Scavenging systems, capnography, CO2 absorption Flashcards
What is the definition of scavenging?
collection of excess gases from equipment used in the admin of anesthesia or exhaled by patients.
removal of these gases to an appropriate place of discharge outside working environment
What is the NIOSH recommended level of anesthetic gas alone in the OR?
2ppm
What is NIOSH recommended level of nitrous in the OR?
25ppm
What is the NIOSH recommended level of volatile anesthetic and nitrous in the OR?
0.5ppm
What are the 5 basic components of the scavenging system?
- gas collection assembly
- transfer means
- scavenging interface
- gas disposal tubing
- gas disposal assembly
What is the role of the gas collecting assembly?
captures excess gases at the site of emission and delivers them to the transfer means tubing
What is the size of the outlet tubing for the gas collecting assembly?
usually 30mm male (19mm on older machines)
What is the importance of the size of outlet tubing?
size of connections are important so that it doesnt connect to other components of the breathing system
Describe the role of the transfer means component?
conveys gas from gas collecting assembly to interface
What are some other common names of the transfer means component?
exhaust tubing or hose and transfer system
Describe the transfer means tubing.
usually female fitting connectors on both ends; tubing is short w/ large diameter; must be kink resistant; must have different characteristics from breathing tubes (ie colored coded yellow and more stiff)
Why does the transfer means tubing need to be short w/ large diameter?
to carry high gas flow w/o significant increase in pressure
What is the role of the scavenging interface?
prevents pressure increases or decreases in scavenging system from being transmitted to the breathing system
What are some other common names for the scavenging interface?
balancing valve; balancing device
The scavenging interface limits ___________, immediately downstream of the _____________to between ______________ and ____________
pressure
gas collecting assembly
-0.5 to 0.5cmH2O
What is the size of the scavenging interface?
inlet 30mm male
Where should the scavenging interface be situated?
as close to the gas collecting assembly as possible
What are the 3 basic elements of the scavenging interface?
- positive pressure relief
- negative pressure relief
- reservoir capacity
What is the importance of the positive pressure relief?
protects patient and equipment in case of occlusion
What is the importance of the negative pressure relief?
limits subatmospheric pressure
What is the importance of the reservoir capacity?
matches the intermittent gas flow from gas collecting assembly to the continuous flow of the disposal system
What are the 2 types of scavenging interface?
open and closed
T/F: The open interface has valves
false; no valves; it is open to atmosphere via holes in reservoir; avoid build up of neg or pos pressure
What does the open interface require?
requires use of central vacuum system and reservoir (open canister; size should allow for high waste gas flows)
How does the open interface work?
gas enters system at top of canister and travels through narrower innertube to base
T/F: the vacuum control valve can be adjusted
true: varies the level of suction on the canister/reservoir; must be greater than or equal to excess gas flow rate to prevent OR pollution
The closed interface is broken down into what type of systems?
positive pressure relief only or positive pressure and negative pressure relief
How does the positive pressure only closed interface work?
single positive pressure relief valve opens when a max pressure is reached and passive disposal
no vacuum used and no reservoir needed
How does the positive pressure & negative pressure relief closed interface work?
has both +/- pressure relief vavles w/ reservoir bag and gas is vented to the atmosphere when pressure exceeds 5cmH2O
for the closed interface w/ +/- pressure: what happens if the pressure falls below -0.5cm H2O?
room air is entrained
what happens if the primary negative pressure relief valve becomes occluded?
a backup negative pressure relief valve opens at -1.8cm H2O
What is the function of the gas disposal tubing?
it connects the scavenging interface to the disposal assembly
T/F: the gas disposal system should be long and thin?
false: short and wide because its a passive part of the system
How can you prevent the accidental occlusion/kinking of the disposal tubing?
by running the tubing overhead
What is the gas disposal assembly?
consists of components used to remove wastes from the OR
The gas disposal assembly is broken down into 2 types: what are the major differences?
active: mechanical flow inducing device that moves gases (produces neg pressure in disposal system; must have neg relief)
passive: pressure raised above atmospheric by patient exhalation, manual squeezing, or ventilator (needs pos pressure
What 3 ways does the passive system evacuate the waste gases?
