Anesthesia Breathing Circuits Flashcards
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
Resistance
“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
Compliance
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)
Rebreathing
-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?
- FGF
- 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
- filters
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
