gas exit from a breathing system unless a ventilator is being used
-used to control the pressure in a breathing circuit
-ASTM requires that clockwise motion increasing the limiting pressure and ultimately closes the valve
-ALWAYS set on OPEN during spontaneous respiratory
-can be used to add CPAP
-close as needed during assisted respiration to enable gas to be directed to patient
-isolated form breathing circuit during mechanical ventilation
-needs to be around 20mmHG when ventilator is on


Maples Breathing Systems

-No CO2 absorption (FGF washes out CO2)
-NO unidrectional valves
-no clear separation of inspired and expired gas
-rebreathing WILL occur depending on: minute vent, respriatory waveform, CO2 production (septic patients, epileptic patients, MH already have high CO2*), unresponsiveness, stimulation, physiologic dead space


What type of Mapelson circuit is used for pediatric patients?

F: Jackson Reese
Pediatric < 25 kg
2.5-3 X MV= FGF
minimum of 4 lpm


Mapelson A- Magill system

-FGF enters at the back of the system near the reservoir bag
-corrugated tubing between bag (FGF) and patient
-Lack modification: expiratory limb from patient to APL
-easier to scavenge but INCREASES work of breathing
1. Spontaneous ventilation
APL open--- gas exits during late expiration

2. Assisted/Controlled:
APL partially closed---- intermittent positive pressure to the bag


Mapelson E and F

E is a T-piece only, no bag, tubing may be attached to create a reservoir, difficult to scavenge

F (Jackson-Reese): functions like D
-FGF to prevent rebreathing similar to D
-offers slightly less work of breathing than in meds than ac circle system does (breathing through valves is hard for babies)


Advantages of Mapelsons

1. Simple, inexpensive, rugged
2. Variations in minute volume have less effect on PaCO2 than circle
3. Low resistance to breathing, light weight, easy to use
4. Lower compression and compliance volume losses than circle
5. FGF changes= rapid changes in inspired gas
6. No problems associated with CO2 absorbants


Disadvantages of Mapelsons

1. High FGF: cold patients, lots of wasted gas
2. Difficult to determine ideal FGF
3. APL location in A, B, C is awkward
4. Difficult scavenging
5. CANNOT be used with possible MH patient because you may no the able to blow off enough CO2****


Basics of the Circle System

-CO2 absorption
-unidirectional valves
-fresh gas inlet proximal to APL valve
-APL valve
-reservoir bag: ventilator- bag/vent switch
-gas monitoring
-airway pressure/volume monitoring
-PEEP valve, filter, humidifier


CO2 absorption

-granule size 4-8 mesh
-channeling reduces efficiency
-base neutralizing an acid
-carbonic acid is formed by the reaction of CO2 and water (EXOTHERMIC)


CO2 Absorbents

Older absorbents: high amounts of K or Na hydroxides led to CO and Compound A (sevo)
-newer absorbents don't have K hydroxide and or little Na hydroxide
-today mostly calcium hydroxide, NO CO formation,
little compound A and does not lose color when dry
-FICO2 monitor it the BEST indication of exhausted


If using Na or K hydroxide absorbents

-turn gas flow off when not in use
-turn vaporizers off when not in use
-change absorbent at least weekly
-do not use fresh gas to dry breathing circuits
-monitor temperature of canister (more of an issue when barium hydroxide was used)


Unidirectional valves

Two: inspiratory and expiratory
-Increase resistant to breathing
may become incompetent= rebreathing (more likely on expiration valve)
Barotrauma is inspiratory valve gets stuck*


What happens if expiratory valve is stuck?

1. elevated baseline CO2on waveform (will not return to zero in between breaths)
2. delayed downslope of CO2 waveform


Goals of circle system

-minimize absorbent desiccation
-maximum inclusion of FGF in the inspired limb and maximum venting of alveolar gas in the expiratory limb: results in faster induction and emergence
-important in the use of lower FGF rate
-accurate readings for spirometer monitor: placing the monitor close to FGF may alter readings
-maximum humidification of gases
-minimal dead space
-low resistance


Issues with circle circuit?

-increased resistance when compared to mapleson-- has not really been shown
-dead space- not really an issue unless extensions used at y port
-heat and humidity increased
-gas concentrations: no breathing, gas vapor concentration in the inspired mixture are the same as those in fresh gas
-low flow/closed circuit