- open window
- pipe passing thru outside wall
- extractor fan vented to outside air
What are the adv/disadv to passive system of gas disposal assembly?
adv: inexpensive, simple to operate
disadv: impractical in some buildings
How does the active system of gas disposal assembly function?
these systems connect the exhaust of the breathing system to hospital vacuum system via interface controlled needle valve
What are the adv/disadv of the active gas disposal assembly?
adv: convenient in large hospitals where many machines are used in diff locations
disadv: vacuum system and pipework is major expense; needle valve may need continual adjustment
T/F: the passive system of the gas disposal assembly is most commonly used in hospitals
false; active is
how do you perform a scavenging system check: 4 steps
- ensure proper connections btw scavenging system and both the APL valve and vent relief valve and waste gas vacuum
- fully open APL valve and occlude y piece
- with minimal O2 flow; allow scavenger reservoir bag to collapse completely and verify that pressure gauge read zero
- w/ O2 flush activated; allow scavenger bag to fully expand and then verify that the pressure gauge reads below 10cm H2O
T/F: capnography is the gold standard for confirmation of ETT placement?
true
What are some purposes of capnography?
confirm ETT placement; determine if patient is being ventilated; guide vent settings; detect abnormalities
T/F: there are some contraindications for using capnography
false: THERE ARE NO CONTRAINDICATIONS
What are some clinical uses of capnography?
estimate PaCO2 ( PaCO2 > PeCO2 by ~ 2-5mmHg); used to eval dead space
What methods are used to measure CO2 in expired gas?
colorimetric; infrared absorption spectrophotometry
how does colorimetric method work?
rapid assessment
uses metacresol purple impregnated paper which changes color in presence of acid
H2O + CO2= carbonic acid and paper changes color
How does the infrared absorption spectrophotometry work?
the amount of co2 is measured by detecting its absorbents at specific wavelengths and filtering the absorbents related to other gases
What are the 2 measurement techniques of capnography?
mainstream and sidestream
How is mainstream capnography used?
heated infrared measuring device placed in circuit
t/f: mainstream capnography is also called flow through?
true
t/f mainstream capnography has a longer time delay than side stream?
false; less time delay than sidestream
What do you want to make sure w/ mainstream capnography?
sensor window must be cleared of mucus
What are some disadv of mainstream capnography?
potential burns and weight of it can kink ETT
How is sidestream capnography used?
aspirates fixed amount of gas/min (50-500ml)
transports expired gas to sampling cell and uses IR analysis
compares sample to known quantity
what does sidestream capnography require?
calibration of known quantity to compare sample to
usually 5% or 35mmHg
T/F: the best placement for the sidestream is closest to the anesthesia machine
false: closest to ETT
What are some disadv to sidestream capnography?
time delay; potential disconnect source; pediatric sampling- lower Vt= dilution
water vapors/condensation: traps and filters must be used
how many phases are in the capnography waveform?
4
phase I is the ________ baseline
inspiratory
phase I is considered to be___________and the first part of ____________
inspiration; expiration
In phase I there is no _____________
CO2
What type of gas is considered to be exhaled in phase 1?
dead space exhaled gas
Phase II of the capnography wave form is the _________
expiratory upstroke
What does phase II represent?
sharp upstroke that represents rising CO2 level in sample
How is the slope of phase II determined?
evenness of alveolar emptying
What type of gas is considered to be exhaled in phase II?
mixture of dead space and alveolar gas
Phase III of capnography wave form is the ___________
alveolar plateau
What is the characteristic of the phase III plateau?
constant or slight upstroke; long phase
What type of gas is considered exhaled during phase III?
alveolar gas
Where is end tidal CO2 measured?
peak a the end of phase III plateau
What is the normal range for end tidal CO2?
30-40mmHg
End tidal CO2 can be considered a reflection of _______ and __________
PACO2 & PaCO2
Phase IV of the capnography wave form is the ________
beginning of inspiration
What is characteristic of Phase IV waveform?
rapid decline in CO2 concentration to inspired value
during end tidal tracing interpretation, what are the 5 things to look at?
- frequency
- rhythm
- height
- baseline
- shape
Presence of stable CO2 waveforms for __________breaths indicated tracheal intubation.
3
T/F: end tidal CO2 indicates proper position of ETT in trachea?
false; must listen for bilateral breath sounds
What are some other observations one can make from end tidal tracings?
frequency of Ve; disconnet indicator; quality of CO2 absorption; changes in perfusion or dead space
What changes will one see in the CO2 waveform of a patient w/ obstructive lung disease?
slow rate rise in phase II and little or no phase III
What are some examples of obstructive lung disease?
