Lecture 7 9/5/24 Flashcards
What are the boundaries of the breathing circuit?
-starts at the junction with the common gas outlet
-connects to ET tube/mask
What are the functions of the breathing circuit?
-deliver O2 and volatile agent to patient
-allow intermittent positive pressure ventilation
-deliver waste gases to scavenging system
-allow measurement of airway pressure
What is rebreathing?
inhalation of previously exhaled gases, from which CO2 may or may not have been removed
What is equipment dead space?
space in the breathing circuit where inhalation and exhalation can coincide
What is the importance of equipment dead space?
it makes it possible for patients to inhale unfiltered exhaled gas
Where are the four critical places where equipment dead space is found?
-portion of ETT that extends out of the trachea
-elbow on the breathing circuit
-any connector used between the end of the ETT and the breathing circuit
-Y piece at the end of a Y circuit
What is tidal volume?
amount of air that moves in or out of the lungs with each respiratory cycle
What is minute volume?
sum of all gas volumes either inhaled or exhaled in one minute; tidal volume x resp. rate
What are the basic components of the breathing circuit?
-CO2 absorbent canister
-unidirectional valves
-APL/pop-off valve
-anesthesia breathing tubes
-reservoir bag
-pressure manometer
What are the characteristics of a non-rebreathing circuit?
-for patients smaller than 5-7 kgs
-O2 flow rate of 100-600 ml/kg/min
-more expensive
-no CO2 absorption system; fresh gas only
-faster speed of anesthesia depth changes
-minimum resistance
What are the characteristics of a rebreathing circuit?
-for patients larger than 5-7 kgs
-O2 flow rate of 10-40 ml/kg/min
-less expensive
-CO2 absorbed via soda lime
-slower speed of anesthesia depth changes
-larger resistance
Which components are absent from a non-rebreathing circuit?
-unidirectional valve
-CO2 absorbent
What are the benefits of a non-rebreathing circuit?
-no previously expired gas is re-breathed
-all gases exit the scavenge
-low resistance to ventilation
-altering the vaporizer settings results in rapid depth changes
How could a patient end up re-breathing CO2 on a non-rebreathing circuit?
hyperventilation
What are the cons of a non-rebreathing circuit?
-requires high fresh gas flows to push expired gases out to scavenging system
-high fresh gas flow contributes to patient’s heat and fluid losses
-more costly
-increases environmental pollution
How is the flowmeter setting chosen?
according to patient’s minute ventilation
What is minute ventilation?
resp rate/min x tidal volume
What is a Bain coaxial tube?
dual tube in which an inner tube carries fresh inspired gas and an outer tube carries expired gas
What are the disadvantages of the Bain non-rebreathing system?
-requires high fresh gas flow
-if inside tubing leaks and communicates with outside tubing, there is increased dead space and CO2 re-inhalation
What is a Bain block?
a mounted component of an anesthesia machine consisting of an APL valve and a manometer to monitor pressure in the breathing circuit
What are the benefits of a rebreathing circuit?
-more economical
-less polluting
-fresh gas flow just needs to meet metabolic oxygen requirements
-warms and moistens inspired air as heat and water are generated by CO2 absorption
-minimal apparatus dead space
What are the cons of a rebreathing circuit?
-recycling of exhaled gases dilutes fresh gas and volatile anesthetic coming from the vaporizer
-slower changes in anesthesia planes
-lengthy tubing offers more resistance and work of breathing
What is the primary function of the breathing tube?
convey gases to and from the patient
How does the length of the breathing tube impact resistance and dead space?
increased length adds to resistance, but does not increase dead space
Why are plastic breathing tubes better than rubber?
rubber can absorb the anesthetic
Why should a coaxial tube not be used with an ET tube larger than 10mm?
there is too much resistance due to the inside tube being smaller than the ET tube
What are the characteristics of the Y piece?
-connection between the ETT and a parallel breathing circuit
-increased mechanical dead space
-mixture of inspired and expired gases
What are unidirectional valves?
one-way valves, both inspiratory and expiratory, that ensure a unidirectional flow of air to and away from the patient
When does the inspiratory valve open and close?
-opens during the beginning of inspiration
-closes during expiration
When does the expiratory valve open and close?
-opens during expiration
-closes during inspiration
What is the main function of the unidirectional valves?
prevent the mixing of fresh gas with the CO2 rich exhaled gas
What are the functions of the reservoir bag?
-allows accumulation of gas during exhalation
-helps monitor breathing
-allows IPPV
-adds compliance to the system
How is reservoir bag size calculated?
patient’s weight in kg x tidal vol. x 6
Why is it important to have an appropriate reservoir bag size?
-too large of a bag leads to slower changes in anesthetic depth
-too small of a bag leads to increased risk of barotrauma
Why can a reservoir bag be used to supply patients with breaths?
it contains recycled gas, but the CO2 has already been filtered out
What is the APL/pop-off valve?
-site of communication between the reservoir bag and the exhaust system
-principal security system of the circuit
What is the function of the APL/pop-off valve?
allows the evacuation of gases in excess in the circuit over a certain pressure in order to avoid overpressure
When should the APL valve be open vs closed?
-completely open during spontaneous ventilation
-closed when providing manual or mechanical positive pressure ventilation
What happens if the APL valve is closed while the patient is spontaneously ventilating?
-pressure increases in circuit
-reservoir bag becomes distended
-pressure increases in resp. airways to very high positive pressure
-venous return, cardiac output, and arterial blood pressure decrease
-cardiac arrest occurs
What are the characteristics of the pressure manometer?
-pressure gauge attached to the top of the absorber assembly
-calibrated in cm of water
-used to assess pressure during assisted/controlled ventilation as well as during ETT and machine leak checks
What aer the characteristics of CO2 absorbent?
-removes CO2 from all expired gases, allowing rebreathing of anesthetic gases and O2 only
-contact between CO2 and soda lime produces a color change related to pH
What is generated during the chemical reaction that removes CO2?
water and heat
How can exhaustion of the CO2 absorbent be determined?
-pH indicator/color change
-canister cold to touch
-increased inspired CO2/hypercapnia
-increased RR, HR, and BP
-granules are hard and not crumbly
What is the purpose of the exhaust system?
reduce personnel exposure to waste anesthetic agents vented from the breathing system
What are the characteristics of passive scavenging without a filter?
-simple duct goes from exhaust outlet to outside air
-simple and inexpensive
-not practical in all buildings
-more polluting
What are the characteristics of passive scavenging with a filter?
-activated charcoal canister connected to exhaust outlet
-removes halogenated anesthetics
-mobile and convenient
-can increase resistance to evacuation with high flow rates
-limited spare life
-does not eliminate N2O
What is active scavenging?
-exhaust outlet is connected to vacuum system via an interface that controls suction with a needle valve
-has positive and negative pressure relief valves in case reservoir bag becomes empty or too full