COPD, asthma, bronchospasm, acute obstruction
What changes in the CO2 tracing would one see if the esophagus was intubated?
any CO2 in stomach will quickly vanish, usually w/in 3 tidal volumes and the waveform will become essentially a flat line
How would one be able to distinguish if a patient is rebreathing via the CO2 tracing?
the CO2 tracing remains above the baseline (zero) at the end of phase IV
What are some causes of rebreathing?
equipment dead space, exhausted CO2 absorber, inadequate FGF
What would a waveform look like when a patient is beginning to spontaneously breath/recover from NMb?
there will be a curare cleft in phase III
What are cardiac oscillations and what does it do to the CO2 tracing?
the pumping of the heart can repeatedly press on the lungs and disturb the capnography graph causing oscillations in phase IV. It is not a concern.
What are some cuases of rising CO2 if the ventilation is unchanged?
MH, release of tourniquet; release of major vessel that was clamped; IV bicarb admin; insufflation of CO2 into peritoneal cavity; equipment defects
What are some causes of a decrease in ETCO2?
hyperventilation, PE, cardiac arrest, sampling error
What would the characteristics of the ETCO2 waveform be if the decrease in ETCO2 was caused by hyperventilation vs. PE?
hyperventilation: gradual decrease reflects increase in MV
PE: rapid decrease, increase in PaCO2 and PECO2 gradient
What is the purpose of the CO2 absorber?
chemical neutralization of CO2, base neutralizes acid
What is the acid and how is it formed?
H2CO3 is formed from CO2 and H2O
What is the base of a CO2 absorber?
hydroxide of an alkali or alkaline earth metal
What is the end product of the reaction in a CO2 absorber?
H2o, carbonate, heat
What are the 2 common types of CO2 absorber?
soda lime and amsorb plus (Ca hydroxide lime)
What are the components of soda lime?
4% sodium hydroxide 1% ptassium hydroxide 15% H2O 0.2% silica 80% calcium hydroxide
Why is silica added to soda lime?
for hardness and to prevent dust
What is the absorbent capacity of soda lime?
26L of CO2/100g of granules
Why is H2O in soda lime?
thin film on granule surface and moisture is essential for reaction to take place; it only takes place btw 2 ions if there is H2O
Describe the soda lime reaction
CO2 + H2O=H2CO3
H2CO3 + 2NaOH (KOH)= Na2CO3(K2CO3) + 2H2O + heat (fast)
Na2CO3(K2CO3) + Ca(OH)2= CaCO3 +2NaOH (KOH)
some CO2 many react directly w/ Ca hydroxide but is much slower
What are the components of amsorb plus?
80% calcium hydroxide
16% H2O
1-4% calcium chloride
calcium sulfate and polyvinlypyroolidine for added hardness
What is the absorbent capacity of calcium hydroxide lime (amsorb plus)
10L of CO2/100g of granules
Describe the calcium hydroxide lime reaction
CO2 + H2O =H2CO3
H2CO3 + Ca(OH)2 = CaCO3 + 2H2O + heat
How do you know if the CO2 canister is exhausted?
an acid or base whose color depends on the pH; color conversion = exhaustion
what color is most common to signify exhaustion?
ethyl violet
When should you change the canister graules?
when it is 50-70% exhausted
What must be aware of w/ the color change in CO2 absorber?
color reverts back to normal w/ rest
What are the diff types of indicators used and their associated color changes w/ regards to CO2 absorbers?
phenolphtalein: white to pink ethyl violet: white to purple clayton yello: red to yellow ethyl orange: orange to yellow mimosa 2: red to white
What is the general size of the granules?
4-8mesh
What is the general shape of granules, and why?
irregular: gives more SA
Small granules _______ resistance?
increase
Why do they blend small and large granules?
to minimize resistance w/ little sacrifice in absorbent capacity
What should the granules hardness be?
75 or greater
What is channeling in regards to the CO2 absorber?
preferential passage of exhaled gas flow through the absorber via pathways of least resistance
How does channeling happen and why is not good?
results from loosely packed granules and the granules along the pathways can become exhausted and you may not notice from the outside
rebreathing of CO2 may occur
How can you prevent channeling?
make sure the granules are tightly packed and/or shake the canister to close any open pockets in the canister
What happens if the soda lime becomes dry from high gas flow?
it can degrade sevo, iso, desflurane and enflurane to CO
What happens specifically to sevo and halothane if it comes in contact w/ dry soda lime?
they degrade to unsaturated nephrotoxic compounds (compound A)
Dry gas may be a _______
fire hazard
What are some recommendations on safe use of CO2 absorber?
turn off all gas flows when the machine is not in use; change absorbent frequently, change when granules are exhausted; change all abosrbent; change granules if uncertain of exhaustion; low flows preserve humidity
